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 -- *************************************************** We specialize in UNIQUE items! Coins shrunk by huge magnetic fields, Lichtenberg Figures (our "Captured Lightning") and out of print technical Books. Visit Stoneridge Engineering at http://www.teslamania.com *************************************************** 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 mailinglist Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
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