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HV capacitor leakage? Answered

I have just built a high voltage glass capacitor for my Tesla coil at a 0.03uF 50KV rating and I have found that the leakage is quite high, the stored energy is almost completely gone in 15 seconds does anyone know how to resolve this?

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LargeMouthBassBest Answer (author)2011-06-05

How are you determining that the energy has leaked out? Do you wait a certain length of time, and then connect a multimeter or other measurement device? Common DMMs have an input impedance on the order of 10 Meg. That would make the time constant of your cap and meter = 0.03*10^-6 Farad * 10*10^6 ohms = 0.3 seconds. The cap will discharge more than 95% of its energy within about 4 times constants, so 1.2 seconds after you apply your meter, the cap would be almost completely discharged. Depending on how fast your meter samples, you may never see much of a voltage reading within that time.

Are you doing this test with the cap charged to a high voltage, or something much lower? If you are doing it at a high voltage, then I assume that you are using some kind of additional high voltage probe with your meter or other measuring instrument. Many common HV probes add about 1000 Meg in series with your meter, because they use a resistor divider with 1000Meg and 1 Meg resistances. This arrangement would result in a much higher time constant, but if there are any other leakage paths in parallel with it that would of course affect the discharge rate.

I don't know if any of this applies to the test you ran, but it is something to be aware of. I still stand by my earlier statement that at any rate, it is not necessary for your cap to remain charged for a length of time much longer than the power frequency for your coil to operate well.

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I simply tested it by waiting a certain time then discharging it with a wire.
After waiting a bit the discharge is a small spark unlike the big high voltage arc it should be.

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steveastrouk (author)2011-06-05

What am I doing wrong here ? The DBV of glass is a lot lower than FR4 PCB laminate, and therefore 1.6mm FR4 can stand 50kV, the resistivity is comparable to glass, so the self-leakage looks OK too.

Steve

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steveastrouk (author)2011-06-05

Test the cap in a dark room. Charge it fully, and look for corona discharges from edges. Round off edges

A. is right, clean it, but you'll have to use solvents to really move contamination.

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iceng (author)2011-06-04

It's only o.03  uF.  LMB makes good sense.
How carefully did you clean your finger prints
   from the edge of your ( old word for that item ) condenser glass ?
How humid is your workshop ?
How thick is your glass ?
How sharp are your foil straight edges ?
Do you live next to a nuclear facility ?

Just some impacts on discharge rate.

A

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The MadScientist (author)iceng2011-06-05

I wasn't that careful at removing my prints but I got rid of most of them.
The humidity was low.
The glass is 5mm (0.197inch) thick.
I am a bit unsure about the sharpness of the foils straight edges all I can say is that they were straight.
No I don't live next to a nuclear facility.

(0.03uF is about average for a Tesla coil tank capacitor)

What if I did live next to a nuclear facility?

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frollard (author)iceng2011-06-04

lol@ possible ionizing radiation causing the leak. Stranger things have happened I suppose. :)

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iceng (author)frollard2011-06-04

Radon Gas drops daughter products at corners.

A

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LargeMouthBass (author)2011-06-04

It shouldn't make any real difference to the operation of the tesla coil, as the capacitor doesn't need to store its energy for a great deal of time. It will be charged and discharged within each half cycle of the power line frequency. For 60Hz, that would be about 8 milliseconds.

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