I've built a 2-stage Cockroft-Walton voltage multiplier with 10nF ceramic capacitors rated at 20kV each and pairs of 12kV diodes attached in series and hooked it up to an NST with a peak output of 10kV. It worked just fine with one small problem-the capacitors began flashing when the input was around 3kV (peak).

So I immersed the multiplier circuit in transformer oil and tested it again. There was a steady arc starting from an input of about 600V all the way till I increased the input to around 5kV peak. At this point, the arc suddenly vanished and I noticed bubbles coming from the ceramic part of my first capacitor (the one connected to the supply). So I brought the input down to zero, waited for a while and gradually increased it again.

This time,however, the arc wasn't steady and increasing the input only increased the frequency with which it struck. I've since replaced the capacitor, but with the same result as before (bubbles coming off the capacitor underside, followed by a non-steady, Marx-generator style arc).

I noticed there wasn't any physical damage on the capacitors once I took them out of the oil, so I thought the capacitor leads were causing Corona discharge and decomposing the oil. I wrapped the leads of the first capacitor in insulating tape and the bubbles began popping up at the the ground capacitor. I have one last spare capacitor left and I don't want to ruin that one as well. I really want to know what's causing these bubbles and sabotaging my output. Any help would be greatly appreciated. Thanks :)


The bubbles are due to oil decomposing. Transformers have a safety switch that detects this gas and then switches the transformer off line. https://en.wikipedia.org/wiki/Buchholz_relay


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