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Registered Member #1806
Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136
I had a 20kV doorknob capacitor connected across an ignition coil and was varying the spark gap. I adjusted the gap to be rather large.. to the point of no longer arcing (big mistake) After that, I just used a normal spark plug and was getting good results until the hot arc in the plug faded and ended up looking like it did before I connected the capacitor. Right now, the doorknob cap has no apparent effect on the spark. The spark is the same with and without the cap attached.
My best guess is that I damaged the capacitor from over voltage when the gap was too large. The iggy will easily crank 13mm/.5" sparks. This is well more than the 20kV the cap is rated at. What is strange is that the cap seems to have failed *open circuit*, *long* (minutes) after the obvious abuse.
If it had suffered dielectric breakdown, that seems like it would have caused an immediate apparent "short" circuit (internal arcing), but what's with the apparent delayed failure?
I have one more cap left that I can use, but until I am confident about why this one failed, I do not want to risk my last cap. Blowing an easy to get $2 igbt is one thing, but these comparatively hard to get $16 caps are not expendable.
It is a surplus Russian part, 4700pF, 20kV, about 1" thick, and 2" in diameter, plastic cup with epoxy end face, with a bolt stud on the cup side and a smooth shaft on the epoxy side.
Registered Member #834
Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644
13 mm between points is not 20 kV. More probably you damaged the capacitor by excessive current in the discharges, that may have destroyed its internal connections.
Registered Member #3610
Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506
If it's dead, take it apart and find out, but other posters are probably correct. The quality of capacitors varies widely so there's no way to know for sure what it will withstand but generally you want to stay at least 50% below the rated voltage of a capacitor if you want it to last a long time. This is especially true if you're using it in ways it was never designed for, like pulse discharge with a standard capacitor.
Registered Member #1806
Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136
I'm a bit reluctant to open it up; who knows what those silly Russians put in it.... it's probably chock full of radium or something equally annoying. (no, not really, but still... who knows?) Not that this is an electrolytic, but I never would have guessed that "they" would have put toxic oil (pcb) in capacitors. I'm sure there's an "In Soviet Russia..." joke in there somewhere.
It's kind of a bummer because I thought I had seen others using a doorknob cap in this way with no apparent problems.
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I think I fried my doorknob capacitor
