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From: olney@fortune.UUCP (John Olney)
Newsgroups: net.misc,net.physics
Subject: Re: Why don't thermostats work?
Message-ID: <2688@fortune.UUCP>
Date: Mon, 5-Mar-84 19:30:45 EST
Article-I.D.: fortune.2688
Posted: Mon Mar  5 19:30:45 1984
Date-Received: Tue, 6-Mar-84 03:48:30 EST
References: <213@unisoft.UUCP>, <244@heurikon.UUCP>, <4376@amd70.UUCP>, <212@cbneb.UUCP>
Organization: Fortune Systems, Redwood City, CA
Lines: 30

Here's an analysis of how an electric blanket works
that I haven't seen on the net yet.
(Of course, I've only been on the net about a week....)

The temperature of the blanket is actually regulated in the control box;
it works on the same principle as modern electric stoves and u-wave ovens.
I don't know what the technical term is, but it's like a switching
power supply, switching at a very low frequency.  (It's also like a
D-class audio amplifier, using pulse width modulation.)

If you want your bed to be very warm, the control box keeps the heating wires
going most of the time.  If you want to be a little bit less warm, the box
keeps the wires going a little bit less of the time.  If you want to be
relatively cool, the heating wires are turned on every once in a while.

It's a duty-cycle kind of thing:  the more time that the control box
lets current flow through the wires, the warmer the blanket gets.
The actual current stays constant; it's the time span that counts.
The person who dissected his control box
and found a heating coil surrounding a bimetal strip
was looking at the on/off timer.

The thermostats inside the blanket are for safety.
If any portion of the blanket gets too hot,
the internal thermostats will cut it off.
I think this is mandated by the Federal Government.

Regards to those in netland.

	-- jho