Re: purpose of a variac?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: <dgoodfellow@xxxxxxxxxxxxxx>

It depends on how you define optimized performance. I think my coils 
are at their best when the spark gap is set conservatively, and the 
spark output is very continuous and flowing. The coil sounds smooth 
like a saw cutting steadily. When the gap is set this way, I see 
activity at the spark gap even when the variac is only at 20% of max. 
There may be no breakout from the topload at this level of input, but 
the transformer clearly has enough voltage to start arcing across the 
gap. In my last coil (ebay# 6226319288 ) The spark gap (made from 
3/8" tungsten electrodes) was set at 13/64" (.203") for the spark 
photos. I have a paper that was written by Mark S. Rzeszotarski PhD. 
that defined a coil using the same gap electrodes, operating at 
.200". In this paper, the breakdown voltage is 17.6 kv. Remember, I 
am using a 9kv 60 ma transformer, with LTR caps, and according to 
this paper, the gap is firing at nearly double the transformer voltage.
If I run a TC with the gap open wider than .200" the sparks are not, 
to me, as pleasing to the eye. The variac will have to be turned to 
1/2 to 3/4 of it's full output before the gap fires. The sparks off 
the topload are less continuous, and one can hear the slower bang 
rate as the caps can not cycle as quickly as they did with the gap 
set closer. This state of operation "feels" to me, that the coil is 
running at the brink of destruction.
   If anyone has different views about the breakdown voltage at 
.200" I would like to hear what you have to say.

Dave Goodfellow

----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Sunday, January 22, 2006 9:19 PM
Subject: purpose of a variac?


> Original poster: "Qndre Qndre" <qndre_encrypt@xxxxxxxxxxx>
>
> Hey coilers,
>
> I've read several documents on the internet which claim that the 
> power throughput of a TC can be adjusted using a variable 
> transformer to power the supply transformer of the coil. I don't see 
> how this could be done under regular circumstances. If the input 
> voltage of the supply transformer is reduced, it's output voltage 
> reduces as well. The voltage across the capacitors can never exceed 
> the supply voltage (unless you use resonant charging which may 
> destroy the secondary winding of your supply transformer). Since the 
> spark gap in the tank circuit is best to be tuned just to fire at 
> the absolute voltage peak for optimized performance it will never 
> fire with reduced input voltage to the primary circuit.
>
> Regards, Q.