Bimetal Disc Thermostat APPLICATION NOTES

Bimetal Disc Thermostat APPLICATION NOTES

Operating Principle

Bimetal disc thermostats are thermally actuated switches. When the bimetal disc is exposed to its

predetermined calibration temperature, it snaps and either opens or closes a set of contacts. This

breaks or completes the electrical circuit that has been applied to the thermostat.

There are three basic types of thermostat switch actions:

• Automatic Reset: This type of control can be built to either open or close its electrical contacts

as the temperature increases. Once the temperature of the bimetal disc has returned to the

specified reset temperature, the contacts will automatically return to their original state.

• Manual Reset: This type of control is available only with electrical contacts that open as the

temperature increases. The contacts may be reset by manually pushing on the reset button

after the control has cooled below the open temperature calibration.

• Single Operation: This type of control is available only with electrical contacts that open as the

temperature increases. Once the electrical contacts have opened, they will not automatically

reclose unless the ambient that the disc senses drops to a temperature well below room

temperature (typically below -31°F).

Temperature Sensing & Response

Many factors can affect how a thermostat senses and responds to temperature changes in an

application. Typical factors include, but are not limited to, the following:

• Mass of the thermostat

• Switch head ambient temperature. The “switch head” is the plastic or ceramic body and terminal

area of the thermostat. It does not include the sensing area.

• Air flow across the sensing surface or sensing area. The “sensing surface” (or area) consists of

the bimetal disc and metal disc housing

• Air flow across the switch head of the thermostat

Sensing surface of

thermostat

Switch head portion

of thermostat

• Internal heating from carrying the application electrical load

• Disc cup or housing type (i.e. enclosed, as on left in picture below, or exposed, as on right)

• Rate of temperature rise and fall in the application

• Intimacy of contact between the thermostat sensing surface and the surface it is mounted on.