Re: Polyester Film Capacitors

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>

> Purely out of idle curiosity, has anyone ever seen high voltage high
> current rated polystyrene caps?  Based on the dielectric properties of
> polystyrene they ought to be pretty good too.  I have an illustration
> in a book from 40+ years ago showing metal-cased polystyrene caps of
> generous dimensions with the usual massive ceramic standoffs on them
> and evidently intended for HT valve circuitry, but the British company
> which made them (TMC in Orpington) no longer exists.  Just wondered if
> anyone still produces such things.  The only polystyrene caps I've
> seen recently are the small signal, low loss 160V jobs for your
> average silicon circuitry.  I'm just wondering if there's a good
> reason why we don't see hefty polystyrene caps, but from a polymer
> chemistry view of things I can't think of a single reason why they
> shouldn't be made.
> 
> Dunckx

	Polystyrene is an excellent capacitor dielectric, with very low loss
tangent and very low dielectric absorption.  Used to use them for analog
differentiators and integrators, long, long, long ago.  Problem is that
it also has a relatively low dielectric constant and softening
temperature.  There are lots of 200 V polystyrend capacitors up to at
least 0.01 mfd.  The ones I have are of the extended foil type, ideal
for high current.  Unfortunately, the leads are about #28.  Doubt if
anyone is going to the trouble of making really big ones when there are
dielectrics which work OK and are a lot easier to handle.

Ed