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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 Malcolm,

Yep, agree with the current limiting and this is what I meant by the
recommended approach. Rob just
sparked my curiousity on the Blumlein effect and matching the impedance.
I've read about this before,
but never really thought much about it. I'll have to do some reading up on
this to fully understand it.

Take care,
Bart

Tesla list wrote:

> Original poster: "Malcolm Watts by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>
>
> Hi Bart,
>           I would use a resistance which limited peak discharge
> current to the cap rating. That should easily produce a damped
> response. Simple Vpk/R = Ipk.
>
> Regards,
> malcolm
>
> On 5 Aug 01, at 12:20, 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.
> msnip...