Re: purpose of a variac?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>

Q,

You are sort of right. Your reasoning applies to a coil with a static 
spark gap. Using a variac with this will probably cause it to either 
not work at all, or fire at full power. Unless the tank capacitor is 
near resonant with the ballast inductance, in which case the voltage 
always rings up to the gap firing voltage, and the variac ends up 
adjusting the break rate. You can adjust it right down to about 1 bps.

This is not a good way to control power though: the power factor is 
very poor at low break rates and the transformer draws a heavy 
current and gets very hot. I think I destroyed an NST by 
experimenting with this.

Rotary gaps are a different case- they are set up so that the 
electrodes present as close as possible without crashing. So they 
should fire fine with reduced voltages and give you lots of room for 
adjustment. I found that my DC coil with ARSG fired at about 50% on 
the variac, and then could be t urned down to 25% before it went out. 
(this was due to the charging voltage doubling once firing began.)

However if you're using a sync rotary with an AC charging system, 
adjusting the supply voltage could spoil the phasing. A higher 
voltage can jump a bigger gap, so it will fire earlier while the 
electrodes are still some distance apart.


Steve Conner
http://www.scopeboy.com/
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.