Re: [TCML] mikes cap failure

["Quarkster" <quarkster@xxxxxxx>]

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.

Regards,
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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