Re: red capacitors

To: tesla-at-pupman-dot-com
Subject: Re: red capacitors
From: Bert Hickman <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Sun, 13 Jun 1999 18:23:32 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Tesla List wrote:
> 
> Original Poster: "Kevin Wahila" <knw2-at-lehigh.edu>
> 
> I have a box of red, 2500pF, 15kV, 1" diameter capacitors.they are of the
> type D70X5R25M15KV and the T.C. is X5R.  well that doesn't mean a whole lot
> to me, i am not familiar with them, but maybe someone on the list is.
> would the voltage rating on them be a DC rating?  if they are DC and i
> wanted to employ them in my coil would parallel strings of 3 in series be
> suitable to take care of and AC supply voltage of 9kV?
> 
> thanks
> kevin wahila

Hi Kevin!

Do they have threaded connections on the ends, is the body a molded red
plastic compound, and are they about 1" or so in length? If so, then
these are commonly called "doorknob" caps - high dielectric constant
ceramic capacitors used for DC applications. You could use them in a
series/parallel configuration to get the desired capacitance and
breakdown voltage. However, most doorknob-type caps tend to rapidly heat
up in use since the high-k dielectric is also fairly lossy at RF
frequencies, and the capacitance also tends to change with increasing
temperature. This may cause your coil to go out of tune as you're
running it. Also, the thin metallization (typically a thin layer of
evaporated silver onto the ceramic ends) tends to "disappear" with time
under the heavy tank currents seen during Tesla Coil operation, and
eventually these caps may become unusable. They are, however, a quick
and dirty way to get capacitors for small coil use. They are DC-rated,
and using 2-3 in series should provide sufficient voltage standoff
capability for a 9 kV NST source. 

However, if the caps are longer, typically 3-4" for 15 kV, and have a
glass body, then they are most likely HV Mylar caps (sometimes called
"Glassmikes"). I really wouldn't recommend using them in your system if
they are of this type, since they also tend to overheat quite rapidly
and may explode in Tesla Coil service. 

Safe coilin' to you!

-- Bert --