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    Complete Helium Neon Laser Power Supply Schematics

    Schematics for power supplies of all sizes:
    ------------------------------------------
    
    This chapter provides a variety of circuits of both line powered and inverter
    types for the basic power supply, some with regulators, modulation inputs, and
    other enhancements.  Several have been reverse engineered from actual working
    commercial products.  Some of these have been modified or enhanced to provide
    additional capabilities like current sensing or modulation.  I have designed
    (and in most cases, constructed and tested as well) the others (those with
    names starting with "Sam's") for various power ratings and capabilities using
    commonly available parts in most cases.
    
    Be warned: There are so many complete HeNe power supply schematics in this one
    document that their combined mass may cause a singularity to form inside your
    computer :-).
    
    AC line operated power supply schematics:
    ----------------------------------------
    
    The first 5 circuits described in the following sections were reverse
    engineered from commercial HeNe power supplies.  There may be errors in
    transcription as well as interpretation.  In many cases, the transformer
    secondary voltage was not marked and where the actual hardware was not
    available for testing, an estimate of its value was made.
    
    Many of these designs are quite old since modern commercial units tend toward
    inverter designs since they can be more compact and have higher efficiency.
    Unfortunately, these are nearly always potted in Epoxy and impossible to
    analyze.
    
    The last one I built using the scrounged power transformer from a long dead
    and cannibalized tube type TV and a pair of high voltage capacitors (for the
    main filter on the doubler) that had been sitting in a box minding their own
    business for the last 15 years.  My total cost for the remaining components
    was about $5.
    
    AC Line operated power supplies will drive HeNe tubes just as well as fancy
    inverters and are somewhat easier to construct and troubleshoot.
    
    The line side circuitry is not shown for any of these.  See the section: "AC
    input circuitry for HeNe power supplies" for details.
    
    Edmund Scientific HeNe power supply:
    -----------------------------------
    
    There were some inconsistencies in the component values of this circuit when
    I first saw it.  I have adjusted the RMS value of the transformer down from
    710 to 650 VRMS so that the numbers work out closer to what one would expect.
    
    Estimated specifications (Edmund Scientific):
    
    * Operating voltage: 1,700 V.
    * Operating current: 4 mA.
    * Starting voltage: around 5,300 V.
    * Compliance range: NA - no regulation.
    
    (Portions from: Steve Nosko (q10706@email.mot.com)).
    
    This is the power supply I traced out and measured which is in an Edmund
    Scientific 0.5 mw. Laser circa probably around 1975.  I bought a 1 mW. tube
    (1986) when the old one broke.  It is still running just fine.  I think it
    is a rather clever design and I don't think they come any simpler.
    
           X                   C5                      C7
           +-------------------||-----------+----------||-----------+---o HV+
           |                       D7  D8   |  D9  D10     D11 D12  |   R5
           |                    +--|>|-|>|--+--|>|-|>|--+--|>|-|>|--+--/\/\--+
           |  D1  D2  D3   Y    |          C6           |               18K  |
       +---+--|>|-|>|-|>|--+----+----------||-----------+               1 W  / R6
    ||(    |               |    |                                            \ 33K
    ||(    |          C1 +_|_   / R1                                         / 1 W
    ||(    |      4.7 uF  ---   \ 1M                                         |
    ||(    |       450 V - |    /                                            / R7
    ||(    |               |    |                                            \ 33K
    ||(    |               +----+ W   Transformer: 650 VRMS, 20 mA           / 1 W
    ||(    |               |    |       (primary not shown)                  |
    ||(    |          C2 +_|_   / R2                                         / R8
    ||(    |      4.7 uF  ---   \ 1M                                         \ 33K
    ||(    |       450 V - |    /                                            / 1 W
    ||( T  |               |    |     D1-D7: 1N4007 or similar               |
       +-------------------+----+                                            / R9
           |               |    |                                            \ 33K
           |          C3 +_|_   / R3  C1-C4: 4.7 uF, 450 V                   / 1 W
           |      4.7 uF  ---   \ 1M  C5-C7: .001 uF, 2 KV                   |Tube+
           |       450 V - |    /                                          .-|-.
           |               |    |     R1-R4: 1M, 1 W                       | | |
           |               +----+ Z   R5-R9: (ballast, 18K+4x33K, 1W)      |   |
           |               |    |                                      LT1 |   |
           |          C4 +_|_   \ R4                                       |   |
           |      4.7 uF  ---   / 1M                                       ||_||
           |       450 V - |    \                                          '-|-'
           |               |    |                                            |Tube-
           +--|<|-|<|-|<|--+----+--------------------------------------------+---o
              D4  D5  D6                                                    _|_ HV-
                                                                             -
    
    Note that there are no equalizing resistors across the 1N4007s.  While I have
    been building similar supplies without them, the use of 10M resistors across
    each diode to equalize the voltage drops is recommended.
    
    The 650 V transformer output feeds a voltage doubler (D1 to D6 and C1 to C4)
    resulting in about 1,750 V across all the electrolytics.  (Slightly less than
    2 times the peak value of 650 VRMS.)
    
    D7 to D12 and C5 to C7 form a classical voltage multiplier ladder which
    generates a peak of up to 4 * V(peak) + 2 * V(peak) or 6 * 880 = 5,300 V.
    This seems somewhat low but the power supply is for only a .5 mW tube.  See
    the section: "Voltage multiplier starting circuits" for a description of its
    design and operation.
    
    The 150K ballast resistor is actually constructed from 4 - 33K resistors
    and one 18K resistor in series.  It doesn't have to be, but this is convenient
    and allows the ballast to be changed easily (or just tap off the appropriate
    point for your tube.  My notes show 600 V across the ballast resistor-combo.
    
    The ballast resistor should be located close to the tube with as short a lead
    as possible and as little capacitance to surroundings as possible.  The tube
    needs to see a high impedance source.  This isn't super critical, but keep the
    wire down to 1 to 3 inches and the first few resistors away from any case or
    ground material. 
    
    Since there is no active regulator, the tube current will depend on the power
    line voltage and other factors like temperature.  However, the relatively large
    ballast resistor in this power supply should minimize excessive variation.
    
    Spectra Physics model 155 HeNe power supply:
    -------------------------------------------
    
    This is the schematic for another simple line operated power supply.  This
    one includes a current regulator which can easily be modified for any typical
    tube requirement.  It can also be converted to a modulator in a number of ways.
    
    Estimated specifications (Spectra Physics 155):
    
    * Operating voltage: 1,700 V.
    * Operating current: 3.75 mA.
    * Starting voltage: greater than 10,000 V.
    * Compliance range: 1,400 to 1,700 V at top of ballast resistor.
    
    High voltage diodes and capacitors are used in this design.  An alternative
    is to use inexpensive 6 - 1,000 V diodes for each 6 KV diode shown here, and
    to use 6 - .003 uF, 1 KV capacitors in series for each 6 KV capacitor.  I
    would recommend 10 M ohm equalizing resistors across each lower voltage
    device though for the diodes at least, this appears not to be essential.
    
    
           X             C103
           +--------------||-------------+
           |             C100            |       C101
           +--------------||-------------+--------||---------+---o HV+
           |                      CR101  |   CR102    CR103  |   R107
           |                   +---|>|---+---|>|---+---|>|---+---/\/\---+
    T100   |    CR100    Y     |        C102       |             33K    |
       +---+-----|>|-----+-----+---------||--------+             2 W    |
    ||(                  |     |                                        |
    ||(           C103 +_|_    / R100                                   |Tube+
    ||(          10 uF  ---    \ 470K   T100: 1,300 VRMS, 20 mA       .-|-.
    ||(          450 V - |     / 1 W      (primary not shown)         | | |
    ||(                  |     |                                      |   |
    ||(                  +-----+ W      CR100-CR103: LMS60 (6 KV)     |   |
    ||(                  |     |                                      |   | LT100
    ||(           C104 +_|_    / R101   C100-C103: 560 pF, 6 KV       |   |
    ||(          10 uF  ---    \ 470K   C103-C105: 10 uF, 450 V       |   |
    ||(          450 V - |     / 1 W                                  ||_||
    ||( T                |     |        R100-R103: 470K, 1 W          '-|-'
       +---+             +-----+        R107 (ballast): 33 K, 2 W       |Tube-
           |             |     |                                        |
           |      C105 +_|_    / R102                                   |
           |     10 uF  ---    \ 470K                +------------------+
           |     450 V - |     / 1 W                 |                 _|_
           |             |     |    R103           |/ C Q100            -
           |             +-----+----/\/\------+----|    MJE3439
           |             |     Z    430K      |    |\ E
           |      C106 +_|_         1 W       |      |
           |     10 uF  ---                  _|_,    /
           |     450 V - |            CR104 '/_\     \ R106
           |             |          1N5241B   |      / 2.74 K
           |             |                    |      \
           |             |                    |      |
           +-------------+--------------------+------+---o HV-
    
    The 1,300 V transformer output feeds a half wave rectifier (CR100) and filter
    resulting in about 1,750 V across all the electrolytics.  (Slightly less than
    the peak value of 1,300 VRMS.)
    
    CR101 to CR103 and C100 to C103 form a classical voltage multiplier ladder
    which generates a peak of up to 4 * V(peak) + 2 * V(peak) or 6 * 1,750 = 10,600
    but losses in the diode-capacitor network probably reduce this somewhat.  See
    the section: "Voltage multiplier starting circuits" for a description of its
    design and operation.
    
    Q100, CR104, and R106 form a constant current regulator which will attempt
    to maintain the tube current at (Vz - .7)/R106 or about 3.75 mA in this case.
    Its compliance range is about 300 V.  This can easily be adapted to your
    requirements by either changing CR104 or R106 appropriately.
    
    Spectra Physics model 247 HeNe power supply:
    -------------------------------------------
    
    This one appears to be capable of driving higher power tubes and to have a bit
    more sophisticated constant current regulator with wider compliance than the
    Model 155.  The regulator is in the positive feed instead of the return but
    otherwise, the basic power supply design is similar.
    
    Estimated specifications (Spectra Physics 247):
    
    * Operating voltage: 3,200 V.
    * Operating current: 2.3 to 10 mA.
    * Starting voltage: greater than 10,000 V.
    * Compliance range: 2,200 to 3,200 V at top of ballast resistor.
    
    
           X    R1       C1                   C11
           +---/\/\------||----------+---------||--------+
           |   680K           CR3    |   CR4       CR5   |  CR6      
           |              +---|>|----+---|>|---+---|>|---+---|>|---+
    T1     |   CR1   Y    |        C10         |    C12            |
       +---+---|>|---+----+---------||---------+-----||-----+------+----+---o HV+
    ||(    |         |    |                                 |      |    |
    ||(    |    C2 +_|_   / R2                              |  R11 /    |
    ||(    | 10 uF  ---   \ 680K  T1: 1,200 VRMS, 20 mA     | 120K \    |
    ||(    | 500 V - |    / 1 W     (primary not shown)     |  2 W /    |
    ||(    |         |    |                                 |      |  |/ C Q1
    ||(    |         +----+ W     CR1-CR6: SCM60, 6 KV      |      +--|    MJE3439
    ||(    |         |    |                                 |      |  |\ E
    ||(    |    C3 +_|_   / R3    C2-C9: 10 uF, 500 V       |  R12 /    |
    ||(    | 10 uF  ---   \ 680K  C1, C10-C13: 500 pF, 6 KV | 120K \    |
    ||(    | 500 V - |    / 1 W                             |  2 W /    |
    ||(    |         |    |       R2-R9: 680K, 1 W          |      |  |/ C Q2
    ||(    |         +----+       R11-R14: 120K, 2 W        |      +--|    MJE3439
    ||(    |         |    |                                 |      |  |\ E
    ||(    |    C4 +_|_   / R4    Q1-Q4: MJE3439            |  R13 /    |
    ||(    | 10 uF  ---   \ 680K                            | 120K \    |
    ||(    | 500 V - |    / 1 W                             |  2 W /    |
    ||(    |         |    |                                 |      |  |/ C Q3
    ||(    |         +----+              +------------------+      +--|    MJE3439
    ||(    |         |    |              |                         |  |\ E
    ||(    |    C5 +_|_   / R5           |                     R14 /    |
    ||(    | 10 uF  ---   \ 680K         |                    120K \    |
    ||(    | 500 V - |    / 1 W          |                     2 W /    |
    ||( T  |         |    |              |             R10 48K     |  |/ C Q4
       +---|---------+----+              |            +----/\/\----+--|    MJE3439
           |         |    |              |            |            |  |\ E
           |    C6 +_|_   / R6           |            |          |/ E   |
           | 10 uF  ---   \ 680K         |            +----------|      |
           | 500 V - |    / 1 W          |            |       Q5 |\ C   |
           |         |    |              |      ZD1  _|_, 2N5086   |    |
           |         +----+         C16 _|_ 1N5245A '/_\           +----+
           |         |    |     .047 uF ---           |                 |
           |    C7 +_|_   / R7     6 KV  |            |   R17    R16    |
           | 10 uF  ---   \ 680K         |     Adjust +---/\/\---/\/\---+
           | 500 V - |    / 1 W          |            |    | 5K  1.5K      
           |         |    |              |            +----+
           |         +----+              |            |   R15    R18
           |         |    |              |            +---/\/\---/\/\---+
           |    C8 +_|_   / R8           |                20K    20K    |Tube+
           | 10 uF  ---   \ 680K         |                2 W    2 W  .-|-.
           | 500 V - |    / 1 W          |                            | | |
           |         |    |              |                            |   |
           |         +----+ Z       C15 _|_                           |   | LT1
           |         |    |     .047 uF ---                           |   |
           |    C9 +_|_   / R9     6 KV  |                            |   |
           | 10 uF  ---   \ 680K         |                            ||_||
           | 500 V - |    / 1 W          |                            '-|-'
           |         |    |              |                              |Tube-
           +---|<|---+----+--------------+------------------------------+---o HV-
               CR2                                                     _|_
                                                                        -
    
    The 1,200 V transformer output feeds a voltage doubler consisting of rectifiers
    CR1 and CR2 and filter capacitors C2 through C9 resulting in about 3,200 V
    across all the electrolytics.  (Slightly less than 2 times the peak value of
    1,200 VRMS.)
    
    CR3 to CR6 and C1 and C10 through C12 form a classical voltage multiplier
    ladder which generates a peak output of 4 * V(peak) + 2 * V(peak) or
    6 * 1,700 = 10,200 V but losses in the diode-capacitor network probably
    reduce this slightly.  See the section: "Voltage multiplier starting circuits"
    for a description of its design and operation.
    
    C15 and C16 provide some additional filtering to the output so unlike the
    previous supplies whose outputs include the last multiplier diodes without
    filtering, this one is more pure DC.  This would be better for laser
    communications, for example, as the tube current will have less ripple.
    
    Q1 through Q5, their associated resistors, and ZD1 (15 V zener) maintains a
    constant voltage of 15 V across the combination of R16 + R17 so the tube
    current will be 15/(R16 + R17).  For example, with the R17 set for 1.5 K, the
    tube current will be 5 mA.  The adjustment range is approximately 2.3 to 10 mA.
    The voltage compliance range of this power supply should be over 1,000 V.
    
    Aerotech model PS1 HeNe power supply:
    ------------------------------------
    
    This one appears to be suitable for higher power tubes but is running at
    very conservative voltage levels with the transformer that is provided.  It
    uses low-side regulation with a fixed output of about 2,000 V at 4 mA.
    
    (Model number PS1 is arbitrary - supply was unmarked).
    
    Estimated specifications (Aerotech PS1):
    
    * Operating voltage: 2,000 V.
    * Operating current: 4 mA.
    * Starting voltage: nearly 10,000 V.
    * Compliance range: 1,500 to 2,000 V at top of ballast resistor.
    
    
           X    R9     C9            C11             C13             C15
           +---/\/\----||-----+-------||------+-------||------+-------||------+
           | 100K, 1 W  CR3  |  CR4     CR5  |  CR6     CR7  |  CR8     CR9  | HV+
           |         +--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--o
    T1     |   CR1   |Y              |               |               |       |   
       +---+---|>|---+----+----||----+-------||------+-------||------+       |
    ||(    |         |    |   C10           C12             C14              |
    ||(    |    C1 +_|_   / R1                                               |
    ||(    | 10 uF  ---   \ 510K  T1: 750 VRMS, 20 mA                        |
    ||(    | 450 V - |    / 1 W     (primary not shown)                      |
    ||(    |         |    |                                                  |
    ||(    |         +----+       CR1-CR9: (3 KV)                            |
    ||(    |         |    |                                              R10 /
    ||(    |    C2 +_|_   / R2    C1-C8: 10 uF, 450 V                    47K \
    ||(    | 10 uF  ---   \ 510K  C9-C15: .005 uF, 3 KV                  5 W /
    ||(    | 450 V - |    / 1 W                                              \
    ||(    |         |    |       R1-R8: 510K                                |
    ||(    |         +----+                                                  |
    ||(    |         |    |                                                  |
    ||(    |    C3 +_|_   / R3                                            +--+
    ||(    | 10 uF  ---   \ 510K                                          |
    ||(    | 450 V - |    / 1 W                                           |Tube+
    ||(    |         |    |                                             .-|-.
    ||(    |         +----+                                             | | |
    ||(    |         |    |                                             |   |
    ||(    |    C4 +_|_   / R4                                          |   |
    ||(    | 10 uF  ---   \ 510K                                        |   | LT1
    ||(    | 450 V - |    / 1 W                                         |   |
    ||( T  |         |    |                                             |   |
       +---|---------+----+                                             |   |
           |         |    |                                             ||_||
           |    C5 +_|_   / R5                                          '-|-'
           | 10 uF  ---   \ 510K                                          |Tube-
           | 450 V - |    / 1 W                                           |
           |         |    |                                               +----+
           |         +----+                                               |   _|_
           |         |    |                                               |    -
           |    C6 +_|_   / R6                                            |
           | 10 uF  ---   \ 510K                                          |
           | 450 V - |    / 1 W                                           |
           |         |    |                                               |
           |         +----+                                               |
           |         |    |                                          +----+
           |    C7 +_|_   / R7                            MJE2360T   | C  |
           | 10 uF  ---   \ 510K                                   |/     |
           | 450 V - |    / 1 W                        +-----------| Q1   |
           |         |    |              R8            |           |\     |
           |         +----+-------------/\/\-----------+             | E  |  
           |         |    Z             470K           |         R11 /    / R12
           |    C8 +_|_                 1 W      ZD1  _|_,      3.6K \    \ 375 K
           | 10 uF  ---                       1N4744 '/_\            /    / 2 W
           | 450 V - |                          15 V   |             |    |
           |         |                                 |             |    |
           +---|<|---+---------------------------------+-------------+----+---o HV-
               CR2 
    
    Note: the laser head itself may have an additional ballast resistor (not
    shown).
    
    The 750 V transformer output feeds a voltage doubler consisting of rectifiers
    CR1 and CR2 and filter capacitors C1 through C8 resulting in about 2,000 V
    across all the electrolytics.  (Slightly less than 2 times the peak value of
    750 VRMS.)
    
    CR3 to CR9 and C9 through C15 form a classical voltage multiplier ladder
    which generates a peak of 2 * V(peak) + 8 * V(peak) or 10 * 1,000 = 10,000 V
    but losses in the diode-capacitor network probably reduce this somewhat.
    See the section: "Voltage multiplier starting circuits" for a description of
    its design and operation.
    
    Q1, ZD1, R8, and R11 form the low-side current regulator.  The tube current
    will be (15-.7)/R11 or just about 4 mA.  So, for a different current, select
    R11 to be 14.3/I.
    
    Since the voltage compliance range of this power supply is only around 500 V,
    the ballast resistor will still need to be selected carefully to achieve stable
    regulation for your particular tube.  See the sections beginning with:
    "Selecting the ballast resistor" for further info.
    
    Enhancements to Aerotech model PS1 HeNe power supply:
    ----------------------------------------------------
    
    Since the component values are all quite conservative, it should be possible
    to safely boost the output of this supply by driving it with a Variac that
    will go to 140 VAC.  This will result in up to 2,400 VDC - enough to power
    most laser tubes of up to 5 mW.
    
    The modified circuit provides a current adjustment control, modulation input,
    'Beam On' indicator, and tube current sense test points.  I have implemented
    these changes to the Aerotech PS1 and installed the current adjust pot, jacks
    for Ground/Test+, Test-, Signal in, and Signal ground, and the Beam On LED on
    the power supply case.
    
    
           |       (Remainder of circuit                                  |Tube-
           |        identical to Aerotech PS1)   +----+-----------+-------+---o +
           |                                     |   _|_          |       |
           |         |    |                      |    -      ZD2 _|_  R13 / Test
           |         +----+                      |        1N4742 /_\   1K \ 1 V/mA
           |         |    |                      |          12 V  |       /
           |    C6 +_|_   / R6                   |                |       | 
           | 10 uF  ---   \ 510K                 |                +-------+---o -
           | 450 V - |    / 1 W                  |                      __|__ IL2
           |         |    |                      |                      _\_/_ Beam
           |         +----+                      |                        |   On
           |         |    |                      |                    +---+
           |    C7 +_|_   / R7                   |         MJE2360T   | C |
           | 10 uF  ---   \ 510K                 |                  |/    |
           | 450 V - |    / 1 W                  |       +---/\/\---| Q1  |
           |         |    |                      |    T2 |   R15    |\    |
           |         +----+ Z                    +--+    +   15K      | E |  
           |         |    |                          )||(             |   |
           |         |    / R8                       )||(             |   / R12
           |         |    \ 470K                     )||(             |   \ 375 K
           |    C8 +_|_   / 1 W      Signal in o----+    +            /   / 2 W
           | 10 uF  ---   |                          1:1 |        R11 \   |
           | 450 V - |    +------------------------------+       1.5K /   |
           |         |                                   |            |   |
           |         |                             ZD1  _|_,    R14   /   |
           |         |                          1N4744 '/_\     5K +->\   |
           |         |                            15 V   |  Adjust |  /   |
           |         |                                   |         |  |   |
           +---|<|---+-----------------------------------+---------+--+---+---o HV-
               CR2
    
    Each of the new and improved features is described below:
    
    * To provide adjustable current, R11 is replaced with a fixed and variable
      resistor in series.  Using a 1.5K resistor and 5K potentiometer results in
      a current range of approximately 2.2 to 9.5 mA.  A heat sink should be used
      on Q1 as it may be dissipating up to 5 W at maximum current and maximum
      voltage (this power is higher for certain settings than in the original
      Aerotech design).
    
    * The Beam On indicator (IL2) is a high brightness LED in series with the tube
      so it will glow whenever current is flowing.  Its brightness provides a rough
      indication of tube current as well.
    
    * The test points permit the use of a multimeter to monitor beam current.
      Using a voltage setting, the sensitivity is 1 mA/V.  Current can also be
      monitored directly between the test points (the current in R13 will be
      negligible).  ZD2 protects the multimeter should the sense resistor go
      open-circuit for some reason :-(.
    
    * The modulation input (Signal in) is coupled via T2, a small audio transformer
      to provide HV isolation and permit the Tube- terminal to be earth ground.
      The insulation rating on T2 should be at least 1,000 V.  Assuming a 1:1
      transformer, current sensitivity (percentage change of current with respect
      to voltage) is about 7%/V relative to the set-point.  The maximum input
      should be limited to about 14 V p-p which will result in a current between
      about 50% and 150% of the set-point.  Thus, adjust R12 for 67% of the nominal
      tube current with no signal.  If the tube goes out on the negative peaks,
      increase the set-point and decrease the amplitude of the input.
    
      The phone line coupling transformer from a long forgotten 2400 baud modem
      served nicely for this application resulting in a useful frequency response
      from about 100 and 10,000 Hz.
    
    * Another desirable enhancement (not shown) would be to provide a selection
      of ballast resistor values in increments of 20K or 25K up to 150K.  In
      conjunction with the current control and optional Variac, this will provide
      additional flexibility in matching the tube, supply voltage, and modulation
      capabilities resulting in a 'universal' HeNe power supply.
    
    With a small HeNe tube requiring about 1,200 V at 4 mA and additional 33K 5 W
    ballast resistor, it was possible to adjust/modulate the current between about
    2 and 6 mA.  For testing, I used a Heathkit audio signal generator to drive
    the modulation input and the simple circuit described in the section: "IR
    detector circuit" with a scope across the C-E leads of the transistor as a
    receiver.  While this IR detector design is not really very good for linear
    operation, with a little care in positioning the photodiode with respect to
    the beam reflected off of a piece of paper, it was possible to display the
    received signal on an oscilloscope.  One could clearly observe the effects
    of adjusting the current set-point and modulation signal amplitude and of
    modulating beyond the rated tube current - the signal inverted (due to reduced
    optical output power).
    
    Stay tuned for exciting future developments.
    
    A similar approach can be used with any of the other HeNe power supply designs
    described in this document which use low-side regulation or which do not have
    any regulation.
    
    CAUTION: Don't try this with power supplies using high-side regulation either
    by modifying the regulator (you would need a 15 KV coupling capacitor or 15 KV
    opto-isolator to hold off the starting pulse) or adding an additional low-side
    modulator (the two circuits will be fighting each other).
    
    Aerotech model PS2B HeNe power supply:
    -------------------------------------
    
    This one is definitely for higher power tubes.  However, the basic design
    is quite similar to those preceding.  The estimated operating voltage is
    3,600 V at 5 to 9 mA with a starting voltage of over 15,000 V.  It includes
    positive (anode) side regulation using an LM723 IC and a cascade of high
    voltage transistors.
    
    There may have been several versions of this model as I have two slightly
    different samples using the same circuit board.  The one described below which
    designate model PS2B uses the higher voltage tap on the transformer.  A nearly
    identical design - model PS3A - runs with a transformer secondary of 1,150
    VRMS yielding 3,000 VDC operating, 12,000 VDC starting, and uses only 8
    electrolytic filter capacitors. 
    
    See the section: "Aerotech model PS2A-X HeNe power supply" for its circuit
    diagram with my modifications.
    
    Estimated specifications (Aerotech PS2B):
    
    * Operating voltage: 3,600 V.
    * Operating current: 5 to 9 mA.
    * Starting voltage: greater than 15,000 V.
    * Compliance range: 2,800 to 3,600 V at top of ballast resistor.
    
    
           X   R11    C11           C13             C15             C17
           +---/\/\----||----+-------||------+-------||------+-------||------+
           | 10M, 5 W   CR3  |  CR4     CR5  |  CR6     CR7  |  CR8     CR9  | HV+
           |         +--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--o
           |         |               |               |               |       |   
           |         |    +----||----+-------||------+-------||------+       |
    T1     |   CR1   |Y   |   C12           C14             C16              |
       +---+---|>|---+----+                                        +----+----+
    ||(    |         |    |                                        |    |
    ||(    |    C1 +_|_   / R1                                 R12 /    |
    ||(    | 10 uF  ---   \ 510K  T1: 1,380 VRMS, 20 mA        62K \    |
    ||(    | 500 V - |    / 1 W     (primary not shown)        2 W /    |
    ||(    |         |    |                                        |  |/ C Q1
    ||(    |         +----+       CR1-CR9: (5 KV)                  +--|    MJE2360T
    ||(    |         |    |                                        |  |\ E
    ||(    |    C2 +_|_   / R2    C1-C10: 10 uF, 500 V         R13 /    |
    ||(    | 10 uF  ---   \ 510K  C11-C17: .005 uF, 5 KV       62K \    |
    ||(    | 500 V - |    / 1 W                                2 W /    |
    ||(    |         |    |       R1-R10: 510K                     |  |/ C Q2
    ||(    |         +----+       R11-R14: 62K, 2 W                +--|    MJE2360T
    ||(    |         |    |                                        |  |\ E
    ||(    |    C3 +_|_   / R3    Q1-Q3: MJE2360T              R14 /    |
    ||(    | 10 uF  ---   \ 510K                               62K \    |
    ||(    | 500 V - |    / 1 W   U1: LM723                    2 W /    |
    ||(    |         |    |                                R24     |  |/ C Q3
    ||(    |         +----+                             +---/\/\---+--|    MJE2360T
    ||(    |         |    |                             |   3.3K   |  |\ E
    ||(    |    C4 +_|_   / R4                          |        |/ E   |
    ||(    | 10 uF  ---   \ 510K                        +--------|   Q4 | 2N4126
    ||(    | 500 V - |    / 1 W                         |        |\ C   | (PNP)
    ||(    |         |    |                             |    C18   |    |
    ||(    |         +----+         +-------------------+----||----+----+
    ||(    |         |    |         |                   |  .005 uF
    ||(    |    C5 +_|_   / R5     _|_, ZD1             | 
    ||(    | 10 uF  ---   \ 510K  '/_\  1N4744          +----------------------+
    ||(    | 500 V - |    / 1 W     |   15 V, 1 W                              |
    ||( T  |         |    |         |                                          |
       +---|---------+----+         |             R15 15K   1N4148   |\        | C
           |         |    |         |            +--/\/\--+----------|+ \*   |/*
           |    C6 +_|_   / R6      |  +------+  |R14 15K |  D1      |Err >--|  
           | 10 uF  ---   \ 510K    |  | Vref*|--+--/\/\------+---+--|- /    |\  E
           | 500 V - |    / 1 W     |  +------+ 7.15 V    |   |   |  |/        |
           |         |    |         |                     |   |   |        R21 /
           |         +----+         |                 +---+   |   \ R16    10K \
           |         |    |         |                 |   |  _|_  / 82K        /
           |    C7 +_|_   / R7      |            C19 _|_  /  /_\  \      ZD2   |
           | 10 uF  ---   \ 510K    |          .1 uF ---  \   |   |   1M4733  _|_,
           | 500 V - |    / 1 W     |                 |   /   |   |    5.1 V '/_\
           |         |    |         |                 |   |   |   |            |
           |         +----+         +----+------------+---+---+----------------+
           |         |    |              |             R17 15K    |       
           |    C8 +_|_   / R8           |   R20      R19         |      
           | 10 uF  ---   \ 510K         +-+-/\/\-----/\/\--------+-------+
           | 500 V - |    / 1 W            |   | 1.5K  1.8K               |
           |         |    |                +---+                      R25 /
           |         +----+       Current adjust: 6 to 11 mA          47K \
           |         |    |                                           5 W /
           |    C9 +_|_   / R9         Note: Components marked            |Tube+
           | 10 uF  ---   \ 510K        with '*' are part of            .-|-.
           | 500 V - |    / 1 W         U1, LM723.  (Compensation       | | |
           |         |    |             not shown.)                     |   |
           |         +----+ Z                                           |   | LT1
           |         |    |                                             |   |
           |   C10 +_|_   / R10                                         |   |
           | 10 uF  ---   \ 510K                                        ||_||
           | 500 V - |    / 1 W                                         '-|-'
           |         |    |                R23                            |Tube-
           +---|<|---+----+-------------+--/\/\--+------------------------+---o HV-
               CR2                      |   1K   |                       _|_
                                      - o  Test  o +                      -
                                          1 V/mA
    
    Note: the laser head itself may have an additional ballast resistor (not
    shown).
    
    The 1,380 V transformer output feeds a voltage doubler consisting of rectifiers
    CR1 and CR2 and filter capacitors C1 through C10 resulting in about 3,600 V
    across all the electrolytics.  (Slightly less than 2 times the peak value of
    1,380 VRMS.)
    
    CR3 to CR9 and C11 through C17 form a classical voltage multiplier ladder
    which generates a peak starting voltage of up to 2 * V(peak) + 8 * V(peak)
    or 10 * 1,800 = 18,000 V but losses in the diode-capacitor network probably
    reduce this somewhat.  See the section: "Voltage multiplier starting circuits"
    for a description of its design and operation.
    
    Q1 through Q4, their associated resistors, and U1 (LM723) maintain a constant
    voltage of 22 V across the combination of R19 + R20 so the tube current will
    be 22/(R16 + R17).  For example, with the R17 set for 750 ohms, the tube
    current will be 6.3 mA.  The adjustment range is approximately 5 to 9 mA.
    The voltage compliance range of this power supply is about 800 V at 5 mA
    (possibly a couple hundred volts greater at higher currents).
    
    Aerotech model PS2A-X HeNe power supply:
    ---------------------------------------
    
    This is the other version of the Aerotech model PS2.  I have modified it by
    replacing the original (fried) high side regulator (identical to the one in
    the model PS2B) with a wide compliance low side regulator using PNP
    transistors instead of the more conventional NPN type.  The advantage of using
    PNPs is that the controls can be near ground potential (rather than floating
    at the top of the transistor cascade) and mounted directly to the metal case.
    As drawn, the compliance is about 800 V. The poor little panel mount pots
    might not be very happy with that sort of voltage on them!
    
    Estimated specifications (Aerotech PS2A-X):
    
    * Operating voltage: 3,000 V.
    * Operating current: 3 to 9 mA.
    * Starting voltage: greater than 12,000 V.
    * Compliance range: 2,200 to 3,000 V at top of ballast resistor.
    
    
           X   R11    C11           C13             C15             C17
           +---/\/\----||----+-------||------+-------||------+-------||------+
           | 10M, 5 W   CR3  |  CR4     CR5  |  CR6     CR7  |  CR8     CR9  | HV+
           |         +--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--o
           |         |               |               |               |       |   
           |         |    +----||----+-------||------+-------||------+       /
    T1     |   CR1   |Y   |   C12           C14             C16              \ Rb
       +---+---|>|---+----+                                                  /
    ||(    |         |    |       T1: 1,150 VRMS, 20 mA                      |Tube+
    ||(    |    C1 +_|_   / R1      (primary not shown)                    .-|-.  
    ||(    | 10 uF  ---   \ 510K                                           |   |
    ||(    | 500 V - |    / 1 W   CR1-CR9: (5 KV)                          |   |
    ||(    |         |    |                                                |   |
    ||(    |         +----+       C1-C4, C6-C9: 10 uF, 500 V           LT1 |   |
    ||(    |         |    |       C11-C17: .005 uF, 5 KV                   |   |
    ||(    |    C2 +_|_   / R2                                             ||_||
    ||(    | 10 uF  ---   \ 510K  R1-R4, R6-R9: 510K                       '-|-'
    ||(    | 500 V - |    / 1 W   RX1-RX3: 100K, 2 W                         |Tube-
    ||(    |         |    |                                                  |
    ||(    |         +----+       QX1-QX3: MPSU60        +---------+-------+-+--o +
    ||(    |         |    |                             _|_        |       |
    ||(    |    C3 +_|_   / R3                           -   ZD2  _|_, R12 / Test
    ||(    | 10 uF  ---   \ 510K                          1N4742 '/_\   1K \ 1 V/mA
    ||(    | 500 V - |    / 1 W                              12V   |       /
    ||(    |         |    |                           Beam On      |       |
    ||(    |         +----+             +-----------+---|<|--------+-------+----o -
    ||(    |         |    |             |           | IL2 LED  R13    R14
    ||(    |    C4 +_|_   / R4          |           |    +---/\/\---/\/\---+
    ||(    | 10 uF  ---   \ 510K        |           |    |    | 5K   1.5K  |
    ||(    | 500 V - |    / 1 W         |           +----+----+ Range      |
    ||( T  |         |    |             |           |    |                 |
       +---|---------+----+             |           |    |        Q1  +----+
           |         |    |             |           |    |    2N3904  |    |
           |    C6 +_|_   / R6          |     ZD1  _|_,  \    (NPN) |/ C   |
           | 10 uF  ---   \ 510K        |  1N4744 '/_\   /<---------|      |  
           | 500 V - |    / 1 W         |     15V   |    \ R15      |\ E   | 
           |         |    |             |           |    | 500K       |  |/ E QX1
           |         +----+             |           |    | Adjust     +--|  MPSU60
           |         |    |             |           |    |            |  |\ C (PNP)
           |    C7 +_|_   / R7          |           +----+----/\/\----+    |
           | 10 uF  ---   \ 510K        |                |  R16 10K        |
           | 500 V - |    / 1 W         / R17            |                 |
           |         |    |             \ 100K           |    RX1        |/ E QX2
           |         +----+             /                +----/\/\----+--|  MPSU60
           |         |    |             |                   100K      |  |\ C (PNP)
           |    C8 +_|_   / R8          |                   2 W   RX2 /    |
           | 10 uF  ---   \ 510K        |                        100K \    |
           | 500 V - |    / 1 W         |                         2 W /    |
           |         |    |             |                             |  |/ E QX3
           |         +----+            _|_ C18                        +--|  MPSU60
           |         |    |            --- 100 pF                     |  |\ C
           |    C9 +_|_   / R9          |                         RX3 \    |
           | 10 uF  ---   \ 510K        |                        100K /    |
           | 500 V - |    / 1 W         |                         2 W \    |
           |         |    |             |                             |    |
           +---|<|---+----+-------------+-----------------------------+----+--o HV-
               CR2 
    
    Note: the laser head itself may have an additional ballast resistor (not
    shown).
    
    The 1,150 V transformer output feeds a voltage doubler consisting of rectifiers
    CR1 and CR2 and filter capacitors C1 to C4 and C6 to C9 resulting in about
    3,000 V across all the electrolytics.  (Slightly less than 2 times the peak
    value of 1,150 VRMS.)
    
    CR3 to CR9 and C11 through C17 form a classical voltage multiplier ladder
    which generates a peak starting voltage of up to 2 * V(peak) + 8 * V(peak)
    or 10 * 1,500 = 15,000 V but losses in the diode-capacitor network probably
    reduce this somewhat.  See the section: "Voltage multiplier starting circuits"
    for a description of its design and operation.
    
    Current adjust (R15) and current range (R13) pots have been added, the latter
    being set by a screwdriver.  This allows fairly linear control of tube current
    up to the set limit from the front panel.  The minimum current is determined
    by what bypasses the transistors and passes through the base resistors.  This
    will be up to 3 mA depending on operating conditions.
    
    As desribed in the section: "Enhancements to Aerotech model PS1 HeNe power
    supply", a current test point and 'Beam On' indicator have also been added.
    
    The NPN transistor (Q1) buffers the reference voltage so that the very low
    current source from R15 can drive the base of the pass transistor cascade.
    
    The base resistors, RX1 through RX3 equally distribute the voltage across the
    3 PNP pass transistor, QX1 to QX3.  The respective transistors act as emitter
    followers and maintain approximately the same voltages across their C-E
    terminals.  Within the compliance range, the voltage across R13+R14 will be
    nearly equal to the voltage on the wiper of R15.
    
    R17 and C18 act as a snubber to protect the transistor cascade from the initial
    over voltage when the tube fires but before the regulator can turn on.  I do
    not know whether this is needed or how much if any it would protect the pass
    transistors when operating near their maximum ratings.
    
    Three pass transistors are shown here only because that particular number fit
    conveniently into the drawing :-).  A greater or fewer number could be used
    with their associated base resistors.  I will probably use 4 to provide a
    greater compliance and permit the same supply to drive a wider range of tubes.
    If only one particular tube is to be driven, a single stage in conjunction
    with a ballast resistor selected to set the operating current at the mid point
    of the range may be adequate.
    
    Sam's line powered HeNe laser power supply:
    ------------------------------------------
    
    This one is quite similar to the two Aerotech models PS1 and PS2 but is
    constructed entirely with parts that are readily available and inexpensive.
    Well, that is, except for the power transformer which you will still have
    to scrounge from somewhere.  See the section: "AC line operated power
    supplies" for possible sources for these boat anchors.  Also, due to low
    demand, the prices of high voltage electrolytic capacitors seem to be quite
    high (about $1.00 each for 10 uF at 450 V).  I had a pair of surplus 1 uF,
    1,500 V oil filled capacitors so I used them instead.  A pair of microwave
    oven HV capacitors could also be used since these are typically around 1 uF at
    a minimum of 2,000 VAC (greater than 3,000 VDC).  The cost of the remaining
    components (diodes, capacitors, and resistors) was less than $5.
    
    * The high voltage rectifiers for the doubler are each constructed from five
      1N4007s in series.
    
    * The main filter on the doubler is a pair of 1 uF, 1,500 V oil filled
      capacitors with 10M bleeder resistors on each.
    
    * The high voltage rectifiers for the multiplier are each constructed from four
      1N4007s in series.
    
      The high voltage capacitors for the multiplier are each constructed from four
      .001 uF, 1,000 V ceramic disk capacitors in series.
    
      The series resistor for the parasitic multiplier is 10 M.
    
    * There is currently no regulator - I may add that at a later time.  For now,
      a Variac is used to adjust beam current.
    
    I have left room for equalizing components on the diode and capacitor stacks
    but so far am running without them without any problems up to 2,500 VDC for
    the operating voltage.
    
    It took me roughly 3 hours to construct the doubler and starting multiplier on
    an old blank digital (DIP) prototyping board.
    
    I then tested it with a Variac and a current meter with several tubes from
    1 mW to 5 mW:
    
    * 1 mW Spectra Physics (3.2 mA): 1,400 V with Rb=100K.
    
    * 1 mW Aerotech (4 mA): 1,900 V with Rb=100K, 1,700 with Rb=22K (additional
      ballast resistor in laser head).
    
    * 5 mW Aerotech (6 mA): 2,300 V with Rb=22K (additional ballast resistor in
      laser head.
    
    The Variac was quite effective at adjusting tube current.
    
    At 115 VAC the output of the power supply is about 2,500 VDC.  This design
    appears to behave in all respects similarly to the commercial power supplies.
    
    Estimated specifications (Sam-L1):
    
    * Operating voltage: 1,500 to 2,500 V.
    * Operating current: 0 to 10 mA.
    * Starting voltage: 7,500 to 12,500 V.
    * Compliance range: NA - no regulator as yet.
    
    
           X     R3     C3            C5              C7              C9
           +----/\/\----||----+-------||------+-------||------+-------||------+
           |  10M, 1 W   CR3  |  CR4     CR5  |  CR6     CR7  |  CR8     CR9  | HV+
           |          +--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--o
    T1     |    CR1   |Y              |               |               |       |    
       +---+----|>|---+----+----||----+-------||------+-------||------+       |
    ||(    |          |    |    C4            C6              C8              |
    ||(    |      C1 _|_   / R1                                               |
    ||(    |    1 uF ---   \ 10M   T1: 900 VRMS, 100 mA                       |
    ||(    | 1,500 V  |    /         (primary not shown) (1,9)             R3 /
    ||(    |          |    |                                              47K \
       +---|----------+----+       CR1-CR2: (5 KV) (2)                    5 W /
           |          |    |       CR3-CR9: (4 KV) (3)                        \ 
           |      C2 _|_   / R2    C1-C2: 1 uF, 1,500 V, oil filled           |
           |    1 uF ---   \ 10M   C3-C9: 250 pF, 4 KV (4)                    |
           | 1,500 V  |    /                                  LT1             |
           |          |    |  IL2 LED      R4     Tube- +-------------+ Tube+ |
    HV- o--+----------+----+----|<|---+---/\/\---+---+--|-|          -|-------+
                              Beam On |    1K    |  _|_ +-------------+
                                      o - Test + o   -
    
    Notes for Sam's line powered HeNe laser power supply (Sam-L1):
    -------------------------------------------------------------
    
    1. T1 is from (approximately 40 year) old tube type TV.  By using the lowest
       line voltage tap and its 5 V and 6.3 V filament windings anti-phase in
       series with the line input, its output has been increased from about 750
       VRMS to 900 VRMS.
    
    2. CR1 and CR2 each consist of five 1N4007s in series:
    
             o--|>|--|>|--|>|--|>|--|>|--o
    
    3. CR3 through CR9 each consist of four 1N4007s in series:
    
             o--|>|--|>|--|>|--|>|--o
    
    4. C3 through C9 each consist of four .001 uF, 1,000 V ceramic disc capacitors
       in series:
    
             o--||--||--||--||--o
    
    5. Construction is on a blank digital prototyping board which just has pads
       for 28 DIP locations (16 pins each).  Perforated or other insulating board
       could have been used as well.  Smooth rounded connections and a conformal
       insulating coating are essential to minimize corona in the high voltage and
       starting circuitry.
    
    6. A Variac is used to adjust current - I will eventually add a low side
       regulator similar to the one described in the section: "High compliance
       cascade regulator".
    
    7. Output is about 2,500 VDC at 115 VRMS input and 3,000 VDC at 140 VAC input.
    
    8. There is audible evidence of HV breakdown near maximum output before the
       tube starts.  I suspect this is on the board itself since I have not coated
       it as yet with HV sealer.  This is not surprising since the output can
       exceed 10,000 V.
    
    9. WARNING: the power transformer is capable of much more than the 20 mA
       required for even higher power HeNe laser tubes making it particularly
       dangerous - take extreme care not to touch (or even go near) the high
       voltage terminals of this or any other high voltage power supply.
    
    Sam's mid-size line powered HeNe laser power supply:
    ---------------------------------------------------
    
    This one uses an oil burner ignition transformer and will drive tubes rated
    between about 5 and 20 mW of beam power.  I have not added a regulator to it
    since due to the severe voltage droop of the transformer, a compliance range
    of several KV would be needed - this is not really practical, at least not
    easily.  I just run the supply on a Variac while monitoring HeNe tube current.
    
    An oil burner ignition transformer rated at 10 KVAC and 23 mA drives a full
    wave rectifier using microwave oven HV diodes.  The DC filter consists of 4
    oil filled .25 uF, 3,500 WVDC capacitors.  A 100K resistor (between the two
    pairs of caps in a pi configuration) was added to reduce ripple and improve
    stability at low tube currents.
    
    The centertap of the transformer's HV winding is connected to its metal case
    internally and to earth ground for safety (via a 3 prong wall plug).  Since
    the negative of the supply is therefore grounded, the HeNe tube cathode will
    end up being a few volts above ground if the normal current sense resistor and
    'Beam On' LED are included.  This is usually acceptable unless the cathode of
    the HeNe tube is connected to the metal case of a laser head and cannot be
    removed - the laser head should be grounded for safety unless it can be
    totally insulated from human contact.  Floating the transformer is probably
    not a great idea since an internal fault (short) could result in line voltage
    on its case - and this could find its way into the power supply wiring.
    
    Starting voltage is provided by a small high frequency inverter.  In fact,
    originally, I was using the same inverter that is the main power source in:
    "Sam's inverter driven HeNe power supply 2".  In this case it was just used
    for starting!  At present, I am using the HV module from a long ago retired
    Monitronix workstation monitor.  It is rated at 25 KV but more than 30 KV is
    actually available if needed as a result of some careful tweaking.  Thus,
    starting any HeNe tube is simply not a problem :-).
    
    Originally, I was using a 15 KV, .5 A microwave oven HV rectifier as the
    blocking diode.  After I smoked that with some overzealous application of
    excessive starting voltage, I replaced it with a stack of 20 1N4007 general
    purpose 1 KV, 1 A diodes soldered together enclosed in a thick plastic tube
    for insulation.  I will have to add some more 1N4007s if I decide to really
    crank up the starter :-).
    
    The inverter output is introduced across a high voltage blocking diode to
    bypass current around the inverter once the tube starts.  Voltage builds up on
    the stray capacitance of the HV diodes, wiring, and HeNe tube until the tube
    fires.  A pair of 10M ohm series resistors rated for 15 KV isolates the
    starter (for safety) and eliminates the annoying tendency for the inverter
    pulses to shut the tube *off* after it has started due to capacitive coupling
    bypassing the HV rectifier - it only takes a few volts to kill the discharge.
    
    Note that the inverter HV return must be isolated from ground since it is
    attached to the main power supply output to gain the added benefit that the
    operating voltage provides in starting.  Take care if this is attached to the
    flyback core!
    
    Starting is not automatic though this feature could be added.  I just power
    the inverter until the tube fires - typically less than a second.  To automate
    this, just add a transistor to disable the inverter which is switched on by
    sensing current flow through the HeNe tube.  See the section: "Inverter based
    starters" for more info.
    
    Estimated specifications (Sam-L2):
    
    * Operating voltage: 2,000 to 4,000 V.
    * Operating current: 4 to 8 mA.
    * Starting voltage: greater than 16,000 V.
    * Compliance range: NA, no regulator.
    
    Primary side components are not shown.  See the section: "AC input circuitry
    for HeNe power supplies" for more info.
    
    
             T1      CR1                    R5               CR3      Rb
           ||==|| +--|>|--+---------+--+---/\/\---+--+----+--|>|--+--/\/\--+
           ||  ||(  15 KV |         |  |   100K   |  |    | 20 KV |        | Tube+
           ||  ||(        |         |  /   10 W   |  /    /       /      .-|-.
           ||  ||(        |     C1 _|_ \ R1   C3 _|_ \ R3 \ R6    \ R7   | | |
     H o-+ ||  ||(        | .25 uF --- / 10M     --- /    / 10M   / 10M  |   |
          )||  ||(        | 3.5 KV  |  \ 1 W      |  \    \ 1 W   \ 1 W  |   |
          )||  ||(        |         |  |          |  |    |       |      |   | LT1
          )||  |+-+-----------+     +--+          +--+  A o -   + o      |   |
          )||  ||(        |   |     |  |          |  |     Starter       |   |
          )||  ||(        |   |     |  /          |  /          - o B    ||_||
     N o-+ ||  ||(        |   | C2 _|_ \ R2   C4 _|_ \ R4        _|_     '-|-'
           ||  ||(        |   |    --- /         --- /            -        | Tube-
           ||  ||(        |   |     |  \          |  \                     |
           ||  ||(   CR2  |   |     |  |          |  |     R8        IL2   |
           ||==|| +--|>|--+   +-----+--+----------+--++---/\/\---+---|<|---+ 
           |   |    15 KV     |                       |    1K    | Beam On
     G o---+-+-+--------------+                       o - Test + o   LED
            _|_
             -       C1-C4: .25 uF, 3.5 KV
                     R1-R4: 10 M, 1 W equalizing/bleeder resistors
    
    
    Notes for Sam's mid-size line powered HeNe laser power supply (Sam-L2):
    ----------------------------------------------------------------------
    
    1. T1 is rated 10,000 VAC, 23 mA, current limited.  This is typical of the
       type of transformer found on a residential oil burner.
    
    2. CR1 and CR2 are standard replacement microwave oven rectifiers rated at
       15 KV PRV, .5 A (gross overkill on the current at least!).
    
    3. C1 through C4 are .25 uF, 3,500 V oil filled capacitors.  Each capacitor
       is bypassed with a 10M equalizing/bleeder resistor (R1 through R4).
    
    4. Construction is point-to-point using wire with 10 KV insulation except for
       the HV+ lead which uses wire rated for 30 KV.  Smooth rounded connections
       and a conformal insulating coating are essential to minimize corona in the
       high voltage and starting circuitry.
    
    5. A Variac is used to adjust operating voltage between 0 and approximately
       4,000 VDC (under load - once the tube has started).  HeNe tube current can
       be monitored at the Test jacks.  Sensitivity is 1 V/mA or directly using a
       mA meter.  The 'Beam On' LED provides another confirmation of laser tube
       operation - an additional safety precaution for higher power lasers.
    
    6. The starting inverter is active only during starting and is operated by a
       momentary switch.  It is powered from a separate DC supply.
    
       If the HV return of the starter can be safely isolated from ground (with
       10 KV insulation), then it can be connected to point 'A'.  Otherwise, use
       point 'B'.  However, the advantage of the operating voltage being added to
       the starting voltage is lost in this configuration.
    
    7. CR3 is a stack of 20 1N4007s in series soldered together and enclosed in a
       thick plastic tube for insulation:
    
              o--|>|--|>|--|>|--|>|--|>|--//--|>|--|>|--|>|--o
                 D1   D2   D3   D4   D5  ...  D18  D19  D20
    
       Where the starting voltage will never exceed 15 KV, a microwave oven
       rectifier (like CR1 or CR2) would be adequate.  However, even the 20 KV PRV
       I am using may be insufficient in case the HeNe tube does not start or
       becomes disconnected - especially when driving the larger and/or hard to
       start HeNe tubes for which this power supply was designed.  Despite their
       beefy current ratings, these rectifiers can still be blown by excessive
       voltage - I have done it :-(.
    
    8. WARNING: Even though the oil burner ignition transformer (T1), is current
       limited, 23 mA is still enough to be lethal and the HV filter capacitors
       can pack quite a punch.  This power supply is very dangerous.  Take extreme
       care not to touch (or even go near) the high voltage terminals when it is
       operating.  Even after powering down AND pulling the plug, treat it with
       this same degree of respect until you have confirmed that ALL of the filter
       capacitors are fully discharged.
    
    9. WARNING: the power output of HeNe lasers driven by this circuit is likely
       to be in the Class IIIb safety classification and definitely hazardous to
       vision.  Take appropriate precautions!
    
    Kim's mid-size HeNe power supply:
    --------------------------------
    
    Remember how I said not to use a neon sign transformer?  Well, this one DOES
    but it is a small one so the spirit is more in keeping with an oil burner
    ignition type :-).
    
    This power supply was constructed by: Kim Clay (bkc@maco.net) and has been
    used to drive a 7 mW HeNe tube (so far).  However, it should be capable of
    driving medium size tubes requiring up to 4,000 VDC operating voltage at 8 mA
    operating current - possibly more - with only minor modifications (among other
    things, due to the no-load output of the power transformer, T1, a higher
    voltage filter capacitor and/or shunt pre-regulator may be needed to prevent
    the smoke from being released).
    
    The general design is very similar to "Sam's mid-size line powered HeNe laser
    power supply based on an oil burner ingnition transformer".  It uses a flyback
    type starter based on a 556 dual timer based drive circuit similar to a
    simplified version of the "Adjustable high voltage power supply" described in
    the section: "Sam's inverter driven HeNe power supply 2".
    
    Operating voltage for Kim's mid-size HeNe power supply:
    ------------------------------------------------------
    
    The operating voltage is provided by a 5,000 VRMS, 30 mA neon sign transformer,
    home-made high voltage bridge rectifier, and oil-filled HV filter capacitor.
    A Variac is used to adjust the output voltage for each HeNe tube/ballast
    combination.  There is no current regulator.
    
    
             T1         CR1                               CR5        Rb
           ||==|| +--+--|>|-----+-------+------+------+---|>|---+---/\/\---+
           ||  ||(   |          |       |      |      |         |          | Tube+
     H o-+ ||  ||(   |  CR2     |       |      /      /         /        .-|-.
          )||  ||(   +--|<|--+  |       |   R1 \   R2 \      R3 \        | | |
          )||  ||(           |  |   C1 _|_  5M /  10M /     10M /        |   |
          )||  ||(           |  | 2 uF ---     \      \         \        |   | LT1
          )||  ||(      CR3  |  | 5 KV  |      |      |         |        |   |
          )||  ||(   +--|>|-----+       |   M1 o +    o -     + o        ||_||
     N o-+ ||  ||(   |       |          |  (V) o -      Starter          '-|-'
           ||  ||(   |  CR4  |          |      |                           | Tube-
           ||==|| +--+--|<|--+----------+------+-------------o o-------------+
           |   |                                          M2 - + (I)         |
     G o---+-+-+-------------------------------------------------------------+
            _|_
             -       T1: 5,000 VRMS, 30 mA neon sign transformer.
                     CR1-CR4: 11 KV, CR5: 20 KV (stacks of 1N4007s).
                     M1: 1 mA panel meter, relabeled 5,000 V full scale.
                     M2: 10 mA panel meter, HeNe tube current.
    
    WARNING: This supply can be deadly!  Don't even think about going near any
    part of the high voltage circuitry except with the plug pulled from the wall
    and only after confirming that the main filter capacitor has discharged
    completely.
    
    As with any transformer designed to directly drive gas discharge tubes, T1
    has significant voltage droop.  At a 7 mA HeNe tube current, the no-load and
    operating voltage differ substantially - 4.7 kV versus 3.2 kV.
    
    A shunt regulator consisting of a stack of zener diodes and dropping
    resistor(s) prior to the bypass HV diode could be added to reduce this
    difference by providing a substitute current path - loading down the
    transformer - until the HeNe tube starts.  Kim has a pile of cheap 36 V,
    400 mW zener diodes he may try out in this manner.  However, for this to
    work without meltdown, the number of zeners would have to be matched to the
    voltage setting - else a new stack of zeners might be required :-(.
    
    Since T1 is not a center tapped transformer, a bridge is required to provide
    full wave rectification.  This was constructed from stacks of 1N4007 diodes
    mounted on perfboard, 11 of these for each of CR1 to CR4.  CR5, the HV bypass
    diode, was similarly constructed from 20 - 1N4007s.  See the section: "Custom
    HV rectifiers" for possible construction techniques and considerations.
    
    Both a voltage meter (M1) and current meter (M2) are permanently attached.
    The current limiting resistor for M1 also acts as a bleeder resistor for the
    main filter capacitor resulting in a time constant of about 10 seconds.  This
    5M resistor (R1) consists of 5 - 1M, 2W resistors in series mounted on
    perfboard.  R1 is constructed from multiple resistors in series to handle the
    high voltage across this component without damage.
    
    Starter for Kim's mid-size HeNe power supply:
    --------------------------------------------
    
    The starter uses the flyback from a mono computer monitor driven by an NPN
    darlington power transistor that used to be a solenoid driver from a dead dot
    matrix printer.  By using the darlington, a 556 timer IC can connect to the
    transistor base without any matching transformer or additional active
    components.  Frequency and pulse width are adjustable with optimal values for
    the particular implementation shown in ()s.  (See the calculations below.)
    
    The flyback was modified by adding the drive winding on the exposed leg of the
    core - 20 turns of #24 magnet wire on an insulating sleeve.  The high voltage
    rectifier is built into the flyback.
    
    The Start button, S1, enables the drive on demand.
    
    The entire starter fits on 1/3 of a Radio Shack experimenters PCB!
    
                                                        +12
                                                         o         Flyback
                                                         |           T2  +--|>|--o
                                           S1 Start      +---------+ |:|(        +
                                    R4       _|_  R5 4.7K|          )|:|(
                            +---+--/\/\---+--- ---/\/\---+    D 20T )|:|(  Starter
       (7.3K)              _|_  |  1K .1  |       R6 1.5K|      #24 )|:|(  Output
        10K     1K          -   +---+ uF  |     +--/\/\--+          )|:|(
        R7      R8  .001 uF C2 _|_  | C3  |     | R9 2.2K       +--+ |:|(        -
     --/\/\----/\/\---------+  --- _|_  +-+ +---|--/\/\--+      |        +-------o
        ^                   |   |  ---  |   |   |        |      +--+
        |   +12 o--+----+   +---+   |   |   |   +----+   |         |
        |          |  14| 13| 12| 11| 10|  9|  8|    |   |    +----+------+----+
        +----------+  +-+---+---+---+---+---+---+-+  |   |    |    |C     |    |
        |             | V   Di  Th  Co  Re  O   Tr|  |   | B|/     |      |    |
        |             | c   2   2   2   2   2   2 |  |   +--|      |  C4  | D1 |
        |             | c                         |  |      |\     | .01 _|_  _|_
        |             |    U1 NE556 Dual Timer    |  |        |  |/   uF ---  /_\
        |             |                         G |  |   Q1   +--|        |    |
        |             | 1   1   1   1   1   1   n |  |   2SD1308 |\ E     |    |
        |             | Di  Th  Co  Re  O   Tr  d |  |             |      |    |
        |             +-+---+---+---+---+---+---+-+  |             +------+----+
        v              1|  2|  3|  4|  5|  6|  7|    |            _|_        FR304
     --/\/\--/\/\--+----+   |   |   o   +---|---|----+             -
       R10   R11   |   R12  |   |  +12      |   |               
       50K    1K   +--/\/\--+---|-----------+   |  Q1: Darlington from NEC printer
     (13.9K)           330  |   |               |  D1: Damper diode (high speed)
                           _|_ _|_ C6           |  
                        C5 --- --- .1 uF        |  Note: Additional bypass caps on
                  .0033 uF  |   |               |        +12 source recommended
                            +---+---------------+        near the drive input to
                           _|_                           the flyback (not shown).
                            -
    
    C4 and D1 need a voltage rating sufficient for the spike that results when
    Q1 turns off.  Its magnitude will depend on the inductance of the flyback and
    total capacitance (C4 + flyback).  The value of C4 is one thing that can be
    changed to optimize performance but make sure to monitor the pulse across
    Q1 (when it turns off) as you bring up the input voltage and adjust the
    frequency and/or pulse width to avoid exceeding the transistor's Vce breakdown
    rating.  D1 should be a high speed (fast recovery) type.
    
    The only somewhat critical components are C5 and R10+R11 to set the operating
    frequency, and C2 and R7+R8 to set the pulse width.
    
    In this drawing, frequency is (Timer 1):
    
                       1.44                          1.44
      F = -------------------------------- = ------------------------ = 28.044 kHz
           ((R10 + R11 + (2 * R12)) * C5)     (14900 + 660) * 3.3E-9
     
    and the pulse width is (Timer 2):
    
      T = (1.1 * (R7 + R8) * C2) = (1.1 * 8300 * 1E-9) = 9.13 uS
    
    Optimum frequency and pulse width will depend on the flyback transformer
    actually used.
    
    The +12 V power supply came from the same monitor as the flyback.  It uses a
    full wave bridge rectifier with a 3300 uF filter capacitor and is adjustable
    from around 9.5 V to 13 V.  There are separate V+ feeds from the power supply
    for the 556 and flyback.  I have a 100 uF electrolytic, 10 uF tantalum, and a
    .01 uF disk keeping the 556 happy.  A larger main filter capacitor or post
    filter capacitor may be desirable due to the pulsed nature of the load.
    
    BTW, Kim even tried some 1 mW HeNe tubes on just the starting inverter using a
    HV capacitor made out of aluminum foil and baggies (!!) with good success.
    Thus, for small HeNe tubes, this may be all you need!
    
    Sam's Ultrabeam(tm) line powered HeNe laser power supply:
    ---------------------------------------------------------
    
    This one will drive tubes rated between 5 and 35 mW of beam power - possibly
    more.  I have not added a regulator to it though this is certainly possible
    using something similar to the low side regulator described in the section:
    "High compliance cascade regulator" (though it may need to be expanded to
    provide even wider compliance).  For now, I run the supply it on a Variac
    while monitoring HeNe tube current.
    
    A pair of power transformers (T1 and T2) originally designed for tube-type
    audio amplifier applications provides the input voltage - between 600 and
    1,200 VRMS using a Variac on T2 only (terminal V).
    
    A voltage quadrupler boosts this to the required operating voltage.
    
    I could also have used my boosted TV power transformer (900 VRMS) in place of
    T1 and T2.  This would easily provide 4,800 VDC from a 115 VAC input or over
    6,000 VDC from the 140 VRMS output of a Variac.  See the section: "Sam's line
    powered HeNe laser power supply" for details and the section: "Boosting the
    output of a transformer with multiple secondary windings" for some approaches
    to change the voltage range.
    
    CAUTION: If the operating voltage is increased much beyond 6,500 VDC, the
    voltage ratings of the rectifiers and capacitors will need to be increased
    as well.
    
    An inverter based starter would be appropriate for this power supply.  Power
    for this circuit can be provided by rectifying and filtering the voltage from
    the filament windings on one of the power transformers (T1).  The starter's
    output is introduced via high voltage isolation resistors across a HV blocking
    diode (a microwave oven rectifier) to bypass current around the inverter once
    the discharge is initiated.
    
    A simple transistor circuit disables the drive to the starting inverter once
    the tube fires by sensing tube current and forcing the 555 based controller
    to the reset state.
    
    See the section: "Inverter based starters" for more info.
    
    Estimated specifications (Sam-L3):
    
    * Operating voltage: 3,200 to 6,400 V.
    * Operating current: 5 to 10 mA.
    * Starting voltage: greater than 15,000 V.
    * Compliance range: NA, no regulator.
    
    Primary side components are not shown.  See the section: "AC input circuitry
    for HeNe power supplies" for more info.
    
    
                          C1             C3
               T1 +-------||-----+-------||------+        Starter
     H o-----+ ||(       1 uF    |      1 uF     |       o -   + o
              )||(      3.5 KV   |     3.5 KV    |       |       |
              )||( 600           |               |    R1 /       / R2
              )||( VRMS          |               |   10M \       \ 10M
              )||(               |               |   1 W /       / 1 W
          +--+ ||(               |               |       \       \
          |   _|_ +--+      CR1  |  CR2     CR3  |  CR4  |  CR5  |   Rb
          |    -     |   +--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--/\/\--+
          |          |   | 4 KV    4 KV  | 4 KV    4 KV  | 15 KV          | Tube+
          |    T2 +--+   |               |               |              .-|-.
     V o-----+ ||(       |               |               |              | | |
          |   )||( up to |               |               |              |   |
          |   )||( 600   |               |               |              |   |
          |   )||( VRMS  |               |               |              |   | LT1
          |   )||(       |               |               |              |   |
     N o--+--+ ||(       |       C2      |       C4      |              |   |
               |  +------+-------||------+-------||------+              ||_||
     G o-------+         |      1 uF            1 uF                    '-|-'
              _|_        |     3.5 KV          3.5 KV                     | Tube-
               -         |                             R4                 |
                         |                        +---/\/\---+------------+
                         |                        |   270    |         R5
                         |                        |          |    +---/\/\---o Vcc 
                         | IL2 LED      R3        |    C5    |    |    1K      _
                 HV- o---+---|<|---+---/\/\---+---+----||----+    +----------o R
                           Beam On |    1K    |  _|_  .1 uF  |    |
                                   o - Test + o   -          |  |/ C Q1
                                                             +--|  2N3904
       _                                                        |\ E
       R (low) and Vcc are from 555 based inverter driver.       _|_
                                                                  -
    
    Notes for Sam's Ultrabeam(tm) line powered HeNe laser power supply (Sam-L3):
    ---------------------------------------------------------------------------
    
    1. T1 and T2 are rated 600 VRMS, 50 mA (at 115 V in).  They also include
       5 V and 6.3 V filament windings.  The 6.3 V winding on T1 powers the
       starting inverter.  The others could be used to adjust the output voltage
       if wired in series with the AC input connections.  See the section: 
       "Boosting the output of a transformer with multiple secondary windings".
    
    2. CR1 through CR4 each consist of five 1N4007s in series:
    
             o--|>|--|>|--|>|--|>|--|>|--o
    
    3. CR5 is a 20 KV rectifier also implemented as a stack of 1N4007s.  Make sure
       it is well insulated!  Mine is in a thick plastic tube.
    
    4. C1 through C4 are 1 uF, 3,500 V oil filled capacitors.  (Microwave oven
       HV capacitors rated 1 uF at 2,500 VAC could also have been used.)
    
    5. Construction is on a blank digital prototyping board which just has pads
       for 28 DIP locations (16 pins each).  Perforated or other insulating board
       could have been used as well.  Smooth rounded connections and a conformal
       insulating coating are essential to minimize corona in the high voltage and
       starting circuitry.
    
    6. A Variac is used to adjust operating voltage between approximately 3,200
       VDC and 6,400 VDC by controlling the input to only T2.  HeNe tube current
       is monitored at the Test jacks.  Sensitivity is 1 V/mA or directly using a
       mA meter.  The 'Beam On' LED provides another confirmation of laser tube
       operation - an additional safety precaution for higher power lasers.
    
    7. The starting inverter is active only during starting and is operated by a
       push button switch.  It can be powered from the filament windings of T1
       (the power transformer not controlled by the Variac), 1N4007 rectifier, and
       10,000 uF, 25 V filter capacitor (these not shown).
    
    8. WARNING: the power transformers are capable of much more than the 20 mA
       required for all but the highest power HeNe laser tubes making this power
       supply extremely dangerous.  Take great care not to touch (or even go near)
       the high voltage terminals while it is operating.  Even after powering down
       AND pulling the plug, treat it with this same degree of respect until you
       have confirmed that ALL of the filter capacitors are fully discharged.
    
    9. WARNING: the power output of HeNe lasers driven by this circuit is likely
       to be well into the Class IIIb safety classification and definitely
       dangerous to vision.  Take appropriate precautions!
    
    Spectra Physics Model 255 Exciter:
    ---------------------------------
    
    Well, the name alone, 'Exciter', makes this sound more impressive, doesn't it?
    However, as you will note, it is basically similar to several of the other
    AC line powered HeNe power supplies in this document.  But, it DOES provide a
    much higher maximum current - about 15 mA.
    
    For this model, I have both an original schematic and an actual sample unit.
    My only complaint is that the laser head attaches via a high quality BNC-like
    HV connector rather than the more common Alden type.  Well, I guess you can't
    have everything!
    
    Estimated specifications (Spectra Physics 255):
    
    * Operating voltage: 6,000 V.
    * Operating current: 6.5 to 15 mA.
    * Starting voltage: 12,000 V.
    * Compliance range: 3,000 to 6,000 V at top of ballast resistor.
    
    The only thing possibly preventing this power supply from working with the
    largest HeNe tubes commonly available (25 to 35 mW) is its somewhat anemic
    starting voltage (considering its exceptional 6,000 V maximum operating
    voltage).  For high power or hard-to-start HeNe tubes, a small external boost
    starter may be needed.  Alternatively, additional stages can easily be added
    to the internal voltage multiplier if starting turns out to be a problem.
    
    Note: I have changed the part numbers to be more logically organized on the
    diagram.  Thus, if you are attempting to repair one of these supplies, they
    will not match the Spectra Physics schematic!  Also, the schematic and hardware
    differ in some component values but not anything that appears critical.
    
    
          X       C3           C4
          +-------||-----------||-------+--o HV+
          |                             |               LT1             R9
    T1    |  CR1  CR2  Y      CR5  CR6  |      Tube+ +-------+ Tube- 25K, 10 W  
       +--+--|>|--|>|--+---+--|>|--|>|--+--/\/\------|-    |-|---+-----/\/\---+
    ||(   |            |   |                Rb       +-------+  _|_     R10   |
    ||(   |            |   \ R1                                  -  +--/\/\---+
    ||(   |            |   / 6.8M   T1: 2,200 VRMS, 50 mA           |30K, 5 W |
    ||(   |            |   \ 2 W      (primary not shown)           |    Q1 |/ C
    ||(   |        C1 _|_  |                                     x--+-------|
    ||(   |     .5 uF ---  |        CR1-CR6: SCM60 (6 KV)        |  MJE3439 |\ E
    ||(   |      5 KV  |   \ R2                               Rx / (Repeat    |
    ||(   |            |   / 6.8M   C1-C2: .5 uF, 5 KV       30K \  Qx & Rx   x
    ||(   |            |   \ 2 W    C3-C4: .0047 uF, 5 KV    5 W /  5 times)  |
    ||( T |            |   |                                     |       Qx |/ C
       +---------------+---+        Qx (Q2-Q6): MJE3439     +----x----------|
          |            |   |        Rx (R11-R15): 30K, 5 W  |       MJE3439 |\ E
          |            |   \ R3                             \ R17             |
          |            |   / 820K   (Rb is in laser head)   / 25K             x
          |            |   \ 2 W                            \                 |
          |        C2 _|_  |                                |            Q7 |/ C
          |     .5 uF ---  |                    +---------------------------|
          |      5 KV  |   \ R4                 |           |       MJE3439 |\ E
          |            |   / 820K               |           |   R18           |
          |            |   \ 2 W                |           +---/\/\---+------+
          |  CR3  CR4  |   |    R5     R6       |     ZD2  _|_, 1.47K  |      |
          +--|<|--|<|--+   +---/\/\---/\/\------+  1N970B '/_\         |  R19 /
                       |       820K   820K      |    24 V   |     Q8 |/ C 330 \
                       |       2 W    2 W       |           +--------|        /
                       |                  ZD1  _|_,         | 2N3569 |\ E R20 |
                       |               1N753A '/_\          /          |  500 /
                       |                  6 V   |           \ R21      |   +->\
                       |                        |           / 10K      |   |  /
                       |                        |           |          |   |  | HV-
                       +------------------------+-----------+----------+---+--+--o
                                                                   Current Adjust
    
    The basic circuit consisting of T1, CR1-CR4, and C1-C2, is a standard voltage
    doubler.  R1-R8 provide a bleeder resistance as well as biasing the series
    regulator voltage reference.  A single stage boost multiplier consisting of
    CR5-CR6 and C3-C4, provides a peak starting voltage approximately twice the
    no-load operating voltage - nearly 4 * V(peak) or 4 * 1.414 * VRMS of T1.
    
    The series regulator is in the low side of the power supply and consists of
    a cascade of MJE3439 NPN transistors - a total of 7 in all (Q1-Q7).  The
    combination of the MJE3439s and their associated base resistors labeled as
    Qx (Q2-Q6) and Rx (R11-R15) (the network denoted by the 'x's) are repeated 5
    times stacked one on top of the other to complete the diagram - I was lazy!).
    
    Operating current is set by the Current Adjust pot (R20) and will be equal to:
    Io = 5.3 V / (R19 + R20) within the compliance range of the regulator.  The
    range is about 6.5 to 15 mA.  This could easily be extended to a lower current
    by increasing the R19 or R20 though it would seem like a waste of a nice piece
    of hardware to power a .5 mW HeNe tube!  However, it could be used for this
    purpose if run from a Variac.
    
    With 7 MJE3439s, the compliance range is greater than 3,000 V.  ZD2 provides
    protection to limit the voltage across the regulator to a safe value for the
    transistors (approximately 3,150 V total, 450 V across each) should the
    compliance range be exceeded due to an accidental short circuit, defective
    laser head, or a HeNe tube which is too small.  However, this allows more
    current to flow into the load which may then not be very happy :-(.
    
    There are taps on the two primaries of T1 for 100, 117, and 125 VAC (primaries
    in parallel), and 200, 234, and 250 VAC (primaries in series).  These would
    also provide additional options for the output voltage range when used without
    a Variac.  The actual power supply has an externally accessible switch to
    select 115 or 230 VAC operation.  However, changing the taps requires going
    inside and doing some minor soldering.
    
    I would recommend adding a 'Beam-On' indicator, and voltage and current meters
    (or test points) since this power supply is suitable for some nice high power
    HeNe tubes and you don't want to take chances!  See the section: "Enhancements
    to Aerotech model PS1 HeNe power supply" for some suggestions.
    
    Simple inverter type power supply for HeNe laser:
    ------------------------------------------------
    
    This 2 transistor inverter is capable of driving a variety of medium to high
    voltage applications from a 6 to 12 V, 2 to 3 A DC power supply, or auto or
    marine battery.  I have used variations of this basic circuit to generate
    over 12,000 V for high voltage discharge experiments, test flyback (LOPT)
    transformers, and power normal and cold cathode fluorescent tubes.
    
    Here, the general design has been customized for use with small (.5 to 5 mW)
    HeNe laser tubes requiring between about 1,100 and 2,000 VDC at 3 to 6 mA
    (and possibly higher).
    
    The inverter drive and multiplier starting circuits (if used) are similar to
    plans for a small HeNe power supply found in the book: "Build your own working
    Fiberoptic, Infrared, and Laser Space-Age Projects", Robert E. Iannini, TAB
    books, 1987, ISBN 0-8306-2724-3 [3].
    
    However, with the designs below, all parts should be available without being
    tied to the supplier listed in the book (Information Unlimited, assuming they
    still even have these parts).  However, there is something to be said for not
    having to modify or wind your own transformer!
    
    Also see the section: "Sam's inverter driven HeNe power supplies" for a way
    to use this inverter design without a separate starting circuit.
    
    Simple inverter type power supply schematic:
    -------------------------------------------
    
    Two alternatives are described.  These differ primarily in the details of the
    high voltage secondary winding, rectifier/filter components, and whether a
    separate starting circuit is required:
    
    1. The transformer is totally custom but well specified using the core from
       a small B/W TV or monochrome computer monitor flyback transformer.  Three
       sets of windings are added but this is not really difficult - just slightly
       time consuming for the 1800 turn output winding if you don't have a coil
       winding machine.  Since the output is relatively high voltage, some care in
       distributing and insulating the wire will be necessary.
    
       Lower voltage rectifiers and filter capacitors can be used but a separate
       starting circuit (e.g., voltage multiplier) will be needed for all tubes.
    
       See the section: "Starting circuit for simple inverter type power supply
       for HeNe laser" for a multiplier type starting circuit for this system.
    
    2. The transformer is constructed using the core and high voltage secondary
       (intact) from a small B/W TV or computer monitor flyback transformer.  Two
       sets of windings are added but these are only a few turns each.  Note: the
       flyback must *not* have an internal high voltage rectifier.  If the primary
       windings are not shorted, they can be ignored.
    
       As an added bonus, with the flyback's HV secondary, there may be no need
       for a separate starting circuit.  Since it will have 3,000 or 4,000 turns
       (compared to 1,800 turns for your homemade high votlage winding), the
       no-load voltage will be much greater and should provide enough output
       for tubes requiring less than about 8 KV starting voltage.  Higher voltage
       rectifiers and filter capacitors are required but construction is greatly
       simplified by the elimination of the starting circuit.  Where greater
       starting voltage is required, a smaller multiplier (2 or 3 stages) will
       likely be sufficient.
    
       This is far and away the easiest approach since no tedious and time
       consuming thousand+ turn coil winding is then required.  I recommend you
       try this first as it will save a great deal of time and effort.
    
       See the section: "Sam's inverter driven HeNe power supplies" for details on
       a high compliance design requiring no separate starting circuit.
    
    The basic design including all primary side components is identical for both
    approaches.  The schematic shows D3, D4, C1, C2, specifically for the custom
    wound HV winding (1) above and described in the text which follows.
    
    
          +Vcc                             o T1 (1)   X 
            o          Q1 +----------------+          o  
            |             |                 )|:|      |         D3
            |         B |/ C                )|:| +----+----+----|>|----+-----o Y
            |  +---+----|    2SC1826        )|:|(          | 3 KV (3)  |
            |  | __|__  |\ E          D 15T )|:|(          |           |
            |  | _/_\_   _|_            #26 )|:|(          |           |
            |  |  _|_     -                 )|:|( HV 1800T |          _|_ C1
            |  |   -  D1 1N4148             )|:|( #36 (1a) |          --- .05 uF
            +--|---------------------------+ |:|(          |           |  2 KV (4)
            |  |  _-_ D2 1N4148             )|:|(          |           |
            |  | __|__   _-_                )|:|( T        |           |
            |  | _\_/_    |                 )|:| +---------------------+-----o Z
            |  |   |  B |/ E          D 15T )|:|           |           |
            /  |   +----|    2SC1826    #26 )|:|           |           |
         R1 \  |   |    |\ C                )|:|           |           |
         1K /  |   |      |                 )|:|           |          _|_ C2
            \  |   |  Q2  +----------------+ |:|           |          --- .05 uF
            |  |   |                         |:|           |           |  2 KV (4)
            |  |   |                       o |:|           |           |
            |  |   +-----------------------+ |:|           |    D4     |
            |  |                      F 10T )|:|           +----|<|----+-----o G
            |  |       R2 100, 1 W      #32 )|:|             3 KV (3)
            +--+---------/\/\/\------------+ 
    
             Windings: HV = High Voltage, D = Drive. F = Feedback.
             (Values of C1, C2, D3, D4 shown design using custom wound HV winding.)
    
    Notes on simple inverter type power supply for HeNe laser:
    ---------------------------------------------------------
    
    1a. T1 is constructed on the ferrite core of a small B/W TV or monochrome
        computer monitor flyback transformer or one that is similar.  If using
        a salvaged flyback, remove the core clamp or bolts and separate the core
        pieces.  Save the plastic core gap spacers for later use.
    
    1b. It may be possible to use the high voltage secondary intact if it is in
        good condition.  However, the flyback must *not* have an internal high
        voltage rectifier if a doubler (may be required to achieve sufficient
        output for a high compliance design) is used for the operating voltage
        or multiplier type starting circuit is used.
    
    1c. The D (drive) and F (feedback) windings for T1 are wound using appropriate
        size magnet wire (if available - hookup wire will work in a pinch) close
        to the core.  If possible, these should go on first *under* the high
        voltage secondary.  If not, wind them on the opposite leg of the core.
    
    1d. Insulate the core and then wind the D and F windings adjacent to each
        other.  Bring the coil ends and centertap out one end and insulate them
        from the windings they cross.  Make sure all the turns of each winding
        are wound in the same direction (both halves).
    
    1e. If you are using the original HV winding, depending on its original
        construction and whether you extracted the internal primary windings, it
        may slip over the D and F windings.
    
    1f. If you are adding your own HV (high voltage) winding, use a close fitting
        plastic or cardboard tube or roll of paper on top of the primary windings
        to provide a smooth uniform insulating form for the O winding.
    
        Build up the 1,800 turn HV winding in multiple layers of about 200
        turns where each is a single layer of wire.  Use thin insulating (mylar)
        tape between layers.  Make sure the start and ends of this winding are
        well insulated from all windings, the core, and everything else.  Wrap the
        outside with electrical tape to insulate it as well.   
    
    1g. Reassemble the core with its plastic spacers or add your own.  With a core
        air gap of .25 mm, the switching frequency is about 10 kHz.  Selecting the
        core gap size is one means of fine tuning operation.
    
    2.  The transistors I used were 2SC1826s but are not critical.  Others such as
        the common 2N3055 or MJE3055T types should also work.  Any fast switching
        NPN power transistor with Vceo > 100 V, Ic > 3 A, and Hfe > 15 should work.
        For PNP types, reverse the polarity of the power supply and D1 and D2.
    
        For continuous operation at higher power levels, a pair of good heat sinks
        will be required.
    
    3.  Diodes D3 and D4 must be at least 3 KV PIV for an 1,800 turn HV winding
        or 10 KV PIV using the original flyback's HV secondary.  Fast recovery
        types would be better but normal rectifiers seem to work.  If diodes with
        the required PIV rating are not available, build them up from 1 KV diodes
        paralleled with 10 M resistors to balance the voltage drops.  For testing,
        I have been simply using strings of 1N4007s without apparent problems.
        Your mileage may vary.  Some commercial power supplies seem to omit the
        equalizing components as well and get away with it.  See the section:
        "Edmund Scientific HeNe power supply".
    
    4.  Capacitors C1 and C2 must be at rated at least 1.5 KV for an 1,800 turn
        HV winding or 5 KV using the original flyback's HV secondary.  Where
        capacitors with these ratings are not available, construct them from
        several lower voltage capacitors in series with 2.2 M resistors to
        balance the voltage drops due to unequal capacitor leakage.
    
    5.  Some experimentation with component values may improve performance for
        your application.
    
    6.  When testing, use a variable power supply so you get a feel for how much
        output voltage is produced for each input voltage.  Component values are
        not critical but behavior under varying input/output voltage and load
        conditions will be affected by C1, the number of turns on each of the
        windings of T1, the gap of the core of T1, and the gain of your particular
        transistor.  If the circuit does not start oscillating, interchange the
        F winding connections to Q1 and Q2.
    
    7.  Add a post-regulator to this supply if desired to stabilize the current
        since the inverter itself is not very well regulated.
    
    8.  WARNING: Output is high voltage and dangerous.  Take appropriate
        precautions.
    
    9.
           |                         |           |
        ---+--- are connected;    ---|--- and ------- are NOT connected.
           |                         |           |
    
    
    Starting circuit for simple inverter type power supply for HeNe laser:
    ---------------------------------------------------------------------
    
    A voltage multiplier based design is shown.  Other approaches can be used as
    well - pulse trigger or wide compliance operation.  See the chapter: "Helium
    Neon Laser Power Supplies" and the section: "Sam's inverter driven HeNe power
    supplies".
    
    This is called a 'parasitic multiplier' since it feeds off of the main supply
    and is only really active during starting when no current is flowing in the
    HeNe tube.
    
    See the section: "Voltage multiplier starting circuits" for a more detailed
    description of its design and operation.
    
    
           R1    C1              C3              C5              C7
    X o---/\/\---||------+-------||------+-------||------+-------||------+
        1M, 1 W     D1   |  D2      D3   |  D4      D5   |  D6      D7   |
                 +--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+---o HV+
                 |               |               |               |               
    Y o----------+-------||------+-------||------+-------||------+
                         C2              C4              C6
    
    G o----------------------------------------------------------------------o HV-
    
    
    X. Y, and G refer to the corresponding points on the schematic above or other
    sample circuits in this document.
    
    With 7 diodes, HV(peak) is approximately (X(peak) * 8) + Y and HV(average) is
    (X(peak) * 7) + Y.  For small tubes, fewer stages can be used.  Increasing the
    number of stages beyond what is shown may not boost output that much as the
    losses due to diode and stray capacitance and leakage begin to dominate.
    
    For the high frequency inverter, typical capacitor values are 100 pF.
    
    The voltage ratings of the diodes and capacitors must be greater than the
    p-p output of the inverter.  The value of R1 can generally be increased to
    10M without afffecting starting.  A higher value is desirable to minimize
    ripple in the operating current once the tube fires.
    
    Notes on voltage multiplier starting circuit:
    --------------------------------------------
    
    1. Construction must take into consideration that almost 15 KV (in this case)
       may be available at the output should the tube not start or accidentally
       become disconnected.  Layout the circuitry so that parts with significant
       voltage differences are widely separated and try to avoid sharp points in
       the wiring and solder connections.
    
       Perforated prototyping board or any other well insulated material can be
       used.  Smooth out all HV connections - avoid sharp points by using extra
       solder.  A conformal coating of high voltage sealer is also recommended
       after the circuit has been constructed and tested.  Together, these will
       minimize the tendency for corona - which can greatly reduce the available
       starting voltage (particularly on damp days).
    
    2. Diodes D1 to D7 must be rated at least 3 KV.  Fast recovery types are
       probably required.  (The multiplier described in the section: "Sam's line
       powered HeNe laser power supply" using normal 1N4007s does not appear to
       generate adequate starting voltage when driven by this inverter).  If 3 KV
       diodes are not available, build them up from four 1 KV diodes (to add
       margin if no equalizing resistors and capacitors are used).
    
    3. Capacitors C1 to C7 are 100 pF 3 KV disk type.
    
    HeNe inverter power supply using PWM controller IC:
    --------------------------------------------------
    
    This power supply was found in a bar code scanner driving a .5 mW, 85 mm long
    HeNe tube.  Fortunately, only the high voltage section was potted and some
    icky disgusting rubber material was used which could be removed by picking,
    chewing, clawing, and scraping, without any serious damage to the underlying
    circuitry.  This is a very compact unit with total dimensions of: 3/4" (W) x
    1/2" (H) x 5" (D).
    
    The input voltage range is about 5 to 12 VDC though the minimum will depend
    on the size of the HeNe tube powered.  The output is current regulated and
    fully protected against a variety of fault conditions.
    
    The power supply has been tested on a variety of HeNe tubes up to 2 mW:
    
    * For a .5 mW, 135 mm tube, nothing becomes excessively hot and continuous
      operation is probably possible.  Output voltage is about 1,300 VDC.  This
      size tube is what the power supply came with for its intended use as part of
      a bar code scanner.
    
    * A 1 mW, 150 mm tube caused the driver transistor and ferrite transformer to
      become quite warm but not too hot to touch.   The DC output voltage of the
      supply in this case was about 1,350 VDC.  With at most a small heat sink, I
      would expect the supply to drive this size tube continuously.
    
    * It was able to drive a 2 mW, 200 mm tube (requiring about 1,750 V from the
      supply).  However, a heat sink would definitely be required on the driver
      transistor for continuous operation and the ferrite transformer would likely
      become hot enough to be damaged in a short time.  Therefore, this is not
      recommended.
    
    The current was maintained near the calculated value of 3.2 mA in all cases.
    
    The basic design is quite nice and could be easily modified to drive much
    larger tubes.  The only non-standard part - the ferrite transformer - is also
    relatively simple to construct (as these things go) with only two windings on
    a circular bobbin in a gapped pot core.
    
    The design uses an integrated circuit, the Philips SG3524.  This is a
    Pulse Width Modulated (PWM) switchmode power supply controller chip which
    incorporates a fixed frequency oscillator, ramp generator, error amplifier and
    comparator, and output drivers.  The SG3524 provides regulation as well as
    over-voltage and over-current protection, and other functions.  Through the
    use of these capabilities, this design should be quite robust in dealing with
    a variety of fault conditions.
    
    If you want to construct a power supply similar to this one, the SG3524 is
    readily available from large electronics distributors and places like MCM
    Electronics and Dalbani but shop around - the price seems to vary widely
    ($2.45 to $12.50!).  Additional information on this part may be found in:
    AN126 - Applications using the SG3524.
    
    Estimated specifications (IC-I1):
    
    * Operating voltage: 1,000 to 2,000 V.
    * Operating current: 2 to 4 mA (by changing resistor).
    * Starting voltage: greater than 6,000 V.
    * Compliance range: 1,000 V.
    
    For the bar code scanner application, the HeNe tube and power supply were
    glued together and mounted as a single unit.  The red cap at the far left
    is a feeble attempt to insulate the high voltage to the HeNe tube (not covered
    by the gray rubbery potting material just visible over the left half of the
    power supply.  You can still get zapped from under the circuit board (as I
    found out!).  This unit used a Uniphase HeNe tube.  Another one came with a
    very similar Melles Griot HeNe tube.
    
    ICI1PS Top View shows the component side of the power supply printed circuit
    board after the rubbery potting material covering the high voltage section
    (left half) had been removed.  The pot core ferrite transformer is just to
    the right of center with the IRF630 MOSFET next to it (separated by a filter
    capacitor).  The SG3524 controller IC is located under the IRF630.  The bright
    blue and orange objects are the filter and multiplier capacitors in the high
    voltage circuitry.  The high voltage rectifiers can be seen above and below
    them.  The 99K ballast resistor (3 x 33K) is visible at the far left.
    
    As a result of the sophistication of the SG3524, the overall design is really
    quite simple.  The PWM controller is shown first followed by the inverter:
    
    
          2N3904                                     R3
           Q3 +---+-----------------------------+---/\/\---+
              |   | 2.21K                       |  3.92K   |    R5
            |/ C  /    +------------------------|----------+---/\/\---o CS
      VS o--|     \ R1 |      U1 SG3524         |              6.81K
            |\ E  /    |   +--------------+     |
              |   |    |  1|              |16   |   Input (+5 to +12 VDC)     
              +---+--------|-In   Vref Out|-----+         o 
              |   |    |  2|              |15             |               1 o  T1
             _|_  / R2 +---|+In        Vin|----+----+-----+-----+------------+ |:|
          C3 ---  \ 2.74K 3|              |14  |    |           |         15T )|:|
        .1 uF |   /     ---|Osc Out    E-B|--- |   _|_ C1      _|_ C4     #26 )|:|
              |   |       4|              |13  |   --- 6.8 uF  --- 100 uF   2 )|:|
              +---+----+---|+CL Sense  C-B|--- |    |  16 V     |  16 V   +--+ |:|
                       |  5|              |12  |   _|_         _|_     D  |
                       +---|-CL Sense  C-A|----+    -           -    .|---+ Q1
                       |  6|              |11          D1           G||<--. IRF630
             +---------|---|RT         E-A|---------+--|>|---+-------'|---+
             |         |  7|              |10       | 1N4148 |         S  |
             |     +---|---|CT    Shutdown|---o OV  |        |            |
             |     |   |  8|              |9        |      |/ E Q2        |
          R4 /     |   +---|Gnd       Comp|---      +------|    2N3906    |
        5.1K \    _|_  |   |              |         |      |\ C           |
             /    ---  |   +--------------+         / R6     |            |
             |  C2 |   |                            \ 4.7K   |            |
             |.001 |   |                            /        |            |
             |  uF |   |                            |        |            |
             +-----+---+----------------------------+--------+------------+--o HV-
                      _|_
                       -
    
         3             C6              C8              C9
     T1 +--------------||------+-------||------+-------||------+
    |:|(                       |               |               |
    |:|( 500T             D2   |  D3      D4   |  D5      D6   |  D7       HV+
    |:|( #36           +--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+--|>|--+----o
    |:|(               |               |               |               |
    |:|( o 4           |         C7    |               |      C10      |  R14
        +---+----+-----+----+----||----+   +-----+-----+-------||------+--/\/\--+
            |    |     |    |          |   |     |     |                  33K   |
      CS o--+ R7 /  R8 /   _|_ C5      |  _|_   _|_   _|_                   R13 /
             10K \ 430 \   --- .1 uF   |  ---   ---   ---                   33K \
             SBT /     /    |          |   | C11 | C12 | C13    LT1             /
                 |     |    |          |   |     |     |   +----------+ R12 33K |
     HV- o-------+-----+----+--------------+-----+-----+---|-|       -|---/\/\--+
              R9        R10     R11    |        _|_  Tube- +----------+ Tube+
      OV o---/\/\---+---/\/\----/\/\---+         -
             13K    |   4.7M    4.7M       D2-D7: 2 KV, fast recovery type.
      VS o----------+                      C6-C8, C10: 1 nF, C9: 4.7 nF, all 3 KV.
                                           C11-C13: 1 nF, 6 KV.
    
    
    Notes on PWM controller for HeNe power supply (IC-I1):
    -----------------------------------------------------
    
    1. R4 and C2 set the oscillator frequency, roughly 1/R*C or about 200 kHz.
       This generates a sawtooth/ramp inside the SG3524.  The output of the error
       amplifier (Pins 1 and 2, -In and +In) is then compared with this ramp to
       control the pulse width of the drive to the switching transistor, Q1, which
       is enabled every other cycle resulting in a switching frequency of 100 kHz.
    
    2. The main feedback loop is from terminal CS (Current Sense) which sets the
       output current based on the voltage drop across the parallel combination of
       R7 (SBT or Select By Test) and R8.
    
       With the installed values for R7 (SBT), the sensitivity is approximately
       .4 V/mA.  The voltage on the +In pin of the SG3524 will then be equal to:
       3.24 V - 146 * Iout.  The 3.24 V reference is derived from Vref (5 V) and
       the voltage divider formed by R3, R5, R7, and R8.  The factor of 146 comes
       from the voltage divider formed by R3 and R5 when driven by CS.
    
    3. VS (Voltage Sense) is derived from a point about 1/3 of HV+ and will be
       approximately equal to: 1/3 * HV+ * 24K / 9.4M.  (11K of the 24K is input
       resistance of the SG3524's Shutdown pin) or 8.3E-4 * HV+.  The -In pin of
       the SG3524 will then be VS - .7 V or 2.77 V (Vref through the voltage
       divider formed by R1 and R2) depending on which is greater.  The 2.77 V
       reference will be in effect under normal conditions.  However, if HV+ goes
       above about 4,200 V, the VS input will take over and limit output even if
       no current is drawn (as would be the case before the tube starts or if the
       tube were disconnected or did not start).
    
    4. Once the tube starts, the set-point will be where:
    
                  -In = +In
                 2.77 = 3.24 - 146 * Iout
    
       Thus:     Iout = 3.2 mA (for the installed value of SBT).
    
    5. If OV (Over-Voltage) becomes too great, the Shutdown input will activate
       killing the inverter momentarily - the cycle will then repeat.  Shutdown
       voltage is sensed from a point about 1/3 of the operating voltage, HV+.  If
       this point exceeds about 700 V, the driver will shut down.  During staring
       this limits the rate of rise of output voltage.  Should the tube fail to
       start or become disconnected, excessive voltage will not be generated.
    
    Notes on the inverter for HeNe power supply (IC-I1):
    ---------------------------------------------------
    
    1. This is a flyback inverter where the length of time the driver transistor
       (Q1) is on determines how much energy will be transferred to the high
       voltage circuitry when it switches off.  The SG3524 drives the MOSFET's
       gate via D1.  Q2 is used to turn off the MOSFET quickly by discharging the
       gate capacitance to ground.
    
    2. T1 is a ferrite transformer wound on a pot core.  The overall dimensions
       are 9/16" diameter by 5/16" height.  The bobbin is 7/16" by 3/16"
    
       There is a core gap which is about .005".
    
       Estimated maximum effective V(peak) (since the output is not symmetric,
       this isn't really precisely defined): 1000 V.
    
       I have estimated the turns ratio for now but intend to perform some
       measurements to confirm.  This is very rough at this point!
    
       * Primary: 5 turns #26.  The wire ends of this winding are buried so the
         wire size is based on required current capacity.
    
         The primary appears to be wound first close to the core.
    
       * Secondary: 250 turns, #43 (Yes, #43!  The closest wire sample I have to
         compare it to is #40 so this is what is known as a wild guess since I
         have not disassembled the transformer to measure the wire precisely with
         a micrometer) and is based on the resistivity of #43 wire and the bobbin
         dimensions.
    
         I suspect that like a normal (TV or monitor) flyback transformer, the
         secondary is built up of several (single thickness) layers of windings
         (perhaps 40 or 50 turns each) with mylar insulating tape in between.
    
       To somewhat confirm the the turns ratios, I measured the peak-peak input
       and output of the transformer while operating with a 1 mW HeNe tube: input
       was 15 V p-p; output was 700 V p-p.
    
       Since this is a flyback converter and the transformer has a core gap, I
       don't know how closely this correlates with winding ratios but at least it
       is in the ballpark.
    
    3. This is basically a wide compliance design and all stages of the voltage
       multiplier are active at all times.  I do not know why the main HV filter
       capacitor is not at the output other than not being able to obtain a set
       of capacitors with an adequate voltage rating and somewhat limiting the
       surge current from the filter (due to lower voltage on the large filter
       capacitors) when the tube starts.
    
    4.  WARNING: Despite its compact size, the output is high voltage and
        dangerous.  Take appropriate precautions.
    
    5.
           |                         |           |
        ---+--- are connected;    ---|--- and ------- are NOT connected.
           |                         |           |
    
    
    Sam's inverter driven HeNe power supplies:
    -----------------------------------------
    
    There are two variations on a similar approach which take advantage of the
    high compliance/poor regulation of these inverters for starting.  Thus, no
    separate starting circuit is required.
    
    These are both based on small flyback transformers and run on low voltage DC.
    For this, I use a very basic transformer/rectifier/filter capacitor power
    supply driven from a Variac.
    
    No starting circuit is needed because of the wide compliance of thess circuits.
    With no load (tube not lit), the voltage will climb to 5 to 8 KV or more.  As
    soon as the tube fires, the output drops to the sustaining ballast resistor
    voltage for the operating current.  In essence, the poor voltage regulation of
    the inverter represents an advantage and allows this minimalist approach to be
    effective.
    
    This is one type of design where monitoring of the input voltage to the tube
    is possible with a VOM or DMM requiring at most a simple high voltage probe.
    Parasitic voltage multipliers may not have enough current capability and
    pulse type starting circuits produce short high voltage pulses.  It is
    possible to clearly see the voltage to the ballast resistor/tube ramp up until
    the tube starts and then settle back to its operating voltage.  For small
    tubes, I can safely use my Simpson 260 VOM on its 5 KV range without a high
    voltage probe though it may go off scale momentarily.
    
    The only additional components required for the HeNe laser power supplies may
    be one or two high voltage rectifiers and a high voltage filter capacitor.
    Since this is across the output at all times, it must be able to withstand the
    starting voltage but be large enough to minimize ripple when the tube is
    operating.
    
    CAUTION: I would recommend using higher voltage capacitors than those shown
    unless you know that your inverter is not capable of reaching the capacitor's
    breakdown voltage.  With some of these on a variable supply, 25 KV or more
    open-circuit is quite possible due to wiring problems, no tube connected, a
    bad or high starting voltage tube - or carelessness in turning the knob to far
    clockwise!
    
    I have also tried a 500 pF, 20 KV doorknob capacitor on design #2 (I didn't
    have two such caps as required for design #1).  While this low value works,
    it is a bit too small and results in about 20% ripple at an operating voltage
    of 1,900 V and current of 4 mA with a 15 kHz switching frequency.  The minimal
    tube current setting for stable operation is slightly increased.  At lower
    switching frequencies it will be worse and may prevent the tube from running
    stably at all.  A few of these caps in parallel would be better.  Or, use a
    stack of parallel plate capacitors made from aluminum foil and sheets of 1/8"
    Plexiglass :-).
    
    WARNING: Since the voltage rating of these capacitors needs to be larger than
    for power supply designs with separate starting circuits, it is possible for
    a nasty charge to be retained especially if the tube should not start for some
    reason.  Stored energy goes up as V*V!
    
    Note: The difference in energy stored in the filter capacitor between the
    starting and operating voltages is dumped into the tube when it starts.  For
    long tube life this should be minimized.  Therefore, a smaller uF value is
    desirable for these high compliance designs.  I do not know how much of an
    issue this really represents.  A post-regulator can be used to remove the
    larger amount of ripple which results with samller capacitors.  However, such
    a regulator must have overvoltage protection since at the instant the tube
    fires, it will momentarily see most of the starting voltage.
    
    Sam's inverter driven HeNe power supply 1:
    -----------------------------------------
    
    This one is based on the inverter portion of the design described in the
    section: "Simple inverter type power supply for HeNe laser" but using the small
    B/W monitor flyback transformer option instead of a custom wound transformer.
    (For the doubler, the flyback must *not* have an internal rectifier.)  The only
    differences are in the voltage ratings of the components required for the
    doubler and filter capacitors (to the right of points X and T in that power
    supply diagram).
    
    Thus, it is an extremely simple circuit with no adjustments.  Power output
    is controlled strictly by varying input voltage.  Only a pair of high voltage
    rectifiers and a pair of high voltage filter capacitors for the doubler are
    required to complete the power supply.
    
    It requires between 6 and 12 VDC (depending on HeNe tube power and ballast
    resistor) at less than 2 A and will power small HeNe tubes requiring up to
    about 6 mA at 2,000 V, perhaps more.
    
    Estimated specifications (Sam-I1):
    
    * Operating voltage: 1,200 to 2,000 V.
    * Operating current: 0 to 6 mA.
    * Starting voltage: 5,000 to 8,000 V.
    * Compliance range: NA - no regulator.
    
    Here are sample operating points for two different 1 mW tubes:
    
    * Spectra Physics: (Rb=150K), Output: 1,450 V, 3 mA, Input: 6.25 VDC, 1.0 A.
    * Aerotech: (Rb=50K), Output: 1,500 V, 4 mA, Input: 7.0 VDC, 1.2 A.
    * Aerotech: (Rb=150K), Output: 1,900 V, 4 mA, Input: 7.5 VDC, 1.4 A.
    
    The rectifiers (D3 and D4) should be rated at least 10 KV PIV (possibly higher
    depending on the capabilities of your particular inverter).  (However, don't
    go excessively high as the voltage drop across the diodes could become rather
    substantial.)  In fact, when I replaced each of the high voltage rectifiers I
    had been using with a string of 1N4007s, the tubes would run stably at slightly
    lower output voltage (about 50 V less) and the discharge could be maintained
    at slightly lower current as well.
    
    The filter capacitor must be rated for the *maximum* no load voltage possible
    with your inverter.  For testing, I constructed it from two .25 uF, 4,000 V oil
    filled capacitors in series with equalizing resistors providing about .12 uF
    at 8 KV.  With the components I used, the maximum no load output voltage was
    slightly less than 8 KV with a 12 VDC input which is more than adequate to
    start most smaller tubes.  However, capacitors with at least a 5 KV breakdown
    voltage rating (10 KV total) should really be used.
    
    
                      +--------------+ X       D3                   Rb
         Vin+ o-------|              |---+-----|>|-----+-----+-----/\/\----+
                      |    Simple    |   |             |     |     100K    |
     8 to 12 VDC, 2 A |   Inverter   |   |         C1 _|_    / R3   5 W    |Tube+
                      | Power Supply | T |     .25 uF ---    \ 2.2M      .-|-.
         Vin- o-------|              |------+ 4,000 V  |     /           | | |
                      +--------------+   |  |          |     |           |   |
                                         |  +----------+-----+           |   | LT1
                                         |             |     |           |   |
                                         |         C2 _|_    / R4        |   |
                                         |     .25 uF ---    \ 2.2M      ||_||
                                         |    4,000 V  |     /           '-|-'
                                         |             |     |     R5      |Tube-
                                         +-----|<|-----+-----+----/\/\-----+
                                               D4                  1K     _|_
                                                                           -
    
    The tube current may be monitored as a voltage across R5 (1 V/mA) or directly.
    It may be varied by adjusting the input voltage to the inverter.  Using a
    different ballast resistor value may also help to stabilize operation.
    
    Sam's inverter driven HeNe power supply 2:
    -----------------------------------------
    
    This inverter is the design from: "Adjustable high voltage power supply"
    in the document: Various Schematics and Diagrams which additional info
    about this circuit.  Since I already had the inverter, it took a total of
    about 10 minutes to convert it to a HeNe laser power supply!
    
    It requires between 8 and 15 VDC (depending on HeNe tube power) at less than
    2 A and will power small HeNe tubes requiring up to about 6 mA at 2,500 V,
    perhaps more.  With a 1 mW tube (1,900 V, 4 mA, 150K ballast resistor), the
    input is about 8 VDC (probably about 1.5 A, not measured) and the switching
    transistor heat sink doesn't even get warm :-).
    
    Estimated specifications (Sam-I2):
    
    * Operating voltage: 1,200 to 2,500 V.
    * Operating current: 0 to 6 mA.
    * Starting voltage: 5,000 to 10,000 V.
    * Compliance range: NA - no regulator.
    
    The schematic for the inverter is available in PostScript and GIF format.  The
    Postscript versions have been compressed PKZIP (DOS/Win3.1/Win95) and GZIP
    (Unix).
    
    
  • hvgen32.zip
  • hvgen32.gz
  • hvgen32.gif The high voltage rectifier is built into the flyback transformer. If you have to use an external rectifier, it should be rated at least 20 KV PIV (possibly higher depending on the capabilities of your particular inverter). The filter capacitors shown were just for testing. High voltage types are recommended, again depending on the maximum output of your inverter with no load. For testing, I constructed it from three .25 uF, 4,000 V oil filled capacitors in series with equalizing resistors providing about .08 uF at 12 KV. However, with the design as implemented, the maximum no load output voltage could easily exceed 15 KV with a 15 VDC input. +--------------+ HV+ Vin+ o-------| |---------------+-----+-------------+ | Adjustable | | | | 8 to 15 VDC, 2 A | High Voltage | C1 _|_ / R1 | | Power Supply | HV- .25 uF --- \ 2.2 M / Rb Vin- o-------| |---+ 4,000 V | / \ 150K +--------------+ | | | / 5 W | +-----+ | | | | |Tube+ | C2 _|_ / R2 .-|-. | .25 uF --- \ 2.2 M | | | | 4,000 V | / | | | | | | | | +-----+ | | LT1 | | | | | | C3 _|_ / R3 | | | .25 uF --- \ 2.2 M ||_|| | 4,000 V | / '-|-' | | | R4 |Tube- +-----------+-----+-----/\/\----+ 1K _|_ - The tube current may be monitored as a voltage across R4 (1 V/mA) or directly. It may be varied by adjusting the frequency and pulse width controls on the inverter and its input voltage. Using a different ballast resistor value may also help to stabilize operation. It should be possible to add feedback from a current sense resistor to one of the 555 timers to regulate output current by controlling switching frequency or pulse width. This is left as an exercise for the student :-). Other HeNe power supply circuits on the Web: ------------------------------------------- * The circuit at: http://www.ee.washington.edu/eeca/circuits/F_ASCII_Schem.html#ASCIISCHEM_007 is a 12 V switching supply, apparently identical to the design found in the book by Robert E. Iannini [3]. As expected, there is no information on the special ferrite transformer which they (the author of the book) expect you to obtain from Information Unlimited. If it is still available, I expect the cost to be about $10. See the section: "Simple inverter type power supply for HeNe laser" for details on a similar design that you can build if you are willing to wind your own transformer or use the flyback from a small B/W TV or monochrome computer monitor. * The circuit at: http://www.execulink.com/~cake/circuits/lasersup.htm is *supposed* to run on 120 VAC but didn't have anything which looked like it would give the starting voltage and besides, used a 9 V transformer *backwards* to provide 1,600 V. Yikes! and the words say a resistor passes 100 W! My guess is that this never worked (at least not for more than a few milliseconds) as it depends on putting 115 VAC into the 9 V winding of a little transformer. These are not normally designed with any substantial margins. It probably operates mostly in core saturation likely melting down or blowing up or both in short order. Perhaps (generously) the author was mistaken about the transformer or (more likely) never actually built the thing at all. I include a reference to it here only to warn that I do not recommend this as a viable option. ------------------------------------------------------------------------------
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