Patent ID: 6851270

Claim:
A refrigeration control for a refrigeration system having a compressor, an evaporator, a freezer compartment and a defrost heater comprising a microprocessor, a first temperature sensor thermally coupled to the freezer compartment, the first temperature sensor having a first temperature responsive electrical signal fed to the microprocessor, a second temperature sensor thermally coupled to the evaporator, the second temperature responsive sensor having a second temperature responsive electrical signal fed to the microprocessor, the system being operated in a refrigeration cycle having a cooling mode and a defrost mode, the compressor being connected to line voltage when the temperature of the first temperature sensor rises above a first threshold to operate in the cooling mode for a cumulative compressor run time, an electronic switch for connecting the defrost heater to a voltage source and a zero-crossing detector network coupled to the voltage source and having an input to the microprocessor, the microprocessor programmed to monitor the first and second temperature responsive signals and based upon such signals and zero-crossing detection to energize the electronic switch to provide power to the defrost heater for a defrost cycle, the cumulative compressor run time of a given refrigeration cycle being determined by the length of time the defrost heater was energized in the immediately preceding defrost cycle.