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Registered Member #311
Joined: Sun Mar 12 2006, 08:28PM
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Posts: 253
The main problem I found with film resistors is that they couldn't handle the surge currents and went open-circuit. If you look at the theoretical peak power dissipation when charging an uncharged cap, it is somewhat large (100s of watts ISTR) , and if you're unlucky, the film doesn't have sufficient thermal mass to absorb the power. I'm assuming this was what was happenning ( Philips VR68 1W metal-glaze), however it could also have been arcing over the gaps in the spiral eating the film away (not sure if the VR68s use a spiral construction, but conventional film R's usually do).
From my experience, 1 or 2 watt solid-carbon resistors are as good as you can get for a moderate sized marx. The ones I used (NOS Allen-Bradley) have taken a great deal of abuse with no significant value change. I would advise laquer or conformal coating to reduce risc of arcing over - this is probably better than heatshrink as theer is less risk of trapped air to ionise. Heatshrink as well (after coating is dry) won't hurt though. If you can find some soft silicone sleefing that's a snug fit, this is even better as it will expel air and have decent thickness to resist pinholing.
Wirewounds (if you can get high enough values) may suffer from turn-to-turn arcing, and at the high values involved, the wire will be thin and an arc may well break the wire.
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Marx Generator
