Also known as a termination switch or defrost thermostat. (Not to be confused with the cold-control thermostat discussed earlier.)
In frost-free refrigerators and freezers, every six, eight, or twelve hours (depending on the type of defrost timer being used), a timer de-energizes the compressor and the evaporator fan and, at the same time, energizes an electric resistance heater which is clipped to the underside of the evaporator fin area to defrost the evaporator for no longer than thirty minutes. A defrost thermostat is wired in series with the electric heater. It is clipped to the evaporator. When the temperature of the evaporator rises to 50°F (10°C), the defrost thermostat opens the electrical circuit to the heater to end defrosting. Contacts within the defrost bimetal close at 20°F (-7°C). No later than thirty minutes after the beginning of the defrost cycle, the timer takes the unit into the cooling cycle—at which time the operation of the compressor and the evaporator fan is restored—and the electrical circuit to the heater is opened.
Defrost bimetals play a very important role as the second component to control the defrost heater. If the heater is not de-energized, the excessive heat can cause damage to the unit. If it is not energized, the unit will no longer defrost. Often a bad defrost bimetal is mistaken for a bad defrost heater.