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 Smith
Epoch, Inc. Digital Music Project

www.tiac-dot-net/users/absmith/     Now with MP3!     Musik. Macht. Macintosh.