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    Complete SS Laser Power Supply Schematics

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    Introduction to Complete SS Laser Power Supply Schematics

    This chapter contains circuit diagrams for several power supplies for pulsed solid state lasers. These include units suitable for driving the popular Hughes ruby and YAG rangefinder laser assemblies as well, one using the flash from a disposable pocket camera, and a high energy flashlamp power supply for that 8 inch long surplus NOVA laser rod you have been saving. :)

    The pulse forming network is what determines the performance of a pulsed solid state laser. Thus, there is a great deal of flexibility in the design of the capacitor charger and trigger circuits. Systems designed for other applications can often be adapted for solid state laser power supplies. See the chapter: SS Laser Power Supplies for more information. And the schematics in this chapter can be easily modified for larger, smaller, or different types of solid state lasers.

    WARNING: All of these systems are potentially lethal - some just more lethal than others. Hey, but when you're dead, it probably doesn't matter how well done you are. Before even thinking about building or going near one of these systems, make sure you have thoroughly read, understand, and follow the laser and electrical safety guideline provided elsewhere in this document!



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    Pulse Forming Network 1

    Description of PFN1

    PFN1 (manufacturer and model unidentified) is a combination of a 36 uF, 950 V energy storage capacitor, .03 mH inductor, automatic bleeder circuit, and various connectors and other stuff. The capacitor is marked with its rating but the inductor is not and its value was determined by performing a 'ring test' using both a separate high-Q 1 uF capacitor and then the one in the PFN.

    The original application for PFN1 was most likely to be used with the SSY1 laser head (see the section: A Small Nd:YAG Laser - SSY1). The maximum useful energy into the flashlamp is around 14 to 15 J when charged to just over 900 V. Pulse Forming Network 1 shows the assembly with major components labeled. This unit is/was available from Meredith Instruments.

    When used without modification, the combination of the 36 uF capacitor and 0.03 mH inductor will result in a 50 to 100 us pulse duration (dependent on other circuit parameters, probably closer to 100 us in practice). This is quite well matched to a Nd:YAG rod. With a well designed cavity, 15 J should be enough to threshold a 50 mm x 4 mm Nd:YAG rod (which is what it apparently was intended to pump) and considerably more than enough for a 25 mm rod.

    Note that the capacitor in PFN1 is a very high quality non-electrolytic type. It may be a Polyester film capacitor with an ESR (Equivalent Series Resistance) of around 0.02 Ohm (compared to almost 1 Ohm for a combination of electrolytic photoflash caps with the same uF and V ratings). The extremely low ESR is essential to achieve the required short pulse duration at reasonable efficiency (i.e., maximizing energy transfer to the flashlamp) or at all.

    (From Chris Chagaris (pyro@grolen.com).)

    This power supply does sound right on the money for a small military Nd:YAG laser, as used in rangefinding, target designators, or illuminators. Typically these lasers are designed to run off of batteries or on-board DC power lines. Typical pulse repetition rates for these systems are on the order of 10 pps and power to charge the cap is usually provided by a DC-to-AC converter, step-up transformer, HV rectifier, PFN, and a parallel trigger circuit. Another much used military PSU operates on the principle of a flyback DC-AC converter.

    Your calculations as to the pulse width of the lamp sound just about right. With Nd:YAG's fluorescent lifetime being 230 us, this pulse width would make perfect sense for maximum energy transfer.

    PFN1 Schematics

    The diagram is available in PDF format:

    The automatic bleeder circuit is something that really should be kept intact even if the useless wiring and that pot are removed. The normally closed contacts of the vacuum relay connect a 10K ohm resistor across the energy storage capacitor whenever the relay's coil loses power. The capacitor will be fully discharged in a couple of seconds. This is an important safety feature!



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    Sam's AC Line Power Supply for a SSY1 (SG-SP1)

    Description of SG-SP1

    This is a basic, but complete power supply for driving the SSY1 or simlar small YAG laser head (see the sections: A Small Nd:YAG Laser - SSY1 and Mini YAG Laser using SSY1 Optics and SG-SP1. (For a more polished version of this system, see the section: Sam's Small Adjustable Power Supply for Solid State Pulsed Lasers (SG-SP4).

    I used components from Pulse Forming Network 1 and added a line powered capacitor charger and push button operated trigger circuit. A better than 1 pps repetition rate is possible (and considerably more if R1 were reduced in size). Current, the components are sort of strewn all over my workbench - which I definitely don't recommend for safety reasons. Eventually, it will be built into a proper case.

    The capacitor charger consists of a 700 VRMS output power transformer, a relic of the vacuum tube age though brand new in its original box. :) I used the low voltage outputs (the 5 V and 6.3 V filament windings, not shown on the schematics) anti-phase in series with the line to slightly drop the the output to about 650 VRMS resulting in just over 900 VDC maximum after rectification and filtering by the main energy storage capacitor (C1). A 300K ohm bleeder resistance discharges C1 in about 30 seconds. The original PFN1 had a relay activated fast discharge circuit using a 10K resistor. Whenever it was de-energized, the resistor was placed directly across C1 discharging it in less than a second. I highly recommend using this if possible.

    Unless you have a similar transformer to that Stancor laying around, I'd suggest trying to use something more modern. I doubt you could even buy it today and even if you could, the cost would be ridiculous. One option might be a 230 VAC transformer and a tripler made from 3 diodes and 2 electrolytic caps (around 900 V peak). Or, a small dual primary, dual secondary isolation transformer wired to produce 345 VAC (three windings in series) followed by a voltage doubler.

    The trigger circuit consists of a tap off of the bleeder resistance at the 1/3rd voltage point (so about 300 V when the power supply is operating at 900 V output). This charges a .05 uF capacitor which is dumped into the trigger transformer (type unknown - looks like a large photoflash variety) when the push button switch (S2) is depressed. It's possible that one of the tiny units from a disposable pocket camera won't work reliably as drawn. However, you can adjust the size of the trigger cap and/or trigger cap charging voltage to optimize performance for the trigger transformer of your choice. The use of a piezo barbeque igniter is also a good solution if you don't need something fired electrically. :)

    I will be installing SG-SP1 in a handy aluminum case with a connector to permit any small flashlamp pumped laser head to be easily attached. It may be used as described above or as a general purpose capacitor charger for higher energy PFNs.

    WARNING: The AC line input and the energy stored in the PFN can be lethal. An interlock should be included (not shown) to remove input power should the case be opened.

    SG-SP1 Schematics

    The diagram is available in PDF format:

    Don's Comments on the SSY1 Flashlamp

    The following was prompted by my request for opinions on the possibility of increasing the flash energy to the flashlamp in the SSY1 to boost its output pulse energy. Based on Don's comments, I think I won't push my luck!

    (Recall that the SSY1 flashlamp is about 5 mm OD, 3 mm ID, with an arc length of 35 mm. Using PFN1 in SG-SP1, the maximum input energy from SG-SP1 is about 15 J in about 100 us.)

    (From: Don Klipstein (Don@Misty.com).)

    Explosion energy for 100 us is about 1.1 Joules times arc length in mm times bore diameter in mm for quartz. EG&G recommends 30 percent of this for good repeated use of quartz (mere thousands of flashes) so I recommend 15 percent of the explosion energy if it is glass.

    For a length of 35 mm and a bore of 3 mm, explosion energy would then be 115.5 Joules. 15 percent of this is not much more than the 15 joules you're using now.

    The capacitance and inductance and voltage (36 uF, 30 uH, 900 volts) sounds about right for this flashtube and flash duration.

    If you want to push things, for one thing I would increase the flash duration accordingly. Explosion energy increases with the square root of flash duration. For flashes this short, you get that increase with glass as well as quartz. (This rule holds with quartz to a flash duration up to 10 milliseconds; glass levels off much sooner!)

    If you want to push that flashtube, just increase the capacitance and the inductance accordingly. You can probably get away with doubling the capacitance, but that is pushing things.

    If you ruin that tube, then I recommend getting a real flashtube from EG&G Electro-Optics Division, which will sell to small time consultants and probably even obvious hobbyists and the like. But they cost about $300 or something like that. But they're quartz and xenon pressure is known (450 Torr unless otherwise specified) and you can get just about any length and diameter almost off-the-shelf. Their catalog items are actually semi-custom parts.

    Their catalog has 2/4 and 3/5 mm. (bore/OD) tubes in 1, 2, and 3 inch arc lengths. They also have 4 mm bore 6 mm OD tubes in 2, 3, and 4 inch arc lengths. And all sorts of bigger tubes. One of these should do a good job of increasing the danger of your YAG laser. You probably want the ones designated "low power air cooled".



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    Sam's Small Adjustable Power Supply for Solid State Pulsed Lasers (SG-SP4)

    Description of SG-SP4

    After getting tired of SG-SP1 looking somewhat like a random pile of junk, I built it into a nice case and gave it a dedicated Variac and voltmeter, and added a thyristor triggering circuit with pushbutton as well as external (logic level) activation. Both the spiff and safety factors are thus greatly improved. :)

    SG-SP4 still includes the basic PFN1 and trigger output and can drive SSY1 and similar laser heads directly. However, it can also be used as a general purpose capacitor charger for laser heads requiring greater energy input (at up to 900 V).

    SG-SP4 Schematics

    The diagram is available in both PDF and GIF format:



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    Sam's Inverter Power Supply for SSY1 (SG-SI1)

    Description of SG-SI1

    This power supply runs off of 12 VDC and is based on a high frequency inverter driven by a 555 timer. A voltage comparator monitors the voltage on the energy storage capacitor and disables the 555 timer by forcing reset, cycling on and off to maintain full charge. The inverter transformer is similar to the one used in the helium-neon laser power supply described in the section: HeNe Laser Power Supply from HeNe Laser Pointer (IC-HI3) but with slightly thicker wire and correspondingly larger core to handle the potentially higher charging power.

    Before you lament the difficulty in winding the inverter transformer, the design found in the section: Simple Inverter Capacitor Charger Based on HSS1 requires a transformer with a grand total of 32 turns of wire! That circuit can be substituted here for the lazy at heart. :)

    Like SG-SP1, it uses PFN1 as the pulse forming network. A bleeder relay (K1) has been added since the values of the resistors in the original voltage divider circuit (R2 to R4) have been increased with a corresponding increase in discharge time to a value too long for safety. K1 will discharge the main energy storage capacitor in about 5 seconds when power is removed.

    As shown, SG-SI1 may be powered by a 12 V battery pack and mates directly with the SSY1 laser head. The voltage to which the energy storage capacitor gets charged may be adjusted between near 0 and around 900 V. SSY1 will require a minimum of about 650 V to reach threshold. However, SG-SI1 can easily be adapted for a lower or higher input voltage and/or for other small solid state pulsed lasers, photoflash units, or xenon strobes.

    The turns ratio of T1 has been selected so that the voltage on C1 tops off at 900 to 950 V with a 12 VDC input. This is safe for the PFN1 even if the voltage comparator circuit fails to disable the 555 oscillator circuit. As additional protection (especially if your particular PFN has a lower voltage rating for C1), one option would be a string of high voltage zeners (total value selected for your PFN) and current limiting resistor attached to C1 feeding a transistor or SCR to kill input power and set a fault indicator should the normal voltage limiting fail to function for any reason. Increasing the turns ratio of T1 modestly might reduce charge time but then it is even more critical to provide some redundant protection so that the voltage is prevented from climbing above the max voltage rating of C1.

    WARNING: Although SG-SI1 is powered by a low voltage source, the energy stored in the PFN can still be lethal. An interlock should be included (not shown) to remove input power should the case be opened.

    Note: The inverter portion of SG-SI1 has not been tested.

    SG-SI1 Schematics

    The diagram is available in PDF format:



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    Sam's High Power Solid State Laser Power Supply (SG-SP2)

    Description and Schematic of SG-SP2

    (Note: This is the same design as the "Super High power (Laser Pump) Strobe Circuit" found in Sam's Strobe FAQ.)

    Here is a schematic for a high power xenon strobe unit suitable for pumping a medium to large ruby, Nd:YAG, or Nd:Glass laser rod. (However, this would still be considered small compared to those at the Laser Fusion Facilities of the Lawrence Livermore National Laboratory.)

    WARNING: This is only an example. We take no responsibility for either the accuracy or functional correctness of the schematic or any consequences should you attempt to construct this circuit either in its original form or modified in any way.

                                                            L1
         Power         D1   R5                            ::::::
             +-----+--|>|--/\/\--+-----+------------------^^^^^^----------------+
          ||(      |  5kV   5K   |     |                25 uH (est)             |
          ||(      | .5A   25W +_|_    /                                        |
    H --+ ||(      |            ___ C1 \ R1  C1-C4: Energy storage              |
         )||( 600  |           - |     /     capacitor bank,          Flashlamp |
     115 )||( VRMS |             |     |     3600uF, 450V (each!)           FL1 |
     VAC )||( 200  |             +-----+                                       +|
     2A  )||( mA   |             |     |     R1-R4: Voltage drop               _|_
         )||(      |           +_|_    /     equalizing resistors,            | | |
    N --+ ||(      |            ___ C2 \ R2  200K, 1W                Trigger ||   |
          ||(      |           - |     /                              30kV   ||   |
          ||(      |             |     |   R7               + C5 -        +--||   |
             +-------------------+-----+--/\/\--+-------+-----||---+   ::(   ||   |
          T1       |             |     |  1.8M  |       |   3.9uF  |   ::(   || _ |
                   |           +_|_    /    1W  |       |   450V   |   ::(    |_|_|
                   |            ___ C3 \ R3     |       |          |   ::(      |
                   |           - |     /        \       |          +-+ ::(     -|
                   |             |     |        / R8  __|__ SCR1      )::(      |
                   |             +-----+        \ 1M  _\_/_ C107D     )::(      |
                   |             |     |        /     / |   400V      )::(      |
                   |           +_|_    /        |    |  |   4A        )::(      |
                   |            ___ C4 \ R4     |    |  |          +-+    +-+   |
                   |           - |     /        |    |  |          |   T2   |   |
                   |  D2    R6   |     |        |    |  |          |        |   |
                   +--|<|--/\/\--+-----+--------+-------+-----+----+--------+---+
                      5kV   5K                  R9   |   R10  |
                     .5A   25W         Fire o--/\/\--+--/\/\--+
                                       (+5)     100      100
    
    

    SG-SP2 Operation

    1. Power transformer, T1, in conjunction with D1, D2, and C1-C4, provides 1.7 kV DC. The power supply doubler capacitors are also used as the energy storage capacitors. Resistors, R1-R4, equalize the voltage drops across the series capacitors to compensate for slight differences in leakage resistance. R5 and R6 limit inrush current and charge rate.

    2. The trigger capacitor, C5, charges through T2 from the voltage divider formed by R7 and R8.

    3. Ready light and capacitor bank voltage monitoring circuits are not shown.

    4. Applying a 5 V signal to the Fire input turns on SCR1 dumping C5 into the primary of the trigger transformer, T2. This generates a 30 kV pulse which ionizes the xenon gas in the flashlamp, FL1.

    5. The energy storage capacitor bank discharges through L1 and FL1.

    SG-SP2 Notes

    1. WARNING: If you thought line operated equipment was dangerous, this is much much worse. The power transformer output is enough to kill. Once doubled and stored in the capacitor bank, it is LETHAL. The total energy storage is about 1300 W-s (this is not a typo!). Based on one estimate, this is enough energy to KILL 20 adult humans simultaneously with the power supply unplugged from the AC line - and still have some juice left over. TAKE EXTREME CARE!

    2. Fuse, power switch, power-on light, and all other absolutely essential safety interlocks and indicators are not shown. R1-R4 do act as bleeder resistors and will discharge the capacitor bank to safe levels in about 10 MINUTES. However, don't depend on these. Resistors can fail. Use the capacitor discharge tool and indicator.

    3. The power transformer from a tube type (old) TV set would probably be suitable for T1. Microwave oven high voltage rectifiers may be used for D1 and D2. A high power xenon tube like this requires a 30+ kV trigger pulse. Those little tiny trigger transformers will NOT work.

    4. The energy storage capacitors, C1 to C4, must be rated for photoflash rapid discharge operation or else they won't survive and/or won't be able to deliver a fast enough output pulse. In fact, for Nd:YAG (or similar solid state laser mediums with short fluorescence lifetimes, special (non-electrolytic) types will be needed. But, you may need to take out a second mortgage to finance them. :) Electrolytic types will work for ruby pumping with it's 3 ms fluorescence lifetime.

    5. High power strobes require special flashlamps - anything from a pocket camera or electronic flash will explode into a mass of molten bits of glass and metal. This design is suitable for driving 2 of the largest of the EG&G 1300 series flashlamps, the FXQ-1305-6 and -9. These have arc lengths of 6 and 9 inches respectively! See the section: EG&G 1300 Series Linear Flashlamp Specifications and Links for more info.

      Even a properly specified flashlamp may explode and must be operated behind protective shielding or as in the case of a typical laser, fully enclosed in the cavity reflector. Flashlamp cooling must be adequate for desired cycle time.

    6. L1 helps to shape the discharge current pulse. For some high power strobe designs, a series inductor is essential to optimize power output and prevent damage to the flashlamp due to excessively high current and negative voltage (undershoot resulting in reverse current). A damping factor of .8 is generally recommended. The 25 uH value is just an estimate - L1 must be calculated for each combination of energy storage capacitor value, voltage, and the impedance characteristics of the specific flashlamp to be used.

    7. If you are serious about constructing a high energy strobe system (and your life and accident insurance is fully paid), consider some advanced reading first. The flashlamp manufacturer's datasheets and application notes will prove essential. See the section: Other Sources of Information on Solid State Lasers and the sections starting with Xenon Flashlamps.

    8. Maximum flash energy is about 1300 W-s. For a typical flash duration of 250 us, this is an equivalent power input to the flashlamp of 5.2 MW! Adjust component values for the desired application.

    9. DO NOT even think about staring at the flashlamp when fired. The peak light output is equivalent to at least 500,000, 100 W light bulbs! Even when averaged over the 1/40th of a second typical response of the human eye, this is still more than 5,000, 100 W light bulbs. (Note: this estimate takes into account the increased luminous efficiency of xenon flashlamps compared to incandescent light bulbs.)

    10. Make sure all optical components - especially the flashlamp - are cleaned with isopropyl alcohol and a lint free cloth to remove all traces of contaminants.

    11.         |                         |           |
           ---+--- are connected;    ---|--- and ------- are NOT connected.
              |                         |           |
      



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    Sam's High Energy AC Line Power Supply for SSY1 (SG-SP3)

    Description of SG-SP3

    This power supply is designed to push the pedal to the metal (as they say) with respect to SSY1. It was prompted by the truly spectacular performance described in the section: Shawn's High Energy Experiments with the SSY1 Laser Head.

    CAUTION: Although this should be operating the flashlamp within spec when run at low duty cycle, it is still what's known as "pushing the envelope". Use at your own discretion.

    The capacitor charger consists of a 650 VRMS output power transformer driven by a Variac to provide up to about 450 VDC from a full wave rectifier. The transformer I used is extremely overrated current-wise. A smaller one could be used since the cycle time is so long and the average power requirement is only about 10 W.

    A couple of alternatives are also shown in the schematic:

    An inverter based capacitor charger similar to Sam's Inverter Power Supply for SSY1 (SG-SI1) could also be used if modified appropriately for the desired cutoff voltage higher power operation.

    The capacitor bank is a pair of 1,800 uF, 450 V, electrolytics (never used but quite old Mallary "computer grade" units in my implementation). CAUTION: Some means should be provided to prevent the voltage from exceeding the 450 V ratings of C1 and C2. While electrolytic capacitors generally have some safety margin, it is best to obey the rules. Depending on your actual line voltage, the charging schemes described above may be operating close to the edge when set for maximum energy. :)

    The trigger circuit is basically the same one used in SG-SP1 (except for part numbers) but since the maximum voltage is lower, it operates from 2/3rds of the capacitor voltage instead of 1/3rd.

    D3 and D4 are reverse protection diodes for the capacitor bank (C1,C2) and flashlamp (FL1), respectively. They need to have a voltage rating of at least 500 V and a peak current rating of several hundred amps. However, the continuous current and power ratings can be small since the duty cycle is very low - or zero. I don't know if either of these diodes would ever conduct under normal conditions as the circuit is not likely to be underdamped. I would suggest a 600 to 1,000 V plastic cased diode with a 200 A or greater IFSM rating. They don't need to be fast recovery or other special types. Or, leave them out and measure the residual voltage on C1/C2 and FL1 after a trial shot - if they are both positive, the diodes aren't needed.

    The panel meter is calibrated to read 500 V full scale. Of course, it would be really cool to have it display energy in Joules. This would make a nice little project in itself. An A/D to monitor the capacitor voltage along with a PIC or other single chip microprocessor, or maybe just an EPROM or Flash based lookup table could do the conversion. Or, even simpler, an analog multiplier configured to square and scale the voltage. :)

    A relay connects a high power bleeder resistor to the capacitor bus when power is removed or switched off, the case is opened, or the neighborhood experiences a blackout due to the use of this device) and the capacitor bank is thus automatically discharged. However, even with the relatively low ohm resistor, reaching a safe level still takes 10 seconds or so. A 'Live' indicator lamp (IL2) shows when the voltage on the caps is above about 100 V.

    Cycle time is limited by the average power rating of the flashlamp.

    WARNING: The AC line input and the energy stored in the PFN can be lethal - especially the energy stored in the capacitor bank! In fact, conservatively, there are enough Joules there at maximum voltage (360 J) to kill dead-dead, six large adult humans simultaneously with the power off! Take extreme care and don't assume anything!

    SG-SP3 Schematics

    The diagram is available in PDF format: