A refrigerator thermostat has only one function -- controlling the refrigerator's cooling system by turning it on or off to ensure that the temperature inside the fridge stays in the correct range of 33 to 40 degrees Fahrenheit. In frost-free models, a separate thermostat with a timer controls the defrosting cycle. Thermostats typically are located inside the fridge behind the interior walls, with a sensor extending into the cooling compartment. There are three general types -- vapor pressure, bimetallic and solid state.
Vapor Pressure Models
Vapor pressure thermostats are electromechanical devices that sense temperature with a bulb filled with a volatile liquid such as alcohol that expands and contracts in volume as temperature changes. The bulb is connected to a diaphragm by a thin capillary tube. As temperature rises, the fluid expands and pushes out the diaphragm, which closes an electrical switch that starts the cooling system. As the temperature drops, the liquid volume contracts, pulling in the diaphragm and opening the switch, turning off the cooling system.
Bimetallic thermostats use a sensing strip composed of two dissimilar metals that expand and contract at different rates. One metal is very sensitive to temperature changes, and the other is not. A rise in temperature causes the strip to warp in one direction to close an electrical switch and start the refrigeration system. As the temperature falls, the strip warps in the opposite direction, opening the switch and turning off the cooling system.
Solid State Units
Solid state thermostats, also known as electronic or digital thermostats, use an electronic element that becomes more or less electrically conductive as the temperature changes. According to Invensys Controls, the unit also includes a microprocessor that compares the sensing element's readings to the desired temperature setting. The microprocessor then sends a command to the refrigeration system to turn on to lower the temperature and turn off when the temperature is sufficiently low.
A refrigerator thermostat normally gives many years of trouble-free service, but these units can go bad. Thermostats allow you to turn a knob to adjust the interior temperature but a failing thermostat won't be able to keep your fridge in the correct temperature range no matter how you set it. The unit will either keep the refrigerator running too long, causing cold food to freeze. Or it won't run the refrigerator often enough to keep the interior temperature below 40 degrees. In some cases the thermostat will just stop working altogether. A defective thermostat can't be repaired. It must be replaced.