Originally Posted by alien
If you are mounting the heating element to the fridge itself, you could just use an aluminium clad resistor.
Let's say you want 20 W of heat. Get an old laptop power supply. Let's say it provides up to 3 A at 12 VDC. The first thing you need to calculate is how much current you will need at 12 V to supply 20 W of heat:
P = I * V (Power = current * voltage)
So to provide 20 W of heat you will need 20/12 = 1.66 A at 12 V. That is less than 3 A so the power supply can provide it. Next you need to know what value of resistor to choose.
V = I * R (Voltage drop = current * resistance)
You want a resistor that will drop 12 V when passing 1.66 A of current through it, so it will need to be 12 / 1.66 = 7.2 Ohms. Buy an aluminium clad resistor rated as close as possible to 7.2 Ohms that can handle at least 20 W. In this case, the closest you value you can easily get is 6.8 Ohms at 25 W. That's OK, but 6.8 Ohms at 50 W would give you more of a margin.
Now to get your 20 W of heat (actually 21.2 W) just connect your 6.8 Ohm resistor across the terminals of the 12 V power supply.
Don't hook up a resistor to directly to the mains electricity. Use a low voltage power supply.