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Registered Member #62119
Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136
The HV DC power supply for my Marx generator consists of one of those Chinese ZVS drivers connected to a flyback transformer. The low voltage DC power source for the ZVS driver was originally an old laptop 18 volt switching power supply. After about 50 shots, the switching supply was destroyed through high voltage kickbacks from the Marx generator. In order to isolate the Marx generator from the wall power, I replaced the switching power supply with two 12 volt lead-acid batteries in series yielding 24 volts. This works well but now I have a new problem.
In trying to achieve the longest possible spark from the Marx generator, even though the main gap misfires, the small individual gaps do fire. Unfortunately this high voltage has to go somewhere when the main gap does not fire. It kicks back into the flyback transformer which then appears to induce a large voltage spike back into the ZVS driver via the center-tapped coil wound around the flyback core which in turn is connected to the ZVS driver output. After a few of these instances, the ZVS driver was blown, presumably shorting the MOSFETs. The MOSFETs only have a 200 volt rating. I now have replacement MOSFETs, but before I do this again, I had an idea for a protection circuit.
My idea is to connect 3 suitably-rated (<200 volts) varistors across the output of the ZVS driver. One varistor would be connected across the outer leads of the center-tapped coil wound around the flyback core. The 2nd varistor wold be connected across the outer flyback core lead and the center tap. The 3rd varistor would be connected across the other outer flyback core lead and the center tap. I know that varistors have a limited life but considering the very low firing rate of the Marx generator, It seems as if the varistors should last a long time before needing replacement. I have a fuse on the battery power supply so if one of the varistors fails to s short circuit condition, the fuse will blow presumably protecting the MOSFETs of the ZVS driver.
Does this seems like a viable HV protection circuit for the ZVS driver?
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Marx generator power supply protection
