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Re: [TCML] Choosing a Cap Size

Hi Michael,

For a pig, consider power when thinking about Cp sizing. Power is directly related to spark length and to other components. Most pigs are STR mostly due to the cap costs for such a high cap size, but also, the energy across the gap is another factor probably not realized since costs keeps us at bay (most of the time). If an LTR type cap were used with a 14.4kv/10kva piggy or bigger, the energy at the gap would be tremendous and then the rotary would need to be built to handle this power. With power in the rotary comes "costs" (once again). And even after all the money was spent, do you have the facility to run the darn thing? These are the factors to look at in my opinion.

I would keep it around 0.1uF to 0.13uF at max. 0.1uF is running at 1.3 x Cres STR, so slight resonant charging is occurring. At 0.13uF, 100% resonant charging is occurring. For this, the caps in my opinion should be designed for 2.5 to 3 x Vp or maybe higher. This would put out the best spark lengths, but you do risk pig and cap death in this condition. The 0.1uF is smarter for maximizing cap energy along with reducing resonant charging and keeping costs down to a manageable level (including keeping energy down to a manageable level. i.e., 20j versus say 70j).

DC's advice on the rotary sounds perfect to me for the 0.1uF size. This will allow powerful spark lengths making good use of your available power without going overboard. Also, I'm not sure of the coil size you are planning to adapt the pig to, but the coil size is another factor which may play into the cap size. Hopefully, it's in the 10" to 12" diameter range. If you plan for the 2 to 5 kva range, then consider between .05uF and .08uF. As a matter of fact, it might be wise to build it with roughly .05uF now, and then later build a duplicate cap to put in parallel when the RSG is ready.

Take care,

Michael Robinson wrote:
Hi All,

I am about ready to start adapting my coil to run with pig power, but
I have a question regarding the size of Cp.  The "rules" for finding a
cap size for for a NST are pretty well defined, but I'm having a hard
time figuring out what to use on a pole pig coil.

What parameters are used in finding the optimal Cp?  Should I use the
transformer's ratings, or the actual power I'll be running it at? (ie.
it is a 10 kva pig, but I'll most likely run it between 2 and 5 kva)
I'd like to go STR to save on cash, but how low can I go while still
maintaining decent spark length?  I will be running a blower gap, and
moving to ASRSG as time and money permits.

Thanks for any advice,
Michael Robinson, KE7AQL
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