Re: ceramic capacitors

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Mike <megavolts61@xxxxxxxxx>

I have a bank of 20  ceramic transmitter caps that do nicely for TC 
service....at least for a small coil.   They are not high capcitance, 
so the total of them us just a little over 8 nF@ 30kV, but they run 
happily all day if I wanted them to from a NST.
Mike



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


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