Re: safety gaps on capacitors

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

Original poster: "Sundog by way of Terry Fritz <twftesla-at-qwest-dot-net>" <sundog-at-timeship-dot-net>

Hi All,

 I'm guilty of not always using safety gaps.  lost a few trannys to it also.
I always put mine in series with the primary, so when the gap does fire,
it's still dumping through the primary.  That way the cap doesn't see much
more abuse (aside from the slightly higher voltage than it normally gets).
Current limiting built in, no fuss, no muss, no extra crap to build.

Just my 2%

Shad
----- Original Message -----
From: Tesla list <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, August 05, 2001 2:20 PM
Subject: Re: safety gaps on capacitors


> 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
>
>
>
>