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Spark Gap BPS

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> 
> Subscriber: Esondrmn-at-aol-dot-com Tue Feb 11 23:12:17 1997
> Date: Tue, 11 Feb 1997 16:56:13 -0500 (EST)
> From: Esondrmn-at-aol-dot-com
> To: tesla-at-pupman-dot-com

> In a message dated 97-02-11 01:09:33 EST, you write:
> 
> << [snip]
>   
>  > A question for Richard Hull, if 400 to 600 bps is what you see from
>  > experience as a good general number, why does this coil want higher
and
>  > higher rpm?
>  > 
>  > Comments anyone?
>  
>  
>  Ed,
>  
>  Because a higher rpm results in increased output power, which is always
> good.
>  
>  -GL
>   >>
> Greg,
> 
> Part of my original reason behind my discussion was to see if anyone
thought
> the high break rate was maybe a cause of the commercial Condenser
Products
> cap failure.  I didn't get much feedback on that idea.
> 
> Ed Sonderman

Ed,
an attempt to bring the discussion back to the original question:

The BPS rate will definitely have an influence on the capacitor. If you
look at the ratings given to commercial capacitors used for pulse operation
you will see that one of the parameters is the pulse repetition rate. 

Recall that the capacitor is only oscillating during the time the  spark
gap is *on*. During the *on* time the capacitor experiences dielectric
heating that is a function of the frequency of oscillation and the current
"flow". When the BPS is higher, the capacitor spends more time oscillating!
This increases the dielectric heating and the temperature of the
insulation. Solid insulations can be made to melt quite readily using
dielectric heating.

The increased power you see is very much due to the increased *oscillation*
time. Big pulses make longer sparks, but the greater power of a higher BPS
results in more sparks, though many are of shorter length. The increased
oscillation time and the greater overall power are directly related. And
that power is oscilatorry in nature and is creating heating within the
capacitor. This heating is especially aggravated if there is any air or
other gas between a plate and the dielectric. Read what Tesla had to say
about the temperatures that such heated gases could be raised to. The
capacitor has the greatest RF fields of all oscillating around in its
innards. A sobering thought. I hope all of you have your caps protected by
safety gaps and venting. If anything is going to *explode* in a Tesla coil,
it is most likely going to be the capacitor. And this doesn't matter
whether it is commercial, salt water, rolled or flat. Every capacitor has a
point where it will break down.

The poor tank capacitor is actually subjected to two different kinds of
abuse. Besides the RF heating, there are the massive pulse effects. Even a
single large pulse can cause an acoustic (sound) wave that will propagate
through the liquid insulation quite readily. The material inside the
capacitor is literally *punched* by these pulses we are applying. Couple
these physical blows with the softening of the plastic insulation and you
can easily see why a capacitor sometimes fails when used in a Tesla coil.

Tesla coils are often only run for short periods of time, which is good,
because as you increase the run time the heating effects increase. This is
especially true near the center of a rolled capacitor, as the surrounding
heat tends to make the center *very* much hotter than the outside of the
capacitor if you keep it running.



Fr. Tom McGahee