It is an electric resistant heating element clipped to the evaporator in frost-free refrigerators and freezers. The purpose of the defrost heater is to melt the frost accumulated on the evaporator surface during the run cycle.
A timer takes the unit into the defrost mode, causing the defrost heater to become energized and melt the frost accumulated on the evaporator by producing the necessary heat. It stays energized until the evaporator reaches a temperature of about 50°F (10°C).
At this point, a termination bimetal senses the rise in temperature and disrupts the flow of current to the defrost heater, even before the defrost cycle ends. In many models, a length of electrical resistance heating element is also installed on the inlet of the drainpipe. It is energized during the defrost cycle to prevent condensation from the evaporator from freezing and blocking the flow down the drain tube.
In cycle-defrost refrigerators, an electric resistant heating element (wired in series with the compressor) is clipped to the inlet and outlet tubing of the fresh-food evaporator to defrost the frozen food evaporator during the off cycles (see fig. 5). The heater is energized when the temperature control is satisfied.