Re: safety gaps on capacitors

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

Original poster: "Barton B. Anderson by way of Terry Fritz <twftesla-at-qwest-dot-net>" <tesla123-at-pacbell-dot-net>

Hi Rob, All,

I understand that a safety gap is needed to protect the tank cap from an
overvolt condition, and that simply throwing
a simple gap can also damage the cap. The use of a series impedance is
recommended to extend the discharge time
reducing the overvolt stress of the safety gap occurance. You mention, to
reduce the Blumlein effect, a carefully
matched series impedance is needed. Could you elaborate on how to design in
the correct impedance?

Thanks,
Bart

Tesla list wrote:

> 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
> >
> > ----- Original Message -----
> > From: <mailto:tesla-at-pupman-dot-com>Tesla list
> > To: <mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com
> > Sent: Saturday, August 04, 2001 17:52
> > Subject: Re: Awsome first light !! but...
> >
> > Original poster: "Edward Wingate by way of Terry Fritz
> > <<mailto:twftesla-at-qwest-dot-net>twftesla-at-qwest-dot-net>"
> > <<mailto:ewing7-at-rochester.rr-dot-com>ewing7-at-rochester.rr-dot-com>
> >
> > Tesla list wrote:
> > >
> > > Original poster: "Lau, Gary by way of Terry Fritz
> > <<mailto:twftesla-at-qwest-dot-net>twftesla-at-qwest-dot-net>"
> > <<mailto:Gary.Lau-at-compaq-dot-com>Gary.Lau-at-compaq-dot-com>
> > >
> > > While your safety gap may have been set for 12kV, what was your main gap
> > set
> > > to?  It sounds like your main gap was set to something higher and allowed
> > > the safety to fire.
> > >
> > > As a side issue, it's generally not recommended to use a safety gap alone
> > > across your cap.  With no safety gap current-limiting, the discharge
> > current
> > > could potentially be bad for your cap.  Also, it's impossible for the cap
> > > voltage to ever be any higher than the voltage across your main gap.
> > > Whatever your main gap is set to will also limit the cap voltage, and the
> > > cap-gap is redundant.
> > >
> > > Gary Lau
> > > MA, USA
> >
> > Gary,
> >
> > Do you current limit your primary to keep the discharge current from
> > damaging your cap?
> >
> > I have NEVER seen a cap destroyed by a safety gap yet. I HAVE however
> > seen many caps go to the great beyond for the lack of a safety gap!
> >
> > The first question I ask someone who has just blown a capacitor is "were
> > you running a safety gap on the cap?", and almost invariably the answer
> > is no. I personally will use a safety gap every time!
> >
> >
> >
> > Ed Wingate RATCB