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RE: Variac/Isos



Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

Negative temperature coefficient thermistors are made specifically to
address this problem.  I had the breaker-tripping problem with my 15A
variac, and adding a thermistor in series with the line input neatly and
completely fixed the problem.  I use an RTI Electronics part number SG301
(specs available at
http://www.thomasregister-dot-com/olc/rtielectronics/sgspec.htm) surge
suppressor.  Briefly, the device looks like a 1" diameter ceramic disk
capacitor.  The cold resistance is 1.0 Ohm, which limits the turn-on
current to a reasonable value.  The hot resistance (and it does get hot!)
goes down to .015 Ohms, so the insertion loss is minimal.  Maximum current
is 20 Amps.  Just be sure to have the ON/OFF switch on the output side of
the variac, keeping the variac windings energized all of the time.
Otherwise, you may be turning on the variac with a hot (low resistance)
thermistor, circumventing its value.

Gary Lau
MA, USA

Original poster: "Kent Vander Velden by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <graphix-at-iastate.edu>


>	Also, if large, variacs can create false trips, due to
>	inrush current. 

  Is there a good way to fix this problem?  I have a variac that keeps
throwing a 20A breaker.

  Thanks.