GB1194760A - Defrosting Controller for Electric Refrigerator - Google Patents

Abstract

1,194,760. Control systems for defrosting refrigerators. MATSUSHITA ELECTRONICS CORP. 3 Dec., 1968 [14 Dec., 1967], No. 57286/68. Heading H2H. [Also in Division F4] A defrosting arrangement for an electric refrigerator comprises means for terminating the defrosting process when the frost in the freezer has been dispersed, the means for dispersing the frost being either an electric heater or a hot gas device. An electric heater 3 in a defrosting arrangement is connected to an A.C. supply by means of a thyristor 4, the thyristor being made conductive by closure of a switch 13 and non- conductive when the resistance of a thermistor 7 rises due to the absence of frost in the freezer. A transistor 5 is normally kept in conduction due to the bias voltage derived from a potential divider 9, 10 connected across the normally non-conductive thyristor 4 rectified by a diode 11 and charging a capacitor 12. To initiate a defrosting cycle the switch 13 is closed, either manually or by a timing device. Capacitor 12 then discharges through the switch 12 thereby cutting off the transistor 5. A signal thus appears at the collector of the transistor which signal is used to trigger the thyristor 4 into conduction to energize the defrosting heater 3. When the frost in the freezer is removed, the freezer temperature rises so decreasing the resistance of the thermistor 7. As the resistance of the thermistor gradually decreases the capacitor 12 begins to acquire a charge until a point is reached where the transistor 5 is once more rendered conductive whereupon the trigger signal to the thyristor 4 disappears and the heater 3 de-energized.