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Re: [TCML] Stuck at 36"



Hi Bill,

Although most folks now use MMC's to avoid the large physical size and mess of oil immersed caps, properly constructed LDPE or HDPE foil caps can provide similar coil performance. Your caps are fine - they are simply not large enough to deliver the desired bang size with your 7.5 kV NST farm.
My first TC caps used eight layers of 0.004" thick LDPE between foil 
sheets under oil. Although large, these caps were very reliable and 
provided excellent performance. For longest life, using at least two 
thinner layers of dielectric between foil plates prevents material 
defects in one sheet from lining up with defects in the other, reducing 
the odds of punch-through failures. If you have high quality material 
and operate your caps from 7.5 kV NST's, you may be able to get by with 
a single dielectric layer, but I would really recommend using two 0.031" 
layers for 7.5 kV NST operation. A single layer of 0.031" may not be 
sufficient for 7.5 kV operation, since homemade caps using 0.062" 
material have been found to prematurely fail when driven from 15 kV NST's.
Most coilers also tend to use higher voltage NST's - this allows them to 
use smaller tank capacitance to deliver the same "bang" energy to the 
coil (0.5*C*V^2). The spark length of your coil is mainly a function of 
the bang energy, break rate, and quenching capability of your gap. In 
order to get the same bang energy at 7.5 kV, you'll need 4X the 
capacitance as a 15 kV system operating at the same overall power level. 
Lower voltage, higher recharge current systems can also be more 
difficult to quench, especially when used with a smaller sized tank 
capacitor. Increasing your tank capacitance and also increasing the air 
velocity through your gaps should help. You may also want to try 
adjusting your coupling - too high can lead to racing sparks and poor 
gap quenching, while too low may lead to excessive ringup time and 
higher gap losses. The ideal coupling for YOUR particular coil can only 
be determined experimentally.
Good luck and please keep us posted on your progress.

Bert
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b.jbohn@xxxxxxxxxxx wrote:
I appreciate the speedy response, thanks!

I suspected my caps were part of my problem.  I thought that .03 MFD
was enough, guess not.  This cap bank is turning into a monster.  I
thought the problem was mixing caps with different dialectic
thicknesses .06 and .09.  I can reduce this thing with thinner
dialectic plates.  With HDPE in oil, how thin can I go for 7.5-10Kv
application?  .03?

I've been getting HDPE in 4 x 8 sheets relatively inexpensive and
easy to cut on a table saw.  They are good quality with no defects.
Of the several sheets that I've bought I've only found one defect.
Any info helpful, thanks.

Bill _______________________________________________ Tesla mailing
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