Re: Low voltage sparkgaps for ignition coil driver...

• To: tesla-at-pupman-dot-com
• Subject: Re: Low voltage sparkgaps for ignition coil driver...
• From: Bert Hickman <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <terryf-at-verinet-dot-com>)
• Date: Tue, 24 Nov 1998 06:50:44 -0700
• Approved: terryf-at-verinet-dot-com

Tesla List wrote:
> 
> Original Poster: "Antonio Carlos M. de Queiroz" <acmq-at-compuland-dot-com.br>
> 
> Tesla List wrote:
> 
> > The dielectric strength of air is 20k(!!)V per mm not 20v/mil.
> 
> 30 kV/cm, or 3 kV/mm, or 76 kV/inch, or 76 V/mil, is a better figure.
> But I read somewhere that less voltage than about 240 V (or
> something in this range) cannot cause air breakdown, no matter
> what is the distance. (Does someone know more about this?)
> 
> Antonio Carlos M. de Queiroz

Antonio,

There's an experimental relationship which defines voltage breakdown in
a gas versus distance and pressure. Discovered in 1889, it's called
Pachen's Law, after its discoverer and researcher. 

If you plot the voltage breakdown of a small gap at in the y-axis versus
the product of pressure and distance in the x-axis, there's a "knee"
(lowest point) of voltage breakdown for all gases. This knee is directly
related to avalanche breakdown where a single electron can be
acellerated by the E-field sufficiently to create more ions and
electrons due to collision with the surrounding gas molecules or atoms. 
If the gap is very short, it will take a higher voltage to begin the
breakdown process - the breakdown voltage actually rises as you decrease
the gaplength! For air at STP, this "knee" occurs at about 380 volts or
so.

-- Bert --

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