fwd: Spark Gaps

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

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Mike,

At 01:36 AM 7/18/2001 -0400, you wrote:
>Hi Terry,
>    How do you arrive at the high voltage?  I have a high pot tester, which 
>is essentially an unpotted OBIT size transformer and a voltmeter which goes 
>to 10kVAC.  Now, being the crazy that I am,  I have used it numerous times to 
>set spark gaps.  I have placed a cap across the gap and notice quite a 
>difference, but not as you suggest.

The voltage was arrived at using modeling and some observations of like 1.5
inch arcs across NSTs (good one :-))  A lot depends on a lot...  Mileage
may vary...  The 80kV number is about the max that is fairly well trusted
and serves to scare the unwary into being careful about resonating up their
NSTs to destruction.

>   I can set a gap for say 8kV and when the gap is made to fire(without a 
>cap) the voltage drops to around 3kV.  Now when I place a 1nF cap across the 
>gap,  it changes the spark characterisics as expected.  It gets bright and 
>loud.  The interesting thing to me is that the voltage measured by the tester 
>now is back up to 8kV,  but I have not seen any great rise in voltage..  

Spark gaps are funny things.  They seems to need to break in a bit on my
small coil before they get to the right breakdown voltage...  Maybe the
higher current with the cap keeps them clean in that little bumps and dust
stay blown off the electrodes.  I sometimes wonder if it is not time to
dump the spark gaps and go full over to MOVs...  May protection filters
where designed with the best of both worlds and basically to "test" the MOV
methods.  The MOVs seem to work just fine and perhaps don't need the gaps
as backup...

Maybe a MOV protection gap with a light bulb across it to indicate with the
current through it is getting too high.  Since MOVs are silent, they can be
working hard without any indication that something is wrong.  they can be
blown up, but they fail safe (short), but that is a pain...

>   Now, the point I wish to make is that I thought the spark gap was meant to 
>limit the voltage by arcing across when the voltage rises to the set point.  
>Is this not the case?  

That is only the theory :-))  In general, I think spark gaps are pretty
poor at limiting to a "specific" voltage.  However, if your NST is going
nuclear, they will step in around 25, 30, 35... kV which is usually enough
to save the day.  You will note that I always use MOVs next to the NST
since I "know" when they fire...

>I would think that if such a rise occurred, I would at 
>least see it peg the meter wouldn't I?   

I guess I would have to "be there" to see the whole picture...

>Now, perhaps this is the case if the 
>circuit remains open, but doesn't the discharge through the gap take care of 
>that?

When the gap fires, all the resonant energy is drained and the NST is
saved. :-)

>Confusedly,
>Mike
>(you can post this to the list if you wish)

Ok,  Others may find it interesting too.

Cheers,

	Terry


>
><< Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
> 
> Hi CJ,
> 
> A cap connected just to the NST may ring up the voltage really high.  If you
> hit the 60Hz resonant point you can get like 80000 volts!  that will blow
> something up so be careful.  When the cap arced, was there anything to limit
> the current?  If you have 20000 volts but no inductor and no resistance, the
> current will be super high.  20000 volts into 1 ohm is 20000 amps.  That will
> be loud :-)  Super high current like that can damage the cap too.  Best to 
>add
> an inductor to act as a current limit.
> 
> Cheers,
> 
>         Terry >>