Re: Cap - Puncture Voltage

To: "'Tesla List'" <tesla-at-poodle.pupman-dot-com>
Subject: Re: Cap - Puncture Voltage
From: Tesla List <tesla-at-stic-dot-net>
Date: Fri, 5 Dec 1997 23:56:21 -0600
Approved: tesla-at-stic-dot-net

From: 	Bill Lemieux[SMTP:gomez-at-netherworld-dot-com]
Sent: 	Friday, December 05, 1997 3:25 PM
To: 	Tesla List
Subject: 	Re: Cap - Puncture Voltage

Tesla List wrote:

> I have seen info around that vary's polyethylene puncture voltage from
> 240/mil to 1200/mil. All I can say is "WOW", what a variance! I assume
> these to be DC ratings and therefore this would need to be derated for
> AC and especially Tesla Coil use. 
> Bart

I have seen typical DC dielectric strengths for PE of ~400v/mil. This
from a number of plastics manufacturer's publications.

What you must realize is that V/mil specs are good for thicknesses of
one mil.  But the dielectric strength does not multiply linearly.  If
you have ten mils, you do _not_ have 4kV of dielectric strength!  In
fact, it depends on the plastic, but you can usually derate the
dielectric strength by at least 40% for very thick pieces (more than a
hundred mils).

Now then, as soon as you introduce voltage reversal (AC), you need to at
least double the necessary rating, since the dielectric will "see" as
much as twice the theoretically applied _peak_ voltage.  Note that neon
sign transformers are rated at RMS, so a 15kV tranny will put out 21.2
kV.  So now you need to hold off 42 kV, at least.

All this theory aside, I once built a rolled PE/foil cap that used 16
layers of 4mil plastic sheet (clear Visqueen), for a total of 64 mils.
Despite not being immersed in oil, and even being operated in a closed
box where ozone could build up, it worked beautifully under heavy
abusive service, in a largish tesla coil (12kva) _with_no_safety_gap_
for years.  Your mileage may vary.

I attribute its eventual failure to ozone-promoted flashover- it did
flashover at the end, rather than puncturing the dielectric.  (it was
built with 1.5" of dielectric extension on each end since I operated it
in air)

For my test bed coils, I now use 60kVDC oil/paper filter caps in
series/parallel.  They don't even get warm.  For my commercial coils, I
use (expensive) commercial mylar/oil/plate caps, specifically
constructed for 100% duty cycle, pulsed-RF duty. 

A single .01uF at 60kV RF rating runs me about $250 (small coils).

A single .1uF at 60kV RF rating runs me about $500 (big coils).

Hope this helps.

-Gomez

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