a fail - safe electronically controlled defrost system is provided for use in a frost - free refrigerator suitable for home use . the defrost cycle , in the interests of power conservation , is made subject to plural input electronic control in a circuit in which the intrinsic reliability of the defrost system is specifically designed to revert to a non - power conservation mode in the event of failure of the electronics . compressor time , door openings and user settings are typical inputs for control of the period between defrosts .