Re: Home-brew Capacitors

To: tesla-at-pupman-dot-com
Subject: Re: Home-brew Capacitors
From: Sam Laur <slaur-at-sekunda.pp.utu.fi> (by way of Terry Fritz <twf-at-verinet-dot-com>)
Date: Tue, 15 Dec 1998 17:07:21 -0700
Approved: twf-at-verinet-dot-com

> 1) It's been a while since posts have been made about proposed home made
> dry (no oil) caps made by connecting a bunch of them in series to minimize
> corona and breakdown in the individual sections.  Has anyone achieved good,
> long life results yet?  If so, many of us would appreciate the particulars.

Yes - I'm running a 23 nF, 8.5kVAC cap made from individual (39 in total)
100 nF, 660 VAC impregnated-paper capacitors. No sign of breaking, but then
at my power levels (140W) one shouldn't expect anything like that happening
either. I have three chains of 13 caps in series in each chain, and 
10 Mohm equalizing / bleeder resistors in parallel with each cap. 
See the results on my page (http://users.utu.fi/slaur/). 

I'm not completely happy with these caps, as they warm up a bit during the
longer runs (maybe 10 C warmer than ambient) but that's mainly because of the
paper dielectric; proper polypropylene caps would stay cool. And that's what
I plan to do next, but who knows when. I believe Terry Fritz experimented
with polypropylene film capacitors with success, but haven't heard of any
results of testing in a higher-power Tesla Coil, for longer times, 
from him yet. 

There is only one major advantage in using these paper capacitors - if one
of them decides to short itself internally, the cap isn't dead, it just loses
some capacitance. During a short, the metallization vaporizes away from the
paper and so there remains no conductive track between layers. This self-
healing is a property required for any good line filter capacitors.

And besides, they were surplus, so I got them cheap :-)

Sam Laur