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Re: Cap - Puncture Voltage
From: Adam[SMTP:absmith-at-tiac-dot-net]
Sent: Saturday, December 06, 1997 10:22 AM
To: Tesla List
Subject: Re: Cap - Puncture Voltage
>> 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.
>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!
>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.
I did my own little test, by taking a 30mil piece of LDPE sheet 4" by 4"
and putting a 2" x 2" aluminum foil square on each side. I then attached
this to my 15kV 30mA Transformer (rebuilt and designated test
transformer), and started cranking up my variac. The PE punctured at a
variac setting of 100V exactly. This works out to be approx 12500 VRMS
on the capacitor, and 17.7kV peak. That's 590 V/mil, albeit AC peak,
not DC. Strong stuff. I am using 90 mil in my reconstructed capacitor
(3 x 30mil) and I am running at 9kV w/resonant charging. I expect this
to be fairly stable.
-Adam
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Adam Smith
absmith-at-tiac-dot-net
Epoch, Inc. Digital Music Project
www.tiac-dot-net/users/absmith/ Now with MP3! Musik. Macht. Macintosh.
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