Re: Polyethylene Capacitors

To: tesla-at-pupman-dot-com
Subject: Re: Polyethylene Capacitors
From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
Date: Fri, 20 Sep 1996 08:21:34 +1200

Hi all,
        I feel this deserves a reply...

> Gentlemen,
>     There has got to be a better alternative to using 30 mil or
> 60 mil LDPE. Even the HDPE is very expensive. I've just spent my
> lunch hour calling around to the local Indiana plastics suppliers.
> Bloomington, Indianapolis, Cincinatti and even someplace near St. Louis.
> The prices were very high. A single sheet of 48"x96" LDPE 30 mil 
> averaged around $18 a sheet! The 60 mil averaged $34 a sheet. A
> sheet of HDPE 48"x96" 60 mil was $43.96.
> 
>     Please would somebody answer my question regarding 6 mil
> thickness LDPE stacked for 90 mil thickness. Will this work and
> are there any pitfalls? 
> 
>     Stacked plates of plexiglass in large square plastic comforter
> boxes are looking better every day. According to my calculations,
> 16 - 14"x14" sheets of aluminum flashing using 17 - 16"x16" 3/16"
> thick plexiglass will yeild .01 uF. With a Tesla voltage rating of
> 32 KV. The mineral oil I've found was realitively inexpensive, 
> $9 per gallon.
> 
> Any comments are more than welcome.

Many layers of thin sheet are by far the best way to go for a host of 
reasons (defect misalignment, higher V/mil in thin sheets). I think
Richard Hull came up with the main reason why our caps are prone to 
failure though (given thick dielectric). He said that impressing more
than 5kV or so across the _oil_ causes oil breakdown. It seems clear
from this that cap manufacturers build their caps with a number of
series sections to avoid doing just this. To me it explains a lot.

Malcolm

** If it's under 10 Amps it's leakage current **