Re: Variac turn-on surge solution
Subject: Re: Variac turn-on surge solution
From: Terry Fritz <twftesla-at-uswest-dot-net>
Date: Wed, 27 Oct 1999 17:37:09 -0600
In-Reply-To: <D5210908318AD211A3F20000F8062CCD01E627FF-at-excpko-02.pko.dec -dot-com>
Perhaps a slower acting circuit breaker would be the fast and correct fix??
The breaker may also be defective... I have a 15 amp variac too but the
surge current has never been a problem but all my breakers are thermal slow
At 01:13 PM 10/27/1999 -0400, you wrote:
>I use a 15 Amp variac on my system, and after my house was upgraded from a
>fuse box to circuit breakers, I had a problem. About half of the time when
>I turn on the variac, even though the knob was at 0%, the breaker would
>trip. The reason for this is that when the variac was last turned off, the
>core may be left in either of two magnetic orientations. If that
>orientation is counter to the phase of the AC voltage the next time it's
>turned on, the turn-on surge current can be huge, large enough to trip a 20
>To solve this problem, I had been considering having a small series
>resistance that is switched out by a time delay relay, but I recently came
>across a much simpler solution.
>Some of the power supplies that we use at work have surge-limiting
>thermistors on them. These devices look like black ceramic disc capacitors,
>0.9" diameter. At room temperature or below, they have a resistance of
>about 1.0 Ohm, enough to limit turn-on current to a reasonable value. When
>they heat up when 20 Amps (max rating) is flowing through it, the resistance
>drops to 0.015 Ohms. Inserting one of these devices in series with the
>input to the variac completely solved the breaker tripping problem.
>I pulled my part off of a scrap board and can't comment on where others may
>get some, but the part is made by RTI Electronics, part number SG301. It is
>a negative temp coefficient thermistor specifically made for limiting surge
>currents. See www.thomasregister-dot-com/olc/rtielectronics/surggard.htm
><http://www.thomasregister-dot-com/olc/rtielectronics/surggard.htm> for specs.
>Hope this helps,
>Waltham, MA USA