A New Demand Defrost Control for Domestic Forced Draft Refrigerator/Freezers and Freezers

The control described is based on the combination of three techniques to develop an accurate low-cost unit responsive to frost loading on evaporator coils. For sensing, a fluidic type sensor monitors the air pressure drop across the evaporator, responding to signals in the order of 0.1 to 0.5 in of water. To detect the signal from the sensor, a simple yet highly accurate and reliable transistor bridge circuit is used. A motor driven timer, which integrates the detected signal, provides a uniform period for defrost and run off, and switches the relatively high currents of the defrost and compressor circuits. The use of this system can extend the interdefrost interval in low humidity conditions by a factor of 3 or 4.