Refrigerator defrost control with control of time interval between defrost cycles

Claim 1

In a self-defrosting refrigerator having at least one food storage compartment with an access door thereto and a refrigerant system including a motor-operated compressor, an evaporator for cooling the food storage compartments, thermostatic means for turning the compressor on and off to maintain a desired temperature in the food compartments, and defrost heater means for periodically removing frost build-up from the evaporator, the combination comprising: timer operated switch means responsive to the thermostatic means (a) during the compressor-on periods to establish a minimum compressor run time between defrost cycles and (b) during the defrost cycle to activate the defrost heater means for a predetermined maximum defrost period; means for accumulating total compressor run time during the entire elapsed time between defrost cycles; a door open sensor; means responsive to the door open sensor for accumulating the amount of time the door is open at least up to a predetermined amount of door-open time; decision means for providing a first control effect representative of when the accumulation of compressor run time reaches the minimum run time of the timer means and a second control effect representative of when the predetermined amount of accumulated door-open time is reached; and timer control switch means coupled in electrical circuit with the timer means and effective in response to the decision means to interrupt operation of the timer means in response to the sole presence of the first control effect, and to initiate or continue operation of the timer means in response to the simultaneous presence of both the first and second control effects; whereby elapsed time between defrost cycles is extended beyond a minimum compressor run time during relatively light usage of the refrigerator wherein usage is measured by door open time.