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Registered Member #1656
Joined: Wed Aug 27 2008, 03:28PM
Location: Pittsburgh PA
Posts: 53
I was looking around my work shop and came across a cap. Its a big block one. .01mfd 100kvdc. My question is, would this one be good for a TC? I cant seem to find any info about it.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
I doubt it would withstand the continuous pulses. It might, but chances are that it wont. Better to sell it and hope you get enough money for a suitable Tesla coil capacitor.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Arcstarter is probably right. Those big metal cans, even if pulse-rated, aren't often good for the rep rate you want. On top of that, I'm never too excited about the thought of some arcing fault boiling the oil inside one of those things and making it blow like a grenade. One inflated like a balloon in a friend's coil, and I'd never run that fast in my life!
Also, consider the capacitance: .01mfd (.01uF) is smallish at typical coiling voltages and break rates, and yet that looks like one big heavy brick of a cap. Even if suitably rated, it would be a bit silly to have a cap so much bigger and heavier than your power transformer!
Registered Member #1656
Joined: Wed Aug 27 2008, 03:28PM
Location: Pittsburgh PA
Posts: 53
J. Aaron Holmes wrote ...
Arcstarter is probably right. Those big metal cans, even if pulse-rated, aren't often good for the rep rate you want. On top of that, I'm never too excited about the thought of some arcing fault boiling the oil inside one of those things and making it blow like a grenade. One inflated like a balloon in a friend's coil, and I'd never run that fast in my life!
Also, consider the capacitance: .01mfd (.01uF) is smallish at typical coiling voltages and break rates, and yet that looks like one big heavy brick of a cap. Even if suitably rated, it would be a bit silly to have a cap so much bigger and heavier than your power transformer!
Cheers, Aaron, N7OE
Well the cap is rather light. maybe 2-3 lbs, with a plastic outside. The Xformer i would be hooking it up to is a 12kv. 30ma. But if there is any risk of it blowing up on me i might just set it aside for sale or if any one wants it they can have it.
Registered Member #477
Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546
Tigeris wrote ...
Well the cap is rather light. maybe 2-3 lbs, with a plastic outside. The Xformer i would be hooking it up to is a 12kv. 30ma. But if there is any risk of it blowing up on me i might just set it aside for sale or if any one wants it they can have it.
Thanks for the insight and help ^_^
Ah, ok. Looked metal.
Well, 12/30 is about right for that capacitance, but unless I could find a datasheet for it, I'd assume it to be unsuitable. Alternatively, if you post some additional photos (e.g., showing the terminals and all), perhaps somebody will recognize the make, etc.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Where a capacitor is given only a DC rating, as yours is "100 kV DC" (as far as I can make out through flux-spattered spectacles and the beer fog leaking across the image ) then it would be usual to assign to it an AC rating not exceeding 70% - and better 60% for safety's sake - of the DC value, unless otherwise stated on the can. And this would be at line frequencies 50/60 Hz too.
However, given that many Tesla folk are able to get by with capacitors made from polythene sheet and kitchen foil, I'd be inclined to give it a go if it's all you have to hand, and you only have a modest plans.
Make yourself an electronic stethoscope and stick it on the outside of the cannister if you fear the processes going on inside!
If it is an oil-filled unit of the type where you can hear the oil sloshing about you can always drill and tap a hole in the top and put a reversed tyre valve in it if you are worried about venting over-pressure.
I'm not a Tesla man, so it may be there are things I have overlooked, but I'm sure someone will step in to correct me if needs be.
Good luck with it. Most of us would never get anywhere if we weren't prepared to try out 'unsuitable' components now and again.
If I was in your situation, I'd switch the device on from a safe distance - and wearing safety goggles if you are really concerned - and then go out for a quiet beer for a few hours.
If the fuse hasn't failed, and the case isn't significantly warm when you come back full of good cheer, it will probably be alright. Being alive at all involves some risk!
Registered Member #1656
Joined: Wed Aug 27 2008, 03:28PM
Location: Pittsburgh PA
Posts: 53
J. Aaron Holmes wrote ...
Tigeris wrote ...
Well the cap is rather light. maybe 2-3 lbs, with a plastic outside. The Xformer i would be hooking it up to is a 12kv. 30ma. But if there is any risk of it blowing up on me i might just set it aside for sale or if any one wants it they can have it.
Thanks for the insight and help ^_^
Ah, ok. Looked metal.
Well, 12/30 is about right for that capacitance, but unless I could find a datasheet for it, I'd assume it to be unsuitable. Alternatively, if you post some additional photos (e.g., showing the terminals and all), perhaps somebody will recognize the make, etc.
Cheers, Aaron, N7OE
I hope these help. The last one, its hard to see, but there is oil in it. its kinda yellow in color.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Tigeris -
You have one of the FCI caps ("Film Capacitors Inc") that were distributed as "used" parts by Fair Radio Sales quite a few years ago. Both Gary Lau (moderator on the TCML) and I used these caps in spark-gap coils with good results. Although I was never able to confirm this, they were described as having polyester dielectric, and were supposedly originally designed for DC filtering use. They are oil-filled, have 1/4-20 UNC threaded studs on each end, and the cases are heavy-wall UHMW polyethylene.
I used mine in a 6" X 30" static spark gap coil powered by a 15-60 NST. Best performance was around 40" - 44" inches to a grounded rod, if I recall (this was a long time ago). Gary Lau has additional information on this capacitor, and the coil it was used in, at
In short, capacitors identical to yours have worked well in Tesla coils, even though they may use a somewhat "lossy" dielectric. The capacitor is physically large, and has a large thermal mass, which may help it survive runs of moderate length. I'd shut the system down and allow tha capacitor to cool thoroughly when the case feels just warm to the the touch.
I've still got mine somewhere, but I haven't seen it for a few years .......
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Hi Tiger,
looking at your photos I'd use it myself were it all I had. As the Zappster pointed out, it might be a bit lossy, but it looks built to last, and has such a generous voltage over-head I'd be surprised if it packed up unless you are planning a giant TDU-type extravaganza, in which case the price of a new, more suitable capacitor would only be a small part of your total outlay.
Good luck with it! A robust, high voltage capacitor in the hand is worth two in cloud-cookoo land.
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Cap question.
) then it would be usual to assign to it an AC rating not exceeding 70% - and better 60% for safety's sake - of the DC value, unless otherwise stated on the can. And this would be at line frequencies 50/60 Hz too. 
