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Re: [TCML] mikes cap failure

Mike -

It looks like there may be several factors contributing to your cap failures.

1. When you mix different value capacitors in a series string, the applied voltage does not get divided uniformly across all the capacitors. The capacitors with the lower capacitance value (your .02uF parts) will "see" a much higher percentage of the total voltage than the other capacitors.

2. When connecting capacitors in series, the total capacitance will be LESS than the value of the smallest individual capacitor. Since you apparently added two .02uF caps in series (total capacitance .01uf) with your existing string of 18 capacitors (.012uF), you ended up with around .005uF, which would have put your coil WAY out of tune, unless you had specifically adjusted it to be resonant with this small tank cap value. For a tutorial on how to calculate the total value of a series string of capacitors with different individual values, see: http://www.tpub.com/neets/book2/3e.htm .

The fact that they apparently failed after only a few seconds of use suggests that either the voltage or current in your system grossly exceeded the .02uF capacitor's "actual" ratings, regardless of what the seller may claim as far as them being "suitable for Tesla service" (whatever that means).

You never provided any specific data on your .02uF, 15kV capacitors, but I think it's likely that they are not rated for the current that you are seeing with a 60ma power supply.

Start over by designing an MMC (use the MMC design tools at classictesla.com) that meets the needs of your system. Bear in mind that your Cornell-Dubilier 940C caps are NOT the preferred type for use in a spark-gap tesla coil. The 940C series uses only metallized plastic film as the electrode material, while the more robust 942C series used metal foil electrodes.

You MAY be able to get an acceptable MMC by adding a second parallel string of the 940C caps to share the current load, and by using 20 or more caps per string to ensure that you have adequate voltage margin. The best option would be to build a new MMC, using the correct C-D 942C capacitors, and specifically designing the MMC to survive the voltage and current peaks that your 12kV, 60ma power supply will provide. If you intend to use a variac to increase the NST input voltage beyond 120VAC, then you need to provide additional voltage margin.

Herr Zapp

----- Original Message ----- From: "mike osipowicz" <yosip1115@xxxxxxxxxxx>
To: "tesla request pupman" <tesla@xxxxxxxxxx>
Sent: Sunday, October 04, 2009 11:35 AM
Subject: [TCML] mikes cap failure

hello everyone again, this is a reply to all of thee people helping me out with my cap failure... I have two 30MA 12kv NST's in parallel, for a total of 60 MA. my capacitors are 18 in series, rated for 1.6kv each, DC, but these caps are not the problem, the problem was the two capacitors I added onto it, I was upgrading from 30 to 60 MA, the two caps I added onto them fried, they were two 15kv, .02 MFD, they failed quickly, while my 18 cornell dublinier 940C16P22K-F caps are still perfect. so what I really need to know is what caps should I buy to add onto my current MMC, or what caps should I buy to get 21 NF, at about 35kv? thanks guys, Mike o.
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