Timed-off control apparatus and method

Abstract

An improved timed-off control apparatus and method are provided to prevent excessive cycling of an electrically operated device, such as an air conditioning compressor. The timed-off control apparatus provides a predetermined timed-off period (e.g., five minutes), during which the electrically operated device is constrained to remain in an off state after the device is deactivated by a loss of power. For example, in an air conditioning system, fluctuations in power supply voltage and thermostat switch bounces may result in excessive cycling of the system compressor. By constraining the compressor to remain deactivated for the selected timed-off period, the compressor is protected against excessive cycling. In order to achieve consistent and predictable timed-off periods, a field effect transistor (FET) is used in conjunction with a resistance/capacitance (RC) circuit. When the power supply is interrupted, the FET is turned off state and remains off during the timed-off period. When the FET is off, the compressor remains off, even if electrical power becomes available during the timed-off period. The FET monitors the voltage across the capacitor of the RC circuit while the capacitor is being discharged and when the voltage drops below a predetermined threshold, the FET is turned on to allow resumption of compressor operation. The time at which the voltage across the capacitor reaches the predetermined threshold is a function of the capacitor discharge rate. Because the FET does not draw biasing current, the capacitor discharge rate is not affected by the FET. Therefore, consistent and predictable timed-off periods are achieved.