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Re: safety gaps on capacitors



Original poster: "Metlicka Marc by way of Terry Fritz <twftesla-at-qwest-dot-net>" <mystuffs-at-orwell-dot-net>

Hello all\any?
Excuse my ignorance, But couldn't the capacitor safety gap incorporate
two HV\HI resistors of sorts into the leads to the gap?
Wouldn't this effectively drain the discharge at a safe rate and still
operate as the safe for the cap?
If we safely discharge capacitors using this method then i would imagine
a safety gap could do it also.
It may not discharge all V\I in the short time needed, But to bring it
into a low\safe level might keep things together?
Just an uneducated guess from me.
Thank you,
Marc M.
Tesla list wrote:
> 
> 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