On Wed, 24 Dec 2008 18:39:49 -0500, Jamie
t put finger to
keyboard and composed:

Ken Wright wrote:

I have read online that they use a heating wire with
a high positive temperature coefficient of resistance so
that it is not only a heater but also a temperature
sensor. I have monitored the current draw. When turned
on the current starts at 2 amps and gradually drops to
near an amp before the controller switches off.

There must be a PTC (positive Temperature Coefficient) resistor in it.

Measure the value are room temperature and and place a small value
R in series with it to off set the calibration so that it'll operate
at a cooler level.
Or, reposition the PTC closer to a heating wire, it could of slip out
of the original position.

The heating element itself provides the PTC resistance. If the PTC
resistor were external to the blanket (ie inside the controller), then
initially it would present a short circuit (or very low resistance),
in which case the blanket would see 110VAC at 2A. If the current then
gradually decreases to 1A, this would mean that the PTC would be
dissipating 55W (= 55VAC @ 1A).

- Franc Zabkar
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