Re: Self healing caps

To: tesla-at-pupman-dot-com
Subject: Re: Self healing caps
From: Bert Hickman <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Wed, 29 Dec 1999 11:51:25 -0700
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Tesla List wrote:
> 
> Original Poster: Justin Gould <justing.mrl-at-dubbo.nsw.gov.au>
> 
> Does anyone know by what mechanism metallised film poly caps self heal?
> If so, is there any way of incorporating this feature into home made caps?
> Could be handy if at all feasible.
> 
> Justin

Justin,

A thin layer of metal is evaporated onto the dielectric - this layer is
actually so thin that you can partially see through it. If a dielectric
failure occurs, follow-through surge current causes the metallized area
around the defect to evaporate, acting much like a fuse, and
"disconnecting" the defect, instead of charing a hole (as in a foil-film
type capacitor. Every time a defect is "healed", a little bit of the
capacitor's effective plate area is removed, decreasing its capacitance
a bit. 

Because of the thin metal layer, the Equivalent Series Resistance of
this type capacitor is not nearly as low as a metal foil type, and
that's why the maximum dV/dT rating for this type of capacitor is less
than for an extended foil type - they are lossier because of OHMIC
losses. Now, if you purchased rolls of appropriately metallized PP
capacitor film, you could indeed "roll your own" self-healing caps.
However, you can't make them using commonly available materials. Sorry!
:^)

Safe cappin' to you!

-- Bert --