Re: [TCML] Primary Capacitance Calculations

[jimlux <jimlux@xxxxxxxxxxxxx>]

Thomas Schmit wrote:
> Hello All
>  
> I'm suspicious of all these "black box" calculators and equations that I've found for determining the primary capacitances. I'd like to determine a good capacitance to use in my primary tank circuit driven by a 7200 V, 10 KVA pole pig. 
>  

I'd be suspicious too, especially if the equation is derived from usage 
for a neon sign transformer, which is a very different beast from a 
ballasted pole pig.

The idea behind picking a good primary C for a NST is to avoid matching 
the output inductance of the NST (resonant), because that can lead to 
very high voltages which in turn is hard on the NST.  (note that there 
are some older references around that actually encourage this, though). 
 There are two ways to go: "larger than resonant" (LTR) and "smaller 
than resonant" (the usual case).  Which works better depends a lot on 
what kind of spark gap (a STR can fire multiple times per half cycle, a 
LTR generally needs a rotary or triggered gap).  The actual primary C 
chosen is usually determined more by available values of capacitor than 
anything else (e.g. caps come in certain increments, so you pick the 
biggest one that is smaller than resonant, for a static gap)


For all intents and purposes, the pole pig is a "stiff" voltage source 
with very low output impedance. It will charge pretty much any primary C 
you choose in much less time than the half cycle. If you have a very 
small primary C, you can have a very high spark rate (breaks per 
second), so if you want to run a high speed rotary gap with lots of 
electrodes, that's the hot ticket.
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