What is a Thermostat?

This is a switch which is either opened or closed by temperature change. The most common thermostat found in home appliances is the type known as a bimetal thermostat, so-called because of the way in which the mechanical action to open and close switch contacts is derived. In a bimetal thermostat, two dissimilar metals are bonded together. As they are heated or cooled, the differences in their rate of expansion cause a bending or warping action of the piece of metal. This mechanical action is used to operate switch contacts.

Bimetal thermostats are found in a large variety of shapes and sizes and temperature ranges. A few common applications are found in con­trolling heat in electric irons, automatic toasters, electric water heaters, clothes dryers, and motor protecting devices.

In testing a thermostat, it is imperative to know the temperature at which the switch should actuate and also know whether you should read that the switch is closed or open when you put your VOM on it. Specifications are found in service manuals, but in the case of fixed temperature thermostats, the temperature point will usually be stamped on the body of the thermostat. For instance, L 150 found on the body of a bimetal thermo­ stat would indicate that this thermostat opens at 150 degrees.

Another class of thermostat is usually adjustable, the kind found on stoves and refrigerators. These are capillary tube or hydraulic ‘stats.

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