On Wed, 24 Dec 2008 16:07:47 -0600, Ken Wright
put finger to keyboard and composed:

I recently purchased a Sunbeam electric blanket. It
works ok except that its lowest setting is a bit too
high for my liking and I am looking for ways to reduce
its output.

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.

Here are two Sunbeam patents for PTC heating wire and one for PTC
temperature sensing wi

http://www.google.com/patents/pdf/Bl...We ftrxVZtgZQ
http://www.google.com/patents/pdf/El...t2tVm2PWmyvU7A
http://www.google.com/patents/pdf/Te...RJ yPn382DRbA

Here are other Sunbeam electric blanket patents:
http://www.google.com/patents?as_drr...&sa=N&start=20

Is there a patent number on your appliance?

Has anyone opened up one of these controllers? The product
description says it is solid state and noiseless ...

I'm guessing it uses a triac with a zero voltage switching control
circuit.

... and that if
there is an error condition the controller light will flash.
It sounds like it might have a microprocessor but I am
reluctant to open up the controller on a brand new item which
has a 5 year warranty if there is nothing inside which I can
tweak to lower the lowest setting.

My thinking now is that if it is microprocessor controlled it
would be difficult to modify. My first thought was to lower
the input voltage but I think the microprocessor would be able
to compensate.

Any ideas?

thanks
kw

As others have suggested, it may be that your blanket is not working
properly. The min-max current difference (1A versus 2A) seems
inadequate, but then you need to determine whether the duty cycle
remains constant or not. I expect that you will find that your blanket
has a PTC heating element, and that its controller operates more like
a thermostat (or simmerstat) than a dimmer. It seems logical to me for
a temperature controller with 10 settings, say, to turn on the heat
for 1 second and off for 9 seconds on the lowest setting, and on for 9
seconds and off for 1 sec on the second highest setting. But I'm only
guessing. AFAICS, a dimmer control would be unnecessary due to the
thermal inertia (?) of the heating element, and it would also generate
EMI.

- Franc Zabkar
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