Electric blanket

Have you ever wonder how an electric blanket works?

Well, Personally I have not wonder that because I Thought had an idea about the basis of this common device. But some days ago somebody give me a small electric blanket and suddenly it stops working. And, you known me, if something can be striped down, it will be striped down!

Here the pics and the things I saw.

Here the innocent blanket… It does not know how lucky it is	The back side of the controller case. Made in China but sold in Japan
And this is the main controller. It has two levels of power. High, low and off.	Here the electronic circuit responsible for controlling the heat by controlling the current on trough heater.
The heater can be seen here. Very nice distributed in all the blanket	And finally, the heater connected to the controller. Very basic, right?

Well, examining more carefully the circuit, I realized it has a Differential comparator, a power resistor of 360 ohms, 10Amp, SCR which is the device responsible to “cut” a portion of the voltage coming from the main power supply (100 V for Japan) and finally some diodes, resistors and capacitors to complete de “magic”.

The main goal of a typical electric blanket controller is to reduce the voltage from 100V to around 20V or 25 volts which is way safer than the initial 100V. This low voltage supply the heater which basically is a special wire that heats when a current flows trough it.

Here is a simple circuit for a electric blanket. You can go to de source of the diagram by clicking on the figure.

The electric blanket is a very old domestic device. Dangerous if used for several long time (usually more than 10 years). Many fires have been started by electric blankets.

Here you can find a patent of the electronic circuit dated in 1971.

Always interesting to know something else, don't you think so?