Re: Self healing caps
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.
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
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 --