The purpose of a defrost heater is to remove the frost that builds up on a refrigerator evaporator during the cooling cycle. The frost build up is simply humidity from the air that freezes on the evaporator. If the frost is not removed during the defrost cycle, the frost will choke the air flow and cooling will be diminished, especially in the fresh food section. That’s because on most refrigerators, there is only one cooling evaporator, and that is in the freezer. A fan blows air through the evaporator, into the fresh food section. When the defrost heater turns on a resistance wire usually encased inside a metal tube resists the flow of electricity causing friction and generating heat.
Electricity to the defrost heater is controlled either buy a defrost timer, or a defrost control which is usually an electronic control. In series with the electric circuit to the defrost heater is some sort of defrost termination device. The defrost termination device is sometimes called a defrost thermostat or defrost terminator. It’s main purpose is to prevent the heater from generating heat in the freezer section in the event the evaporator is not frosted over and defrosting it is not necessary. In addition to that the defrost terminator will interrupt the voltage to the defrost heater whenever the temperature near the evaporator exceeds its rated amount. When the defrost heater is on the frost on it changes from a solid to a liquid (water) and flows down a tube that runs down to a drain pan located in the base of the refrigerator.
More info: http://www.dallasappliancerepairllc.com/