Inrush current limiters (was Re: Variac turn-on surge solution)

Hello everybody.

You can find a whole line of ceramic disk "Surge-gard" inrush current limiters
from Rhopoint Components. Farnell sells them at page 1562. Component codes for
instance 606-832.

For instance a 30A rated device, Rcold=0.5 ohm, Rhot=0.01 ohm, sells for $7 and
is 3 cm diameter by 5 mm thickness only.


Tesla List <tesla-at-pupman-dot-com> on 28.10.99 01:34:19

To:   tesla-at-pupman-dot-com
cc:    (bcc: Marco Denicolai/MARTIS)
Subject:  Variac turn-on surge solution

Original Poster: "Lau, Gary" <Gary.Lau-at-compaq-dot-com>

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