Re: spark gap spacing

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

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

Trial and error (i.e. empiricism) is how you set the spark gap.  Sure,
there are kV/cm equations (30kV/cm, roughly) (which are for uniform field,
hardly the case with a sphere gap) and lots of tables (derived empirically,
I might add), but even then, an accuracy of 5% in a laboratory setting
(careful control of surroundings, atmospheric conditions, cleanliness,
radioactive sources, etc.) would be doing well.

Rod gaps are usually used for overvoltage protection on HV lines, and have
very high variability (no problem if you are trying to suppress 90 kV
transients on a 14 kV line...). The rods are nominally 1/2" or 5/8" square,
by the way. The more non-uniform the field, the greater the variability, in
general.

The "Handbook of Chemistry and Physics" also has needle gap tables, and I
have rod gap tables on my web site
(http://home.earthlink-dot-net/~jimlux/hv/rodgap.htm).  The site also has the
necessary construction details, if you are interested in building a
"calibrated sphere gap".


In my TC, the gap is right across the NST, so I just disconnect the primary
circuit (i.e. remove the clip from the primary L), and set the gap spacing
so it just stops firing at 120V input to the NST.  If you boost the voltage
with a Variac a bit, it starts to fire.

Once you've figured out the gap for YOUR particular setup, then measuring
it (say, using drill bits or feeler gauges) would be a handy way to reset it.

The important thing to remember with spark gaps is that each and every one
is special in its own way, and has a unique breakdown characteristic.  Even
identically manufactured gaps in identical environments will show a fairly
large variation in initial breakdown voltage (which is why triggered gaps
are popular for all manner of HV switching applications... the trigger
controls the breakdown, not some chaotic random process).

Tesla list wrote:
> 
> Original poster: "Shaun Epp by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<scepp-at-mts-dot-net>
> 
> Hello Again,
> 
> I Have another question,
> 
> Does anyone have more exact information to setup the gap distances on a 12Kv
> at 60mA neon sign powered tesla coil, I've seen the SGapVolt.jpg file on
> Terry's web page under misc and its kinda rough, but interesting.  Anyway,
> I'm looking for the safty gap distances to ground, and the main spark gap
> distance for a 2 gap (I hope 2 is enough) spark gap using 3/8 inch tungsten,
> flat surface, oh and the safty gaps are ~3/8 inch brass balls.
> 
> I know I've seen the equation for  Kv/cm spark jump somewhere, but it
> doesn't include electrode shape, and I figure that there is lots of people
> out there using very similar hardware so I'm hoping I don't have to do it
> trail and error.  :-)
> 
> BTW.  I was thinging of using Drill bits to gap my spark gaps,  does this
> work well??
> 
> Thanx,   Shaun Epp