The circuit is a switching power supply, producing about 500watts of power at about 1500 volts at a frequency of 200kHz. At the heart of the circuit is IC1 - a UC3825 high speed PWM chip (see Unitrode site). The frequency of operation of this chip can be varied from about 150kHz to 220kHz. The width of the output pulse and thus the power can be altered from 0 to 90%. The pwm chip produces two identical but antiphase output square waves. The chip is unstable as there is no current feedback into the ramp - see the data sheet for an explanation of this.
The outputs from IC1 are buffered by IC2 and 3, a pair of TC4422CPA FET driver chips (by TelCom) which switch the output FETs Q1-6. The 10ohm resistors R20-25 and Schottky diodes D4-7 damp any unwanted oscillations caused by the fast switching.
Current limiting is provided by monitoring the voltage across the sense resistor (R14-19?), at 1V the output is switched off. This sense resistor must be a low inductance type - not a wire wound. The current limiting is precariously balanced between being oversensitive and under sensitive.
FETs will switch on and off in about 30ns - the extremely fast switching of large currents coupled with the stray induction in the wiring and in the transformers would cause very high voltages pulses which are dealt with by the RC snubber networks formed by R26-29 and C25-28. At switch on the current rise is limited by the stray induction. At switch off, the capacitors C26,28 can effectively conduct through diodes D10,11. These capacitors discharge through R27,29 when the FETS are switched on. The FETs Q1-Q6 drive step up transformer Tr2 in push pull.
Output transformer Tr2 Primary is 11 turns of 2 * 1mm side by side, with the primary winding repeated on top twice, So that each primary consists of 3 * 1mm in \\. (2 identical primaries for push pull.) Secondary is a single 0.9mm wire. There is room on the bobbin for 3 layers giving a ratio of about 7.5 to 1. The higher ratio's give the bigger sparks even though, because of current limiting, only about 50% of the cycle can be used.
A larger coil can be wound around a bigger former (e.g. a 5 litre polythene bucket sold in Boots for home brewing). Starting 1cm from the bottom and finishing 1cm from the top the bucket can be close wound with 0.4mm wire. A larger toroid then needs to be constructed. This bigger coil has a lower impedance so the circuit has to be modified to be driven by an interrupter (switched on by closing S2) - giving larger peak currents in short bursts. The resonant frequency is around 100kHz, R1 needs to be increased and a larger output transformer is necessary. This modification is capable of giving 14" arcs. The interrupter is built around the CA3524 and is based on a design in the Harris data sheet. With S1 open the timing capacitance is 0.1uf (C14) giving a frequency of 140Hz to 2000Hz. With S1 closed the timing capacitance is 0.6uf (C14 + C30) giving a frequency of 25Hz to 500Hz. The mark space ratio of the interrupter can be altered from 0 to 90% using VR3.
C19 to C24 should be as close to chips IC2 and IC3 as possible. The data sheets for the ICs used strongly suggest the use of a ground plane type PCB, though keeping the lead lengths short by using a compact PCB design may be sufficient to avoid problems.
The operation at 12v only is checked first, by removing the high voltage fuse. The output from each of the buffer chips is then examined with an oscilloscope. Adjustment of VR1 should modify the pulse width from 0 to 45%, and adjustment of VR2 should vary the frequency from 150kHz to 225 kHz. VR1 and VR2 can be calibrated and marked at this stage. If an oscilloscope is not available, measure the voltage at each output. Most DMMs include an internal capacitor to smooth out the high frequency and give an average value which should vary from about 0 to 5V.
Set the pulse width to zero, switch off, insert the high voltage fuse, place a large resistor in the high voltage path, i.e. a 100watt light bulb, and switch on. Check that everything is working as it should, increase the pulse width, and adjust the frequency. Two L.E.D.s connected in anti-parallel can be inserted between the output of the transformer and the coil to tune for resonance. At resonance, the L.E.D.s will glow brightly.
Remove the resistor and the Leds, put a wire onto the toroid - protruding a couple inches over the side, very slowly increase the pulse width until the current meter begins to rise. Now adjust the frequency. A purple spark at the discharge electrode should be seen. Adjust the tuning to maximise this and continue to increase the power until the current is at maximum. Maximum current is 3.2A to keep within the transformers rating.
R1,10 2k2 R2 10k R3,7 4k7 R4 68k R5 200R R6,12,13 100R R8,9 2k7 R14-19 0R05 Low Inductance (Rsense) R20-25 10R R26-29 330R R11 1k VR1 10k Pulse Width Adjust VR2 1k Frequency Adjust VR3 10k Interrupter Mark Space Ratio Adjust VR4 100k Interrupter Adjust
| C1-10 | 300uF 385v electrolytic. |
| C11 | 1500pF |
| C12 | 10uF electrolytic |
| C13-22 | 100nF |
| C23-24 | 10uF Tantalum |
| C25-29 | 1nF |
| C30 | 500nF |
| C31 | 470pF |
| Q1-6 | IRF740 |
| IC1 | UC3825 |
| IC2,3 | TC4422CPA |
| IC4 | CA3524 |
| D1 | |
| D2,3 | 1N4004 |
| D4-7 | SB140 |
| D8,9 | |
| D10,11 | |
| D12-15 | 400V 25A bridge (needs to cope with high switch on currents as capacitors are charged) |
| Tr1 | 240V to 50-0-50 V 500VA Toroidal |
| Tr2 | ETD49 with a 3C85 core (Farnell Electronics UK) |
| F1 | 4A slow blow fuse is used on the mains side. |
| F2 | 5A quick blow |
| S1 | SPST for Interrupter Speed |
| S2 | SPST for Interrupt On\Off |
| M1 | 5A meter. |
| IS1 | Inrush current limiter. |
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