2002-11-03
First Proof-of-Concept Unit
My first Marx generator implementation used resistive charge current limiting. Inductive limiting is quite popular but I didn't feel like winding that many chokes. Inductive loading when done properly is probably superior, it does not burn up energy in the resistors while charging. I just lashed up each cap with a ladder of 1M 1W resistors. The flash-over voltage of the resistors and the unpotted leads was a problem, they were not designed for multi-kV service, but I chose to just ignoring it. The device was charged using a few kilovolt HeNe laser supply. Up to 7kV was acceptable, but much more than that and it arced over now and then.
Gap spacing and charging current are important. The gaps should all trigger at once. In practice the first gap fires and the over-voltage avalanches up the stack. The UV light and RF from each gap helps the next too. Having a gap further up the stack firing first is bad. Make all the gaps above the first wider, and control the trigger voltage from the first gap.
If your charging current is too high the first gap may light continuously. This was initially quite a problem for me, the laser supply has a ignition pulse which was breaking down the gap and when I had only 2M of total resistance in the loop the arc would not extinguish in the first gap. Blowing on it helped, but I just increased the charging limit resistor to 10M and widened the gap slightly. I'd like to try triggering the first gap rather than just letting it break down. That would make the output much less variable.
The dark shot was taken just before the bench PSU died a horrible and expensive death. I'll have to fix it and harden it before I can try this again. Yes, I am pulling the spark from the 'wrong side', not using the last stage. Mainly because I was concerned that I was in the firing line in that direction. I am considering building a dedicated and hardened charging supply. There is a fairly cheap 50VA 12V AC halogen down-light supply available at Jaycar currently. That with a bridge, some caps, and a 555 controlled MOSFET chopper should run a flyback or car coil nicely. Now all I need is some HV diodes or a flyback with an inbuilt one.
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Parent article: Marx Generator.