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Re: Variac turn-on surge solution

Hi Gary,

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
acting...

	Terry


At 01:13 PM 10/27/1999 -0400, you wrote:
>Dear List:
>
>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
>Amp breaker.
>
>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,
>Gary Lau
>Waltham, MA USA
>