Thermal delay demand defrost system

Claim 1

A defrost control system including a timing circuit for a refrigerator of the type having a cabinet containing feezer and fresh food storage compartments separated by a partition; an inlet and outlet passageway in said partition; an evaporator chamber associated with said freezer compartment including an evaporator; air supply means circulating freezing compartment air over said evaporator and for directing a portion of said air through said fresh food compartment between said inlet and outlet passageways; a refrigerator compressor; a defrost heater; a thermostatic switch responsive to the temperature in said fresh food compartment for energizing said compressor; a defrost timing motor connected in series with said thermostatic switch; a defrost switch means in series with said thermostatic switch being operable by said defrost timer after a normal period of timer operation from a first switch position to complete a circuit to said compressor to a second switch position to complete a circuit to said defrost heater; a defrost frequency control means arranged in the path of air passing through said partition outlet for sensing the temperature of air passing therethrough, including a temperature responsive switch means in said defrost frequency control being operable when said thermostatic switch is closed and said air supply means is circulating relatively warm fresh food compartment air through said outlet to energize said defrost timer, and being inoperable when said thermostatic switch is open and said air supply means is inoperative to open as colder freezer air back flows through said outlet to delay energization of said defrost timer motor during initial operation of said fan means to alter said normal period of timer operation until said temperature responsive switch once again senses the relatively warm fresh food compartment air passing through said outlet.