Re: ceramic capacitors

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Ed Phillips <evp@xxxxxxxxxxx>

Hi Matthew:

There are many different kinds of ceramic dielectrics, some more
suitable for Tesla coil caps than others.  Several people have attempted
to use series/parallel strings of inexpensive ceramic disc capacitors,
and these have all failed miserably.  Ceramic doorknob capacitors come
in several dielectric varieties, and some have been used successfully,
and others run only for a short while before heating up.  Even if the
capacitor does not fail outright, if it gets warm during use, it will
change its capacitance and shift its resonant frequency, so it won't
work well.

I'd recommend forgetting about disk capacitors.  If you happen upon a
fantastic deal on some doorknob caps, they're worth a try, but if they'd
cost you any significant fraction of what a good C-D MMC would cost,
save yourself the heartbreak and skip the ceramic caps.

Regards, Gary Lau
MA, USA"

   I agree with most of this but with an exception.  Even the fairly 
lossy doorknob capacitors intended for HV DC filtering have a very 
low internal resistance and, in my experience, work quite well as 
long as cautions about capacitance drift and overheating are 
observed.  Nice and compact and not a bad place to START.  I have a 
small coil running about 700 watts input and using four 20 kV 0.003 
mfd doorknob filter capacitors in series.  Works fine for about a 
minute and then the tuning starts to drift enough to notice.  When I 
first used them I blew up a couple (actually they shorted in spite 
of  the almost 1/2" thick ceramic) while playing with the tuning as 
it drifted.  At the time I had an adjustable roller coil in series 
with the primary so I could tune while the coil was running.  Still 
use that setup as I don't have long run times but eventually plan to 
build some MMC banks when I get a place where I can run coils for a 
longer time at higher power.  They're in a small attic now and I 
don't like strikes to the ceiling!

Ed