Re: purpose of a variac?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: FutureT@xxxxxxx 

In a message dated 1/23/06 9:04:47 AM Eastern Standard Time, 
tesla@xxxxxxxxxx writes:


> >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.


Q,

The maximum voltage point is a timing issue.  The rotary electrodes
align when the voltage is at a max using a 120 bps sync rotary gap.
If the voltage is lowered using the variac, the gap will still fire because
the gap should be set very close when using a rotary.  A wide gap
setting will tend to destroy NSTs, although a Terry filter and
LTR operation will probably protect the NST.  But the coil will
run erratically with a wide gap setting on a rotary.  For best
spark output at the lower voltage the rotary phase will need
to be readjusted.

Using a static gap with LTR, the gap will still fire if the variac
is turned down because the gap should not be made so wide
that it barely fires.

Bottom line is that many coilers use a variac to adjust the
power and spark length and it works perfectly.

John