Re: safety gaps on capacitors

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

Original poster: "Area31 Research Facility by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rwstephens-at-hurontario-dot-net>

Bart,
 
This is a problem I have not yet personally sought a solution, but I have
thought about it and think I know where I would start to try to devise a
solution in the lab by actual experiment.
 
In simple terms the capacitor represents a PFN or transmission line and you
want the load you switch onto it with the spark gap switch to match the
characteristic impedance so that maximum power will be transferred with minimal
reflection. 
 
If this were my problem I suppose I could scope the capacitor terminals and
test the capacitor at something safe like 100 VDC with a charging supply
isolation resistor in the neighborhood of 1 k ohm or higher, and try dropping a
power resistor across the cap while changing it's value until I saw minimal
ringing at the capacitor terminals.  That would be the value to place in series
with the safety gap.  Your next consideration would be the power rating of the
resistor.  Since a cap safety gap fires infrequently if at all in a properly
tuned system the peak power could be high but the average power will be low.  A
resistor that has good thermal mass, but sized perhaps smaller than the Joule
rating of the cap at Tesla service voltage might be sufficient.  If you want to
see the accompanying movie that describes it all much more clearly download
'handwaving.avi'. 
 
Just kidding.
 
Or you could ask someone who knows.
 
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: Sunday, August 05, 2001 14:20
> Subject: Re: safety gaps on capacitors
>
> Original poster: "Barton B. Anderson by way of Terry Fritz
> <<mailto:twftesla-at-qwest-dot-net>twftesla-at-qwest-dot-net>"
> <<mailto:tesla123-at-pacbell-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
> <<mailto:twftesla-at-qwest-dot-net>twftesla-at-qwest-dot-net>"
> <<mailto:rwstephens-at-hurontario-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>mailto:tesla-at-pupman-dot-com>Tesla list
> > > To:
>
> <<mailto:tesla-at-pupman-dot-com>mailto:tesla-at-pupman-dot-com><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>mailto:twftesla-at-qwest-dot-net><mailto:twftesla-at-qw
> est-dot-net>twftesla-at-qwest-dot-net>"
> > >
> <<<mailto:ewing7-at-rochester.rr-dot-com>mailto:ewing7-at-rochester.rr-dot-com><mailto:e
> wing7-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>mailto:twftesla-at-qwest-dot-net><mailto:twftesla-at-qw
> est-dot-net>twftesla-at-qwest-dot-net>"
> > >
> <<<mailto:Gary.Lau-at-compaq-dot-com>mailto:Gary.Lau-at-compaq-dot-com><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
>