Re: cap

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Ed Phillips <evp-at-pacbell-dot-net> 

Tesla list wrote:
 >
 > Original poster: Jim Lux <jimlux-at-earthlink-dot-net>
 >
 > At 08:08 AM 11/21/2003 -0700, you wrote:
 > >Original poster: "RIAA/MPAA's Worst Nightmare" <mike.marcum-at-zoomtown-dot-com>
 > >Yup, they work for a few min, and then the leads just fall off from the
 > >heat. Question: why don't ceramic "doorknobs" fall apart as well? I've seen
 > >those used in SGTC's for whatever reason even though they waste power and
 > >are kind of expensive since you need 10 or so in parallel (around $20 for a
 > >measly .002 uF -at- 40kv).
 >
 > They use doorknobs because sometimes you can get them cheap.  My small
 > spark gap TC uses 3 2700pF 40kV doorknobs because they were free (out of a
 > piece of gear that I bought surplus for $10 for the other parts).  They're
 > easy to use, and have lasted so far.
 >
 > As to why the doorknobs don't fall apart... they're better built than the
 > radial leaded disk capacitors.  Sure, they're not great, but they're still
 > better.

	I assume this discussion is related to use of DC filter or bypass
capacitors, not the much more expensive and larger RF-rated ones.  The
reason they work pretty well is that their series resistance is very
low.  I've been using 3 nominal 4000 uufd 30 kV capacitors for years
with quite satisfactory results.  Coil runs off a 12 kV, 60 ma
transformer with around 400 watts input.  I popped one of them in the
beginning by failing to notice that they do warm up pretty fast (I limit
my runs to less than a minute) and one of them punctured.  That was the
one and only failure.  I cut the bad capacitor apart and found that the
ceramic had actually cracked and that there was a burned line between
the two metallized surfaces.

	You have to be prepared for the fact that these things have a lousy
temperature coefficient of capacitance and tuning may change with
capacitor temperature.  So what?  They work.

Ed