RE: safety gaps on capacitors

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

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

Aren't all pulse-duty caps, including individual MMC-worthy caps, built with
an extended-foil configuration?  While the old rolled poly (shudder) caps
were indeed a pair of long strips, I don't think this applies to most
others.

Gary Lau
MA, USA



>Original poster: "Area31 Research Facility by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <rwstephens-at-hurontario-dot-net>
>
>All,
> 
>Simple safety gaps (not impedance current limited) placed across your tank
>capacitor can cause the capacitor to fail due to a mechanism known as
Blumlein
>Inversion Generator.  The capacitor is usually a long pair of foil strips
with
>a dielectric between.  The fact that it is rolled up is not important.
This
>structure is a form of parallel wire transmission line. If you fully charge
the
>line to some voltage X, then if you place a low impedance short circuit
across
>one end of this line (the safety gap firing) you will create a pulse of
reverse
>polarity which will travel down the transmission line, adding to the
original
>charge and appearing as 2X voltage at the other end.  You have just
subjected
>your capacitors innards to the highest possible discharge current it can
>possibly produce with all the associated hydrodynamic stresses, and you
have
>doubled whatever overvoltage condition you normally subject the capacitor
to in
>Tesla use.  This is a proven formula for capacitor failure!
> 
>Safe use of a capacitor safety gap requires carefully matched series
impedance
>to neutralize the Blumlein effect.
> 
>Rob