Re: Doorknob caps okay for tesla coil???

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

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Jonathan and Ed,

I agree with Ed - any style doorknob caps can be used in low power TC's
if you keep run times short. Here's some further information you might
find useful for HC capacitor selection. 

There are two significantly different ceramic dielectric systems used in
HV doorknob capacitors. The formulation used in most common HV DC
Doorknob caps is based on Barium Titanate (BaTi), while the dielectric
used in high performance RF and pulse caps tends to be based on
Strontium Titanate (SrTi). Of the two, SrTi capacitors are preferred for
TC use. 

Some major differences between these two dielectric systems follow:

                                   BaTi       SrTi
                                   ----       ----
Dissipation Factor                 High       Low
Piezo/Electrostrictive             Yes        No 
Capacitance decrease vs Voltage    High       Moderate
Capacitance change vs Temperature  Nonlinear  Linear
High Pulse Current capability      No         Yes
Suitable for RF applications       No         Yes 

Barium titanate shows a significant decrease in capacitance versus
applied voltage - as much as -50% at full voltage. This dC/dV
characteristic lowers the "Q" when used in an oscillatory LC circuit. In
addition, manufacturers of BaTi HV caps typically use a very thin
coating of evaporated silver as the "plates" for these caps. Under
disruptive coil use, the combination of high RF currents, ohmic losses
and dielectric losses cause significant heating. And, as the capacitor
heats up, it may lose as much as 40-50% of its capacitance at room
temperature, causing significant detuning of the primary circuit's
resonant frequency. Finally, the cumulative effects of electrostrictive
contractions (i.e., the capacitor becomes physically smaller as you
apply high voltage!) and the RF current stress on the thin metal system
often result in progressive, and permanent, decreases in capacitance
over time. 

Strontium titanate caps are made to handle TC abuse much better. Because
of heavier metallization systems and lower dielectric losses, they don't
heat up nearly as rapidly. However, as with other ceramic HV caps, they
will show a significant reduction in capacitance with elevated
temperature, and oil cooling may become necessary for high performance
applications.  

Hope this helped and best regards,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
"Electromagically" Shrunken Coins!
http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<evp-at-pacbell-dot-net>
> 
> Tesla list wrote:
> >
> > Original poster: "Jonathan Peakall by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <jpeakall-at-madlabs.info>
> >
> > Dan,
> >
> > Haven't used any Cera-Mite caps, but my experience with doorknob caps in TC
> > service is they get warm, detune and eventually blow. You can get away with
> > short runs, but it seems like sooner or later they go. I'd use those caps
> > for a Marx generator or something.
> >
> > Jonathan Peakall
> 
>         I go along with that; I use them here with good results, subject
to the
> limitations above.  If allowed to get too hot they short, but that means
> they're quite warm to the touch.  If you keep them cool by limiting the
> running time they work fine, and the low series resistance more than
> compensates for the dielectric hysterisis loss.  I don't know whether
> cooling in oil would extend the safe run time, but someone oughta try
> it, as these things are nice and compact and relatively cheap.  No
> competition for better capacitors, but good for starting out.
> 
> Ed