Design of Defrosting Circui七sfor Refrigerators

The present report deals with two types of transducers for detecting the frost，which are the most important part of the defrosting circuits in refrigerators. These transducers are based upon the following principles. (1)The electric resistance between two parallel plate conduc tors set up with the cooling pipe is decreased by the frost grown between them. The variation of this resistance is measured as the voltage drop across the load resistor which is connected in series with this transducer, (2)The voltage drop across the load resistor connected in series with a photo transistor is decreased because the light path between the 1ight emitting diode and the photo transistor is filled with the frost. The comparator output signal，depending on whether the voltage drop across the load resistor is larger or smaller than the reference voltage，is used as a control signal for TRIAC switches