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RE: [TCML] Variac

Hi Jay,

I also had the problem where often when I plugged in my 15A Variac, regardless of wiper setting or load, it would trip the 20A breaker.

I solved the problem by adding a NTC (Negative Temperature Coefficient) thermistor in series with the Variac line input.  A spec sheet for a family of NTC devices is at http://www.specsensors.com/pdfs/ntc_surge_current_limit.pdf.  The device I used has a 1 Ohm cold resistance, 0.015 Ohm hot resistance.   This small device solved the problem very neatly.  It basically does the same thing as the relay-switched series resistor suggested by Dave, just more compactly (and harder to find!).

If the problem is NOT due to the Variac turn-on surge, do you use PFC caps to minimize the NST current?

Regards, Gary Lau

> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On
> Behalf Of jhowson4@xxxxxxxxxxx
> Sent: Tuesday, October 28, 2008 6:34 PM
> To: tesla list
> Subject: [TCML] Variac
> Ok this is attempt 3 my e-mail keeps saying that there was a sending error. so if 3
> of the same message were in fact sent i am sorry.
> Ok so i think that this so called surge is the culprit since at the time of the breaker
> being blown I was trying to figure out where a weird sound was coming from and
> this entailed turing it on and off.  learing from my mistakes i won't be doing that
> again... ever...
> How much current is the norm if nothing is plugged into it? If the unit is rated for
> 22.5 amps why is it not pulling that much power all the time?
> How does starting the variac with it turned up help.  I understand what you want me
> to do but I am curious as to why it works.
> Even if the surge current was the culprit i want to be sure before I plug it back in
> and the people start asking why stuff blew.
> Thanks,
> Jay. H

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