When opening the refrigerator and freezer doors repeatedly to add and remove food you also let warm moist air enter the interior compartment. The humidity from this warm air gets attracted the refrigerator evaporator coils and will freeze directly to the coils. This will form a uniform frost layer to the outside of coils. If too much frost builds up on these coils, airflow will be restricted and the evaporator cannot absorb heat from the interior compartment. The result will be the freezer operating at a lower temperature and a partial or full restriction in cold air flow to the refrigerator.
To prevent too much frost from collecting on the evaporator coils, a self-defrosting refrigerator will actually stop itself for approx 20 minutes every six to twelve hours to melt off this frost. This defrost cycle is switched off and on by an electric defrost timer or electronic ADC board. The timer stops the compressor and switches current to an electric defrost heater located directly beneath most evaporator coils. When the heater is energized the heat is radiated upwards to melt the frost. The frost water drains into a pan located at the bottom of the evaporator and is diverted through a tube into little pan which is located on the bottom of most refrigerators. Most Maytag, Kenmore, Whirlpool, GE, and Frigidaire models will have this type of system. The condenser fan circulates warm air across the water filled pan and evaporates the drain water before the next defrost cycle.
If all the frost melts before the defrost timer finishes the defrost cycle, a terminating thermostat, or bimetal, will keep the defrost heater from overheating the evaporator compartment. When the freezer compartment reaches a fixed temperature, the terminating thermostat will open the circuit and shut off the electricity to the defrost heater.
Many of the later electronic refrigerators incorporate an electronic device called an adaptive defrost control board. This electronic control board provides the same function of the defrost timer but adds a new element to save energy and to adjust the defrost cycles to “adapt” to customer usage. The adaptive defrost board monitors how many times the door is opened and closed. The more the refrigerator doors are opened and closed, the more humidity has entered the compartment. The more humidity that has entered the refrigerator the more frost that has accumulated thus the “period” between each defrost cycle will be reduced.