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    HeNe Laser Power Supplies

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    Requirements, Types, Choices, Build or Buy, Safety

    Basic HeNe Power Supply Considerations

    Modern small to medium size HeNe tubes require an operating voltage between about 900 and 2,500 VDC at 3 to 6 mA and a 5 to 12 KV starting voltage (but almost no current). Precise values depend on the size and construction of the tube. This assumes something in the .5 to 10 mW range. Larger tubes will required greater voltage and current (e.g., 5,000 VDC at 8 mA, 15 KV to start) but are powered in basically the same way as their smaller siblings. However, a 250 mW monster as well as some very old tubes will likely have significantly different or additional power requirements.

    Unlike laser diodes, the HeNe drive is not nearly as critical to performance and tube life :-). Therefore, even if you do not have a datasheet for your tube, you can probably guess fairly closely as to its requirements and then optimize performance based on optical output alone.

    So Many Choices

    There are any number of ways of constructing these supplies - over a dozen sample circuits are provided in the chapter: Complete HeNe Laser Power Supply Schematics. However, you don't need to build your own:

    SAFETY when Dealing HeNe Laser Power Supplies

    Whether you have constructed your own power supply, are testing an old one, or just checking out a newly acquired HeNe tube, SAFETY must come first:

    See the section: Laser Safety with respect to the optical hazards associated with lasers. While not in the metal cutting class, careless use of especially higher power HeNe lasers can result in permanent damage to vision. Many are only Class II or Class IIIa but some are well into the Class IIIb range.

    HeNe laser power supplies utilize high voltage at low current. The few mA needed to operate the HeNe tube or even a 10 KV starting pulse tube can certainly give you an unpleasant shock but will not likely be enough to do lasting damage. However, there may be much more current actually available and there is always the possibility of collateral damage:

    Read, understand, AND FOLLOW, the guidelines provided in the document: Safety Guidelines for High Voltage and/or Line Powered Equipment.

  • Back to HeNe Laser Power Supplies Sub-Table of Contents.

    Construction Alternatives, Organization

    Constructing Your Own Power Supply

    If you still want to build your own, there are basically two approaches for the operating voltage (AC line operated or high frequency inverter) and three approaches for the starting voltage (diode/capacitor multiplier, pulse trigger circuit, or high compliance design).

    Here are six options for providing the operating voltage. These examples assume an output of at least 1,800 VDC:

    1. Use a 1,300 VRMS power transformer. Then, all that is needed is a rectifier and filter capacitor.

    2. Use a 700 VRMS power transformer with a 2 diode 2 capacitor voltage doubler.

    3. Use a lower voltage power transformer and a multi-stage voltage multiplier. Up to 6 stages should be reasonably easy to construct.

    4. Build a low voltage input inverter using a flyback transformer from a small B/W or color TV computer monitor, or video terminal but running at lower voltage than normal. These usually have a built in HV rectifier but you will need a HV filter capacitor and ballast resistor. While rated at only a mA or so for the CRT HV, more current should be available at reduced voltage. With proper design, it is possible for there to be enough voltage compliance to be self starting. See the section: Sam's Inverter Driven HeNe Power Supplies.

    5. Build a HV inverter based on any of a number of simple DC-DC converter topologies. See the document: Various Schematics and Diagrams for ideas.

      You will probably need to modify these mostly by increasing the number of turns on the output windings of the inverter transformer - which in itself may be a challenge to maintain low capacitance and high voltage insulation.

      See the section: Simple Inverter Type Power Supply for HeNe Laser for one modified for driving a HeNe laser tube.

    6. Build a HV inverter of the type discussed above using a PWM controller integrated circuit - Linear Technology, Maxim, Motorola, National Semiconductor, Unitrode, and others have suitable DC-DC controller chips.
    I would recommend (1) or (2) if portability is not a big issue and you can locate a suitable transformer. These are virtually foolproof (well, at least as long as you don't fry yourself from the high voltage). For small tubes, the design described in the section: Edmund Scientific HeNe Power Supply is about as simple as possible.

    See the sections starting with: AC Line Operated Versus Inverter Based Power Supplies for more discussion of features, basic operation, and design issues.

    Basic HeNe Power Supply Organization

    A typical configuration is shown below. As noted, the starting circuit may be omitted in a high compliance design. The regulator is desirable but the location shown (low-side series) is just one option. Without a regulator, tube current will need to be set by controlling the power input and/or selecting the ballast resistor.
                +----------+     +------------------+ HV+     Ballast Resistor
         o------|          |-----| Starting Circuit |--------------/\/\----+
       Input,   |  Main    |     +------------------+               Rb     |
       AC line  |  Power   |                                               |
       or DC    |  Supply  |     +-----------+   Tube- +-----------+ Tube+ |
         o------|          |-----| Regulator |----+----|-|        -|-------+
                +----------+ HV- +-----------+   _|_   +-----------+
                                                  -      HeNe tube
    See the Typical HeNe Laser Tube Structure and Connections for the basic hookup diagram.

    The ballast resistor (Rb) may sound really boring but it is essential to provide stability and limit the current to the value specified for your particular tube. The most common value is 75K but slightly smaller and much larger values may be used to match a particular HeNe tube to a power supply.

    A higher voltage supply and larger ballast resistor will be more stable if there is no built-in regulation. This results from the fact that the voltage drop across the tube is relatively independent of tube current so it subtracts out from the supply voltage. What is left is across the ballast resistor and changes by a proportionally greater amount when the line voltage varies. The smaller the ballast resistor, the more this will affect tube current.

    You really do want the current to be close to that recommended for your tube both to maximize the life of the tube and because maximum optical power output is produced at the optimal current.

    This can be accomplished by adjusting either the input voltage to the power supply or the output current and/or by selecting the value of the ballast resistor. Excessive current is bad for the tube and will actually result in decreased optical output. If the current is too high by a factor of 2 or 3, there will be no output at all. It is not possible to pulse a HeNe laser for higher power (though pulse drive is possible for modulation - see the section: Pulse Type Drive and Modulation of HeNe Tubes. Without any regulation, the value of the ballast resistor is more critical and power line fluctuations will significantly affect tube current though such variations may not matter for many applications.

    Note that since the tube itself provides a relatively constant voltage drop around its nominal current, a small change in line voltage will affect the tube current to a much greater degree than would be expected by the percent of the actual voltage change. The incremental change in tube current will be closer to: delta(I) = delta(V)/Rb where Rb is the ballast resistor. This will be on the order of 10 to 20 times more sensitive to input voltage changes than with a straight resistive load. Thus, a regulator is often desirable.

    The large (can) electrode of the HeNe tube is the cathode (-) end. It may run if connected backwards but the small anode (supposed to be positive but now incorrectly connected as negative) will get very hot since much of the power dissipation is at the negative electrode due to positive ion bombardment. The resulting sputtering may damage the mirror at that end and tube life will likely be shortened.

    Design considerations and a variety of sample circuits are provided in the chapters: HeNe Laser Power Supply Design and Complete HeNe Laser Power Supply Schematics.

    For detailed plans, see the books listed in the section: References on Laser Principles, Technology, Construction, and Applications.

    However, if you have trouble changing a light bulb, never fear - packaged solutions are readily available at attractive prices. You don't need to build your own HeNe power supply unless you have some special requirements, a really strange HeNe tube, or just for the shear joy of creation!

  • Back to HeNe Laser Power Supplies Sub-Table of Contents.

    Using Commercially Available Power Supplies

    Examples of the Use of Commercial Power Supply Bricks

    The simplest approach to powering HeNe tubes is often to purchase surplus or new power supply 'bricks' - fully self contained inverter type (usually) power supplies from one of the suppliers listed in the chapter: Laser and Parts Sources or elsewhere. These are compact, high efficiency, and reliable.

    Cost may be anywhere from $25 to $100 or more depending on power capability, whether new or surplus, and other factors. Such power supplies (and HeNe tubes to go with them) may even be offered on the Internet, possibly at much more attractive prices. Of course, the quality and reliability (of both the equipment as well as the seller) may be unknown.

    Power supplies may also be packaged along with a small HeNe tube, ballast resistor, and wiring as a complete laser optics assembly. These have become available at very attractive prices as products like UPC scanners and laser disc players have switched to over to the use of laser diodes. Since nearly everything but a wall plug is likely included in such a package, this approach will result in a working laser with minimal effort.

    The following sections lists the wiring color codes for some typical 'brick' type HeNe power supplies and describe the required connections and additional circuitry that I used to make complete lasers using two types of HeNe tubes and power supplies that were available from Herbach and Rademan.

    Common Color Coding of Power Supply Bricks

    Here is a summary of some of the wiring color codes I have come across. In all cases, the outputs are either fat red and fat or thin black wires for the ballast resistor/anode and cathode respectively.

    Note: color1/color2 means color1 with a color2 stripe.

    The CDRH (Center For Devices And Radiologic Health of the Food and Drug Administration) delay mentioned with respect to some of these power supplies prevents the beam from coming on for 3 to 5 seconds after power is applied (i.e., should the ON switch be hit accidentally) and may be needed to meet certain regulatory requirements.

    1. Laser Drive, Inc. (110/220 VAC type). Green or green with a yellow stripe is earth ground and connected to cathode lead internally.

      • 110 VAC: Whites to yellow.

      • 220 VAC: White to white, yellow open.

      • TTL high (+3 to +5 VDC) required between yellow/white (+) and yellow/black (-) to enable laser.

    2. Aerotech, Inc. (110/220 VAC type). Red (or possibly other color) wire loop enables CDRH delay (cut to disable). The AC input wire color may depend on model. Green or green with a yellow stripe is earth ground and connected to cathode lead internally.

      • 110 VAC: Brown and blue (or gray) to black.

      • 220 VAC: Blue (or gray) to brown.

    3. Power Technology, Inc. (110/220 VAC type). Violet wire loop enables CDRH delay (cut to disable). Whether AC inputs are white or gray depends on model. Green or green with a yellow stripe is earth ground and connected to cathode lead internally.

      • 110 VAC: Whites (or grays) to yellow.

      • 220 VAC: White to white (or gray to gray), yellow open.

      • TTL enable: White/black - requires +5 on white/red, high to enable, low to inhibit. These color codes and operation vary by model.

    4. General +9 to +12 VDC type. (Note some of these may use a higher voltage like +24 VDC.). Where covered with a metal foil shield, this will also be connected to the cathode. Red, violet, or other color wire loop may be present to enable CDRH delay. Cut to disable delay. Regulated input voltage may be needed - depends on model.

      • Positive voltage input: Red.

      • Ground (common): Black

    5. Laser Drive, Inc. (12 VDC type), models unknown, from barcode scanners.

      • +12 VDC: Red.

      • Ground (common): Black.

      • Enable: Yellow - ground to turn on laser).

      And another:

      • +12 VDC: Red.

      • Ground (common): Black.

      • Enable: While/yellow - high (+4 V) to turn on laser.

      • Black/white, white/black, and blue/black for scan motor.

    6. Scanner head brick, manufacturer unknown (+12 VDC). This may be a sort of Standard. (My sample may have come from a Symbol Technologies LS-6000/6500 scanner but it had no markings.)

      • +12 VDC: Red.

      • Ground (common): Black.

      • Enable: Yellow - turns laser on when connected to a level between +5 and +12 VDC.

      • -12 VDC output: White - to other circuitry, possibly for RS232 driver used by scanner.

    Initial Testing of HeNe Power Supply Bricks

    Two aspects of power supply operation should be checked as soon as possible after powering up your HeNe tube for the first time: polarity and current.

    If the power supply is a name brand unit or was pulled from a device like a bar code scanner, it is safe to assume that the connections (the output in particular) are correctly labeled. However, some power supplies sold to hobbyists (e.g., from places like Herbach & Rademan or HSC Electronic Supply) have been known to have incorrectly marked high voltage connections. (The models I have seen this on are a low voltage inverter type with red and black wire leads for the 9 or 12 V input and spade terminals for the high voltage marked with hand printed or typed stickers for VA and VC - apparently at random.) If your power supply looks like it was put together in someone's basement, this should confirmed to prevent damage to the HeNe tube from reverse polarity.

    At the same time, the HeNe tube current can be checked.

    Put a 1K, 1/4 W resistor in series with the cathode return and measure across it with a voltmeter for the correct polarity and current. The end of the resistor attached to the HeNe tube cathode should be positive and the current will be 1 mA/V of your reading. Also see the section: Making Measurements on HeNe Laser Power Supplies.

    1 mW HeNe Laser Powered by 12 VDC, 1 A Wall Adapter

    H&R part numbers: power supply - TM91LSR1495, HeNe tube - TM94LSR2631.

    This uses a short (140 mm) HeNe tube and power supply running off of 9 VDC. I built these into a case which was from a 1/8" cartridge tape backup system in its former life. In order to obtain regulated 9 VDC, an LM317 IC regulator on a heat sink was added along with a power switch, power-on LED, and the required ballast resistor of 150K. The ballast resistor was determined by monitoring the HeNe tube current and selecting values until the current was correct. The HeNe tube was mounted on standoffs using a pair of nylon cable clamps and aimed through a hole drilled in the plastic case.

              S1      +-------+                       In+ +--------+ HV+
      Vin+ o--/ --+---| LM317 |---+-------+-------+-------|        |-------+
            Power |   +-------+   |       |       |       |        |       |
                  |       | A     /       |       / R3    |        |       / Rb
                  |       |       \ R1    |       \ 500   | 9 VDC  |       \ 150K
                  |       |       / 240   |       /       | HeNe   |       /
                 _|_ C1   |       |     +_|_ C2   |       | Laser  |       | Tube+
                 --- .1   +-------+      --- 10   |       | Power  |     .-|-.
                  |  uF   |             - |  uF __|__ IL1 | Supply |     |   |
                  |       \ R2            |     _\_/_ LED | Brick  |     |   | LT1
                  |       / 1.5K          |       |       |        |     |   |
                  |       \               |       |       |        |     ||_||
                  |       |               |       |   Gnd |        | HV- '-|-'
      Vin- o------+-------+---------------+-------+-------|        |-------+ Tube-
    It may be easier to locate a 12 VAC, 1 A wall transformer since these were commonly used on with older obsolete modems. In this case, add a bridge rectifier and a 5,000 to 10,000 uF, 25 V filter capacitor to the input.

    2 mW HeNe Laser Using AC Line Operated Power Supply Brick

    Power supply: Laser Drive, Inc. model 4009479; The H&R catalog no longer lists this exact power supply but their TM92LSR2278 appears similar.

    HeNe tube: Siemans LGR7631A with attached ballast resistor and Alden HV connector. Its specifications are similar to the 1-2 mW tube listed in the section: Typical HeNe Tube Specifications. The H&R catalog no longer lists this tube but their TM92LSR1954 or any other type requiring up to 1,500 V at 5 mA could be used.

    This was a power supply and HeNe tube combination. This brick can be wired for either 110 VAC or 220 VAC operation and includes an 'enable' input which must be pulled to around +5 V to turn on the tube. Rather than using a separate power supply just for this, I provided a battery holder with 4 AA cells. Even old tired decrepit ones work fine in this application! The only other parts I added were the line cord, power switch, fuse, light, and enable switch.

    The ballast resistor was already built into the tube mounting so that this was truly a 'plug-and-play' assembly.

            S1    _ F1                      white +----------+ HV+
      H o---/ ---- _----+-----------------+-------|          |--------<<---+
           Power  .5 A  |                 | white |          | red         |
                        /                 +-------|          |             / Rb
                     R1 \                         | 110/220  |             \ 75K
                    47K /      S2 Enable  yel/wht | VAC HeNe |             /
                        |    +-----/ -------------|  Laser   |             | Tube+
                       +|+   |                    |  Power   |           .-|-.
                   IL1 |o|   |   | | | |  yel/blk |  Supply  |           |   |
                  NE2H |o|   +---||||||||---------|  Brick   |           |   | LT1
              Power On +|+      +| | | | -        |          |           |   |
                        |       B1 5-6 V          |          |           ||_||
                        |                  yellow |          | black     '-|-'
      N o---------------+-------------------------|          |--------<<---+ Tube-
                                                  +----------+ HV-
                                                  green |
      G o-----------------------------------------------+

  • Back to HeNe Laser Power Supplies Sub-Table of Contents.

    Ballast Resistors, Function, Selecting

    Selecting the Ballast Resistor

    Selecting a ballast resistor that works with a given tube is usually a trivial exercise. So, don't let the length of the following discussions intimidate you! It is quite possible that for the entire future of the universe (until the big crunch and even beyond), you will have no need to use any other value than the usual 75K ohms. However, there are situations where this will not work well or at all when mating a power supply to a HeNe tube or laser head other than the model or size for which it was originally designed.

    The purpose of the ballast resistor is twofold:

    1. Assure stability: It is used to adjust the total resistance in the HeNe tube circuit so that it is comfortably positive. This is needed for stability to balance the negative resistance characteristics of the HeNe tube itself.

    2. The second funtion depends to some extent on the type of power supply:

      • For power supplies that are not current regulated, it serves to limit the HeNe tube current to the optimal and safe value for proper operation with the desired input voltage to the power supply (e.g., 110 VAC).

      • For power supplies that are current regulated, its value can be chosen to place the operating point near the middle of the compliance range of the regulator. This will maximize immunity from input variations and ripple, and may reduce stress on regulator components as well.
    If you are connecting a power supply to a laser head (not just a bare tube), there is almost certainly a ballast resistor inside the head itself. So, an additional full ballast resistor will not be needed unless you find that the original value is insufficient. Also, the power supply may already have some amount of ballast resistance (partial or up to 75K) and the combination may be too large! The discussions below refer to the total of all resistance in series with the HeNe tube (usually only the anode but some laser heads and/or power supplies may have additional resistance in the cathode circuit as well). Do you know where all your series resistance is hiding? :-)

    Note: Although a single resistance value may be specified below for Rb, it is better to construct the actual ballast resistor from several lower value resistors in series to achieve the required voltage and power ratings - see the section: Power Supply Construction Considerations for more info.

    Some of the procedures below may require measurement of voltage or current in the HeNe tube circuit. Before proceeding, for your own safety and the continued good health of your test equipment, see the section: Making Measurements on HeNe Laser Power Supplies.

    Selecting the Ballast Resistor Using Your HeNe Tube

    Using the actual HeNe tube (rather than a dummy load) is generally the easiest, fastest, and most accurate approach for selecting the ballast resistor. Unless the power supply and HeNe tube are grossly mismatched, the chance of damage to either is small. Here are several possibilities: Don't get carried away - running the tube with slightly excessive current won't damage or destroy it immediately (unlike a laser diode). However, using a regulated power supply near or beyond one end or the other of its rated voltage compliance range could result in overheating or overstress of components and it may be damaged or fail completely. Thus, a regulated power supply designed for a .5 mW tube should generally not be used with a 5 mW tube (or vice-versa) for any length of time even if it appears to operate properly!

    Here are a couple of approaches to selecting the ballast resistor for a small HeNe tube and simple unregulated power supply like the one described in the section: Edmund Scientific HeNe Power Supply.

    (From: Steve Nosko (q10706@email.mot.com)).

    The ballast resistor, the actual voltage at the doubler output and the tube running voltage determine the tube current. Look at the tube ratings for the running voltage and tube current - 1,150 V at 4 mA for the tube I was using.

    One can approach the design from two directions:

    Selecting the Ballast Resistor Using a Dummy Load

    This approach will also permit the limits or compliance range of the power supply to be determined without risking the HeNe tube. This may be necessary to select the ballast resistor to match a HeNe tube up with a power supply that is not marked or determine its limits - what sizes of HeNe tubes it will drive. Even when marked, the ratings on a typical HeNe power supply do not tell you how it will behave under varying load conditions. However, first see the section: Selecting the Ballast Resistor as the approaches described there are usually adequate for the vast majority of situations.

    Where the voltage drop across your HeNe tube is known (from its specifications or having been measured), a dummy load will also permit a ballast resistor value to be selected such that the regulator operates near the middle of its compliance range.

    CAUTION: Determining the limits of some power supplies is like testing a new jet fighter; pushing the envelope entails some risk as the power supply can be damaged or destroyed if run outside its design specifications. This is especially true with power supplies that are current regulated. Damage could result if the design is not adequately protected. Many commercial designs are quite robust but don't push your luck! If your power supply is current regulated, see the section: Determining the Compliance Range of a Power supply.

    The discussion below assumes a power supply which does NOT include a current regulator. If yours does, see the section: Determining the Compliance Range of a Power Supply if you want to determine more about your power supply's characteristics.

    Use a voltage divider constructed from a 500K potentiometer (actually four 100K (5 W) resistors and five 20K, (1 W) resistors to get high enough wattage - not an actual pot since such things probably do not exist), 75K (5 W) resistor and 1K (sense) resistor.

    This load will short out (and thus disable) voltage multiplier type starting circuits or fool any auto start circuit into thinking the tube is running.

    These resistor values should work for most tubes rated between 1100 V and 2,400, and currents between 4 mA and 8 mA with typical power supplies but yours may be the exception. Therefore, the resistor values may need to be adjusted if you cannot obtain meaningful results.

    Connect the components as follows:

                       +----+               +--o Measure o--+
                       |    |               |               |
                    R1 v    |      R2       v       Rs      v  I ->
          PS+ o-------/\/\--+-----/\/\------+------/\/\-----+------o PS-
                    500K 25 W    75K 5 W            1K
    Measure the voltage across Rs to determine current. The sensitivity will be 1 V/mA. Alternatively, simply put a 10 or 20 mA current meter across or in place of Rs. (Then, Rs is effectively 0 for the calculations, below.)
    1. Start with R1 at its maximum.

    2. Apply power and measure the current (I) through Rs as described above. Your goal is for this to be equal to the nominal HeNe tube current (Io).

    3. Kill power and confirm that any capacitors have discharged before touching anything!

    4. If the measured current is still too low, reduce R1 and try again (goto step 2).

    5. Calculate or measure the resistance of the entire R1 assembly. The value of the ballast resistor (Rb) should then be:
                           Rb = R1 + 75,000 + Rs - (Vo/Io);
      where Vo is the operating voltage from the tube specifications.

  • Back to HeNe Laser Power Supplies Sub-Table of Contents.

    Determining Characteristics, Fixed Current Considerations

    Determining the Compliance Range of a Power Supply

    Most commercial HeNe power supplies incorporate some type of regulator to maintain the current through the tube constant. It may be fixed at a set value like 5 mA or 6.5 mA or may be variable via a front panel or internal pot, by changing a resistor somewhere, or even by a control voltage (providing modulation, for example).

    By varying the load using the test circuit (above), and taking measurements of both voltage and current, the limits of such a regulated power supply can be explored. Over the useful compliance range of a power supply, output voltage will automatically vary to maintain the output current nearly constant independent of input and load variations and ripple from the power supply itself.

    However, since the HeNe tube introduces some negative resistance into the equation, actual performance may not be quite the same as with the dummy load. Thus, although the power supply may be able to maintain a specified current over a certain range of dummy load resistance values, some portions of this range (usually at either end) may not result in stable behavior with actual HeNe tubes.

    CAUTION: There is always some risk of damage if the specifications are unknown and the designers didn't provide adequate protection. Keep this in mind if you decide to test your power supply in this manner.

    Using the same setup as described in the section: Selecting the Ballast Resistor Using a Dummy Load the compliance range of a power supply can be determined without a HeNe tube:

    CAUTION: For regulated power supplies (inverters in particular), only leave power applied long enough to take your readings if the current is not being maintained at the specified value as this may be stressful to some designs. Where the specifications include a voltage rating (e.g., 2,500 V), this is generally the maximum that should be output continuously. In this case, the upper limit is already known - going beyond this is asking for trouble.

    1. Start with R1 at a modest value and work first one way and then the other so that the limits in both directions can be determined. A suggested initial value for R1 is around 250K. This is likely to be within the compliance limits of typical power supplies for small to medium size (i.e, .5 to 5 mW) HeNe tubes).

    2. Apply power and note both the current (I) through Rs and the total voltage (Vout) from the power supply.

      CAUTION: For power supplies with linear regulators, it is best to use a variable input source and bring up the supply slowly while also monitoring the voltage across the pass transistor(s) to assure it doesn't begin to approach their breakdown voltage. If this happens, R1 is too small (beyond the lower limit of compliance).

      Current can be measured as described above. Vout can be measured if you have a voltmeter with a high enough range or calculated as:

      Vout = (R1 + 75,000 + Rs) * I;

    3. There will be three possibilities (Io is the value of current that the power supply is supposed to be maintaining):

      • If I = Io, the power supply is within regulation.
      • If I < Io, R1 is (or has become) too large.
      • If I > Io, R1 is (or has become) too small.

      Kill power and confirm that any capacitors have discharged before touching anything. Modify R1 as appropriate and goto step 2. You are finished if the entire range over which current is nearly constant has been explored.

    4. The compliance limits are at the points where the current goes out of regulation at each end of the range of values of R1. As noted above, whether these are actually usable limits depends on design and your particular HeNe tube.

    5. The HeNe tube voltages (range of Vo) that will be supported (assuming a 75K Rb) will be within the limits:
                             (R1(u) * Io) > Vo > (R1(l) * Io);
      where R1(u) is the value of R1 at the upper end and R1(l) is the value of R1 at the lower end of the range where the current (I) is constant.

      For a given HeNe tube with specified Vo and Io, the Rb which has the power supply in the center of its compliance range is given by:

                  Rb = (((R1(u) + R1(l)) / 2) + 75,000 + Rs - (Vo / Io);
    As noted above, for regulated supplies, the current will be maintained at nearly a constant value over some range of output voltage. Therefore, if possible, select the ballast resistor (Rb) for your particular HeNe tube such that the supply is operating near the center of its voltage compliance range. Of course, where a large tube is used on a smaller supply or vice-versa, the usable compliance range will be reduced. In fact, there isn't even any sort of guarantee that the 'optimal' Rb value calculated above will even work with your HeNe tube! Life it not simple :-).

    An alternative is to perform the compliance tests with a HeNe tube installed. The problem with this is that starting may make measurements more difficult and result only applies directly to the HeNe tube that is used. However, this IS a more accurate procedure. The series resistance is varied in the same manner as described above except that it will much smaller since much of the voltage drop is taken up by the HeNe tube. The modified setup is shown below:

                +----+                                +--o Measure o--+
                |    |                   LT1          |               |
             R1 v    |     R2   tube+ +-------+ tube- v       Rs      v  I ->
      PS+ o----/\/\--+----/\/\--------|-    |-|-------+------/\/\-----+------o PS-
             200K 15 W   50K 5 W      +-------+               1K
    The procedure is otherwise similar but take care when making measurements since the starting circuit is no longer disabled and use an initial R1 of 25K instead of 250K. As noted above, if you had previously determined the limits of your power supply using the dummy load, with the HeNe tube installed, the discharge may become unstable before these limits are reached. Details are left as an exercise for the student :-).

    Fixed Current Power Supply Considerations

    Many inverter type power supplies are completely potted (i.e., 'bricks') and will have a fixed current specification (e.g., 5 mA, 6.5 mA) and a maximum voltage specification (e.g., 1,750 V, 2,500 V). They will generally work with a 75K ballast resistor and attempt to maintain their rated current through a variety of HeNe tubes (various sizes and power ratings). Unless there is an adjustment accessible through an access hole, there is no easy way change this current set-point.

    CAUTION: These power supplies may attempt to maintain the set HeNe tube current even when the operating voltage is beyond their specifications as might be the case with a high power HeNe tube on a small power supply or a ballast resistor that is too large. The power supply may overheat or be overstressed under these conditions and fail without warning. See the section: Typical HeNe Tube Specifications to determine if your HeNe tube is likely to be within the capabilities of your power supply brick.

    Where the current rating of your HeNe tube does not match the power supply (and it cannot be adjusted), your options are limited. There is no easy way to increase or decrease current with external circuitry. However, if it is within +/- 10 percent of the optimum current, don't worry about it. If the current is within +/- 20 percent of optimum, it still isn't the end of the world (though a different power supply would be the best solution):

    Where the power supply components are accessible (it is not a potted brick), modifications may be possible but how easy this is depends on the design. See the sections starting: Current Regulators and the chapter: Complete HeNe Laser Power Supply Schematics for examples.

  • Back to HeNe Laser Power Supplies Sub-Table of Contents.

    Starting Problems, Measurements, Testing, Repair

    Starting Problems and Hard-to-Start Tubes

    Some tubes seem to practically start on their own. Other won't perform even when you stand on your head, hold your breath, and provide the proper chants and sacrifices :-). Or, your power supply operating voltage, ballast resistor, and other factors may need modification.

    There are two types of problems. You need to determine if the discharge is being initiated at all. If the starting voltage is adequate, there will be momentary flashes that may be extremely short and weak and only visible in a darkened room. As you approach a stable condition, these will become brighter and longer. At the hairy edge, you may get a nice flashing laser.

    WARNING: If your HeNe tube doesn't start after a reasonable length of time (like a minute), don't leave the power supply on overnight in a futile attempt to get it going. Starting is a stressful time for power supply components, especially some wide compliance designs, and may result in total failure. If the laser is flashing, this may be ultimately bad for the tube as well. Step back and try to determine what is wrong.

    1. Tube does not fire at all - there is no evidence of any beam, even for an instant. While tube manufacturers generally specify a starting voltage of 7 to 10 KV (or higher), typical tubes will fire with 3 to 5 times their operating voltage. Thus, a tube that runs on 1,700 VDC will probably start on 5,400 to 8,500 VDC.

      No action at all generally means the starting voltage is inadequate for the tube, there are other circuit problems, or the tube is bad. Tubes with longer and narrower bores (capillaries) will generally require greater starting voltage.

      • There may be too much leakage in the anode circuit preventing the buildup of adequate starting voltage. The problem may be in the power supply itself or in the wiring to the HeNe tube. Corona discharge or arcing can result from inadequate insulation or component spacing as well as sharp points in the wiring or connections. Dirt and grime may also reduce the insulation resistance. A sizzling and/or ticking sound along with the aroma of ozone are indications of this sort of HV breakdown. Highly humid conditions may make the situation worse. For pulse (trigger) type starters, there may be too much capacitance as well.

        In the case of an enclosed laser head with an Alden connector, HV cable, and internal (potted) ballast resistor, there may be a breakdown in one of these components and it may only show up when starting voltage is applied (not with an ohmmeter). Here are two ways of testing for this situation:

        • Disconnect the anode of the HeNe tube and substitute your own ballast resistor and wiring.

        • Remove the negative connection from the ballast resistor assembly entirely - float it so the starting voltage cannot arc to anything. Connect the negative directly to the cathode of the HeNe tube or laser head case as appropriate.

        If the tube now starts, one of the original components was faulty (most likely the potted ballast resistor assembly if the negative connection runs through it) and this will need to be replaced.

      • If you need to increase the input to start or obtain any sort of response but then must back it off substantially to reduce the tube current to the proper value, low starting voltage or one of the other related problems is indicated.

        Assuming the power supply and wiring check out and the tube is good, the only solution is to boost the starting voltage or use a different type of starting circuit (inverter instead of voltage multiplier, for example).

      • You may have an extremely hard-to-start tube. Whether this is just normal for your particular tube, is due to it being old or unused for a long time, it is just tired with many hours under its belt, or some other problem, the result is that the specified starting voltage does not have any effect.

        One test for this is to try the tube with reverse polarity on its input. Connect the positive output of the power supply to the ballast resistor (don't omit this!) and then to the cathode (can electrode) end of the HeNe tube. Connect the negative of the power supply to the anode of the tube. You are only doing this for testing! Do not be tempted to leave the tube wired this way permanently should it actually start.

        Based on tubes I have tested, the starting voltage is much lower with the anode and cathode connections interchanged. However, the voltage drop across the tube when running with reverse polarity is much higher than with correct polarity. Thus, the tube may not run within the normal operating voltage range of your power supply even if the discharge is initiated - it may just pulse.

        Nonetheless, even if it just pulses, at least you know the tube is not totally dead. If the tube is otherwise undamaged, there should also be an indication of (at least weak) laser output from the business end of the tube. Perhaps, all you need is a power supply with higher starting and/or operating voltage. An inverter type starter using a flyback transformer appears to be particularly good for hard-to-start tubes. Unfortunately, I do not know of any reliable way of determining the likelihood of success without actually trying it.

        I have one 5 mW HeNe tube that requires (depending on its mood) as much as 15 to 20 KV to start (it should be less than about 10 KV). However, once started, it runs with a normal operating voltage of about 1,800 VDC.

        WARNING: Do not let the HeNe tube run for any length of time with reverse polarity as damage may occur due to heating and sputtering at the anode end of the tube.

    2. Tube flashes momentarily but does not 'catch'. What happens is that the discharge is initiated but the voltage drops too much at the tube anode and the discharge goes out.

      To produce a stable discharge, the following must be satisfied:

      • The sum of the effective resistance of the power supply and the ballast resistor and the (incremental) negative resistance of the tube (dV/dI at the operating point) must be greater than 0.

      • The voltage across the tube must be above the minimum for the tube at the operating current.

      • The current must be above the minimum for the tube/power supply/ballast resistor combination.

      These factors are not independent. Since the negative resistance and sustaining voltage of the tube are not normally specified and depend on current, some amount of trial and error may be required to achieve consistent stable operation but in most cases it really is very easy.

      Cycling behavior can be due to several factors:

      • Poor power supply voltage regulation or excessive ripple. Until the tube fires, there is essentially no load on the supply resulting in much greater voltage than under load. Except for a high compliance type of design where this is needed to produce the starting voltage, minimizing this difference will improve stability and reduce the voltage needed for stable operation.

        If the transformer or inverter drops too much under load, the tube voltage may fall below the minimum for the tube/ballast combination as soon as it starts. This cycle will repeat continuously or it occasionally may catch.

        Use a higher voltage and larger ballast resistor, and/or increase the uF value of the main filter capacitor (and/or the one in the DC supply to an inverter type supply as well if it isn't regulated).

        Minimum capacitor values for less than 5 percent voltage ripple (typical voltage and current requirements):

        • Line operated supplies: .5 to 1 uF (2000 V, 5 mA).
        • Inverter output: .005 to .01 uF (10 kHz, 1,800 V, 4 mA).
        • Unregulated inverter input: 15,000 to 20,000 uF (12 V, 1 A).

        Actual ripple in the current to the tube may be several times greater than this since it depends on the change in voltage with respect to the total effective resistance of the PS+tube+ballast resistor combination). However, the resulting ripple in the optical output power will be 2 to 10 times lower than the ripple in the current depending on operating point. The lowest will occur around the tube's optimal current specification.

      • Ballast resistor too large for the operating voltage. The operating current falls too low resulting in increased (magnitude of) negative resistance. Once the total system resistance goes negative, the discharge becomes unstable and goes out. The result is a flashing laser like a neon bulb relaxation oscillator.

        For an unregulated power supply, increase the operating voltage and/or decrease the ballast resistance.

        For a regulated power supply, decrease the ballast resistance so that the voltage for the desired operating current falls within its compliance range.

      • Too much stray capacitance and/or inductance in anode circuit. The system is behaving like a relaxation oscillator as the capacitance charges and then discharges through the tube. The wiring inductance causes the current from the main supply to lag too far behind the starting current and the discharge goes out.

        Shorten the wiring - minimize the distance between the power supply and ballast resistor, the ballast resistor, and tube anode, and don't use long runs of high voltage coax (which may have higher capacitance). Increasing the energy of the starting circuit slightly may help as well.

      • For laser heads in particular, the additional capacitance resulting from the metal case may increase the minimum stable tube current by up to 1 mA or more - and thus require changes in the power supply and/or ballast resistor. So, if you tested the HeNe tube and power supply on your workbench but the enclosed system is unstable, this may be the reason.

      • Power supply polarity is reversed. The voltage drop across a HeNe tube operated with the cathode and anode interchanged is higher than under normal conditions. However, required starting voltage is much lower. The result is likely to be a pulsing laser. Double check your wiring and terminal connections. I have also seen commercial power supplies mislabeled! See the section Making Measurements on HeNe Laser Power supplies if you need to actually test for reverse polarity.
    Also see the section: Testing a HeNe Power Supply and Power supply Construction Considerations.

    Making Measurements on HeNe Laser Power Supplies

    Voltage and/or current measurements on a HeNe power supply may be needed to characterize its performance bounds, to troubleshoot or identify a defective unit, or to monitor conditions during operation or with different HeNe tubes, particularly where there are user adjustments (i.e., a Variac used as an input voltage source). Note: I DO NOT recommend the use of typical DMMs (Digital MultiMeters) for measurements in high voltage power supplies of this type. Many of these are more susceptible to damage from voltage or current spikes than analog VOMs (Volt Ohm Meters) or simple dedicated moving coil (D'Arsenval) panel meters.

    It is difficult to measure the output voltage of a HeNe laser power supply with a multimeter even if it is supposedly within your meter's range. Connecting the meter across the tube while it is on will likely extinguish the arc due to the capacitance of the probe inducing a momentary dip in the voltage. Even if the tube remains lit or restarts, there may actually be oscillation resulting in an erroneous reading. Leaving a multimeter connected during starting may prevent the tube from firing due to its additional capacitance and reduced resistance. And, the meter may be damaged due to arcing from any high voltage starting pulses. A VOM or DMM with a suitable high voltage probe can be left connected to a wide compliance type power supply and possibly on one using a voltage multiplier (though it may load it excessively) but should probably not be used with a pulse (trigger) type starting circuit.

    WARNING: Make sure whatever you have is well insulated and/or grounded where appropriate. Those high voltages can bite! Use proper high voltage cable (rated 20 KV or more - non-resistive type automotive ignition cable and TV or monitor CRT high voltage cable works well) and insulated coverings on the terminals (several layers of plastic electrical tape or Plexiglass barriers). You may be working in the dark (light-wise, at least) or with somewhat subdued lighting, so it makes sense to prevent accidental contact as much as possible. This is especially important with power supplies that may be overkill (no pun intended...really!) as is often the case with home-built equipment.

    Testing a HeNe Power Supply

    Failure of a HeNe laser to lase could be due to a bad tube, bad power supply, bad connections, or you forgot to plug it into the wall socket!

    If there is a continuous glow from the inside of the HeNe tube, the power supply is probably working properly though the current could be incorrect. However, this would result in reduced output power or excessive heating - but not a totally dead laser unless the current was more than 2 or 3 times too high. (In this extreme case, if the tube is good, there would likely be at least a flash of laser light from its output at power-on and/or power-off as the tube current passes through the normal operating range).

    Where there is no sign of a discharge - even momentarily - the problem could be with the tube, wiring, or power supply.

    In either case, it would probably be a good idea to see the section: How Can I Tell if My Tube is Good before getting into heavy troubleshooting of the power supply.

    To determine if the power supply is working requires testing it for both the starting and operating voltage (and proper current if it includes an internal regulator). Of course, the easiest approach is to substitute a known working HeNe tube, but this is not always a viable option.

    Before proceeding, for your own safety and the continued health of your test equipment, see the section: Making Measurements on HeNe Laser Power Supplies.

    If the power supply has a current adjustment, see if this behaves as expected. A control that does nothing could indicate that the load resistor is sized incorrectly and the compliance range of the power supply is being exceeded (low or high).

    In all cases, a defective regulator or control circuit could result in these faults as well.

    Also see the sections: Selecting the Ballast Resistor and Starting Problems and Hard-to-Start Tubes.

    HeNe Power Supply Repair

    Various types of HeNe power supplies may turn up in electronics surplus stores, junk yards, or buried under piles of other stuff in the back of your company or university lab storeroom.

    WARNING: Just because these are often compact units doesn't mean they cannot be lethal. See the document: Safety Guidelines for High Voltage and/or Line Powered Equipment before working on any type of equipment which uses line voltage or produces high voltage.

    CAUTION: Make sure you discharge both the output of the power supply AND the HeNe tube itself with a high value power resistor (e.g., 100K, 2 W) before touching anything. Else, you may be in for a surprise!

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