RE: Cap setup suggestions needed

From: 	Bert Hickman[SMTP:bert.hickman-at-aquila-dot-com]
Sent: 	Friday, June 20, 1997 1:55 AM
To: 	Tesla List
Subject: 	Re: Cap setup suggestions needed

Tesla List wrote:
> From:   Brandon Doughan[SMTP:fiveht-at-well-dot-com]
> Sent:   Wednesday, June 18, 1997 9:48 AM
> To:     tesla-at-pupman-dot-com
> Subject:        Cap setup suggestions needed
> Hey you HV freaks-
>         I was lucky enough to get ahold of 20 or so doorknob caps rated
> at 30KV and 3600 pF (.0036MFD).  I'm wondering about a few things:
>         Firstly, they are paired and incased in rubber blocks with three HV
> leads, coming of each end and in the middle. [they are from a HV
> multiplier device].  On one end of the cap pairs (I've scrapped the
> rubber off a few) there is a resistor.  I don't know the value.  My
> question is, if I use several pair in a coil powered by a 15KV NST, how
> will these resistors affect my capacitance?  It would be nice to keep
> the added insulation of the rubber (and not have to scrap all of it
> off!), but if the resistance is negatively affecting my coil output, as I
> suspect, then I'm gonna have to do it.
>         Second question-  I know you want to have your caps rated 3
> times your primary voltage, but I want to get away with just have my 30KV
> caps with my 15 KV NST.  Anyone have experience with doorknob cap
> failure?  Think I can do it?
>         Last question-  I cracked one of these caps open and found that
> it consists of just two plates each no more than 3 sq. ins. and about an
> inch of that strange ceramic dielectric.  How do these get .0036MFD??
> This must be because of the K value of that stuff.  This amazes me.
> Thanks for your help_  Brandon
> PS-  Visit the High Voltage Camp at Burningman '97!


Congratulations on finding the caps - these have become somewhat scarce
(at least at reasonable prices). Answers to your questions follow:

1. I couldn't quite tell from your post if the resistors were connected
in series (as current limiters) or in parallel (as bleeder resistors to
discharge the caps). If connected in series, they need to be removed,
since they will worsen the Q in your primary circuit, and if relatively
high value, may prevent thye primary from even "ringing" when the gap
fires. BTW, while doorknob caps will work as tank caps for Tesla Coils
for a while, they are quite lossy, and the metalization on the ends
tends to "disappear" under extended usage since it isn't thick enough to
handle the repetitive high currents seen when the gap fires. If you only
expect to use them on very small coils you may have fairly good success.

If the resistors are across the caps, then they are high resistance
bleeder resistors, and could be left in place... however, I'd remove
them as well since they'll only contribute to further losses in the
primary circuit. In either connection, their presence will not change
the effective capacitance, only add to resistive losses in the primary

2. As long as you don't open your gaps too wide, you should be OK. Even
though your transformers may be rated at 15 KVRMS, the actual maximum
voltage the caps will see will be governed by the breakdown voltage of
your gaps. Capacitors in Tesla Coil service lead a very rough life, sice
we purposely subject them to very high voltage reverals when the gap
fires and the primary circuit "rings". If your gaps are set too widely,
you may easily reach voltages of 1.414xVRMS or 21KV - and this can be
much higher if you "size" the tank capacitor to the current rating of
the transformer due to 60 Hz series resonance effects. If we assume the
gap fires at 20 KV, and has 90% reversal, the cap is actually seeing a
voltage swing of over 38KV and will probably suffer a premature death.
However, it will probably die in this application anyway due to
dielectric overheating or metalization failure, so what the heck! 

3. These caps are made from a class of dielectrics (ferroelectrics) that
have a very high k. Unfortunately, the higher the k of the dielectric,
the greater the losses. These high k dielectrics are typically made from
barium titanate, and can have k's in the range of 500 - 3000 and most
typically about 1500 as long as you keep them below their Curie
transition temperature of 120 degrees C. However, these dielectrics also
tend to be very lossy (1000X greater than LDPE) when used in RF
applications. You'll notice that they'll become quite warm fairly
rapidly in TC use. Get them too hot, and they'll prematurely fail. The
capacitance tends to increase as they heat up, causing your coil to
change its tune as they heat up.  

Hope this helps, and safe coilin' to you!

-- Bert H --