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Re: Maxwell 31159 cap on eBay

Original poster: Terry Fritz <teslalist-at-twfpowerelectronics-dot-com>


Pulse caps (big spark but poor RMS current) tend to have paper and other RF 
lossy stuff in them.  They can put a lot of high voltage capacitance in a 
small cap that way.  No problem for big DC pulse applications.

High RMS current RF caps need polypropylene and other very low loss 
dielectrics, but this adds greatly to the size.

I am sure there are little secrets, but there is no "big" one ;-))  At work 
we have water cooled RF (<<1MHz) caps that actually have water pipes in 
them to cool the oil.  Works very well!!



At 07:21 PM 6/20/2004, you wrote:

>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
> >
> >
> >