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HV caps (fwd)
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From: Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
Sent: Monday, May 25, 1998 10:45 AM
To: Tesla List
Subject: Re: HV caps (fwd)
Tesla List wrote:
>
> ---------- Forwarded message ----------
> Date: Sun, 24 May 1998 08:58:02 -0700
> From: Terry Perdue <terryp-at-halcyon-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: HV caps
>
> I picked up three HV caps at a flea market some time back for a few bucks
> each, and wonder if they could be used in a small Tesla coil using a NST.
> The seller told me they are from an xray machine. They are made of
> phenolic, measure about a foot tall by 4" by 1.75", and are oil filled,
> with a screw terminal on top and another near the bottom. They are made by
> Plastic Capacitors, Inc, and are rated at .01uF, 80KVDC. Comments?
>
> Thanks, Terry
Terry,
Nice find on the HV caps! If these are from a x-ray machine, they're
most likely Mylar/polyester dielectric, and are usable in a Tesla coil
for short run-times. The problem with Mylar is that it's quite lossy in
RF aplications, and the energy that goes into heating the dielectric
subtracts from that going into streamers. Because of the dielectric
loss, Mylar caps have a tendancy internally overheat. And, since the
dielectric losses increase with temperature, once you develop a "hot
spot", the capacitor will rapidly fail, sometimes exposively. Because of
their construction, these hot spots typically occur deep inside the
individual capacitor rolls long before any heat can transfer to the
outside case of the cap.
Your caps should work as long as you keep your run-times brief, allow a
cool-down period between runs, and carefully monitor temperature rise by
periodically feeling the outer case. You may also want to put them into
a container to catch any debris and oil "just in case".
Good luck!
-- Bert --