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Re: Maxwell 31159 cap on eBay
Original poster: Ed Phillips <evp-at-pacbell-dot-net>
Tesla list wrote:
> Original poster: "Dr. Resonance" <resonance-at-jvlnet-dot-com>
> Your assumption is correct --- they are built differently. The difference
> is a Maxwell trade secret that they will not reveal.
> Dr. Resonance
> > "If a HV pulse cap is not rated for high % voltage reversals, there is
> > significant corona formation at the edge of the foils. This corona and
> > surface tracking begin to heat up the local dielectric area (di/dt) and
> > will
> > eventually produce enough heat to begin melting through the dielectric.
> > Dielectric breakdown at the edge of the foil is the number one killer of
> > HV
> > pulse duty caps (according to Maxwell Sr. Engineer Bob Cooper). Second
> > on
> > the list are small voids in the dielectric material, which also produce
> > corona in the voids due to the different dielectric constant in the void
> > material, and, again, leading to local intense heating effects that
> > damage
> > the dielectric.
> > Pulse caps not rated for high voltage reversals literally "can't take
> > the
> > heat".
> > Dr. Resonance"
> > I understand the breakdown phenomena and also the problem with corona
> > in voids under AC operation, but don't understand how the construction
> > of the "high voltage reversal" capacitors differs from the "low voltage
> > reversal" ones. Are they built differently or just screened? When I
> > was at Hughes Aircraft we had a group building special capacitors for
> > radar modulator pulse-forming networks (where dV/dT can exceed 10^7
> > volts/sec and operation is at several thousand pps) and they used to
> > screen capacitors after construction by applying a high AC voltage and
> > listening for radio noise as a symptom of corona in unfilled voids. In
> > this case they were pitched out because the construction techniques
> > including vacuum impregnation were the best they knew about. Does
> > Maxwell do something like this but salvages the rejects by issuing them
> > under a different part number?
> > Ed
OK - trade secrets can be very important. Presumably the capacitors
with smaller reverse voltage ratings are built that way because they can
be sold for a lower price and/or smaller in size. Any of that correct?