What is a Defrost Bi-Metal Thermostat?

...
How Do Defrost Bi-Metal Thermostats Work?
...
A bimetal strip is a laminate of two different metals.

Did you ever wonder why your frost free freezer is indeed frost free? It's because inside every one of these appliances is a heater. This heater melts any frost before it can accumulate and evaporates the moisture away. The little device that makes this possible is the bi-metal, or defrost, thermostat. It is called a bi-metal thermostat because at its heart is a strip made of two metals with different expansion rates, which when heated begins to curve. This property is used to make a switch which opens with a temperature change. This switch operates the heater.

Where Does Frost Come From?

...

All that pesky frost comes from the moisture in the air, which condenses out when the air is cooled. The more times the door is opened, especially in the summer time, the greater the potential for frost. In the old days, before frost free freezers, this ice would build up to the point where a freezer door could not be shut. When this happened it was necessary to remove everything from the freezer and melt the ice away.

Defrosting Cycle

...

Working in conjunction with a timer the defrost thermostat controls the heater which turns on briefly when the bi-metal strip closes the circuit. The duration of the heat cycle is controlled by the timer. This action is not triggered by the frost itself, but rather by the freezer temperature. Even if there is actually no frost present the defrost cycle takes place.