GE Dielektrol Capacitor Popping Sound While Charging

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workinprogres
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Hey everyone, so I just bought a used GE Dielektrol capacitor rated at 40,000 volts and 1.85 microfarads, and while I'm charging it up there is a popping sound that appears to be coming from inside the capacitor. It only occurs at voltages over about 2000 volts and only while the capacitor is charging. As soon as I disconnect one of the charging wires it goes silent. I'm using a neon sign transformer driven by a variac and run through a bridge rectifier consisting of 12 kv diodes so there is a ground potential from the charging wires but I don't know if this would have anything to do with it. The capacitor holds a charge and has no problems discharging. Also the capacitance has also decreased since I bought it from 1.88 to about 1.54 but I think that may be due to temperature because it's been in my near freezing garage for a day but before it was kept at room temperature (about 70 degrees). I'm a little scared the the dielectric might be failing but I don't really know. Any advice would be helpful, Thanks in advance!
 
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I think you're right to be concerned - could something be delaminating within the cap? I'd contact GE directly and try to find someone knowledgeable on that design. Don't forget that some HV caps, even when you think they are fully shorted, will 'self charge' to some intermediate voltage. (I had a nasty shock one time...). Make sure both electrodes are shorted to each other and Earth ground to safe them.
 
I sure hope the cap isn't falling apart inside. I'm probably going to call G.E. tomorrow but in the mean time I've brought the cap back inside to see if the capacitance will go back up. Hopefully it will or otherwise I've got even more problems. I'm wondering if maybe the dielectric inside is changing volume with the temperature and moving the conductive plates around or letting holes form or maybe even not absorbing into the dielectric material. And thanks for the tip on HV caps charging themselves with a ground potential! I never would have thought that that might happen but now that I think about it the Earth would make quite the capacitor :)
 
After making a few calls and some experimentation it turns out I couldn't just use the NST's current limiting and needed a charging resistor.
 
Good to hear. Make sure you have not damaged the cap - I'd recommend shielding it with
something to stop shrapnel if it decides to dump all its energy with an internal short...