Three pin refrigerator thermostats?

[Davej]

Can anyone explain the internal circuit of a 3-pin refrigerator thermostat such as a WDF-18 ? Thanks.

[Rod Speed]

Davej <galt [e-mail] wrote

One terminal is the common. That is connected to one or other of the other two terminals depending on whether the temperature is above or below the set point.

[Davej]

Oh, so what would a refrigerator need to apply power to when it shuts off the compressor?

[Rod Speed]

Davej <galt [e-mail] wrote

It doesn’t, its just the way thermostats are usually done.

You see the same thing with oven thermostats too.

Its more obviously useful with room thermostats.

[Davej]

Hmmm, I've just removed one from a dorm-sized fridge and there was a wire on every terminal.

[Rod Speed]

Some thermostats do have an internal heater. That reduces the hysteresis in the thermostat. Bit counter intuitive why that approach works.

Now that you have it out, easy to use a multimeter to see how that one uses the terminals.

[trader_4]

Not likely, I bet there is a wire connected to all three terminals. It may be part of a design so that the defrost heater is only powered when the compressor is off.

It's not useful there, of all places. I never had a thermostat that was more than a simple on/off switch for a home heating system. Some have two stages, but that's another story.