Single phase compressor thermostat with start relay and motor protection

Abstract

The present invention involves a refrigeration system. The refrigeration system comprises an electrical compressor motor, a line adapted to be connected to an electrical power line and a thermostat control circuit. The thermostat control circuit comprises a thermostat responsive logic circuitry coupled to a temperature sensor associated with the compressor and a solid state motor control circuit. The solid state motor control circuit includes a relay circuitry, power switch circuitry, compressor starting circuitry with an overload timer, high voltage protection circuitry, under voltage protection circuitry and run current limiting circuitry. The relay circuitry includes an optical coupler and triac to provide line voltage to the compressor through a start resistor. The power switch circuitry includes a pair of SCR's connected in parallel as a switch to allow current to flow through the compressor. The overload timer comprising a resistor and capacitor masks the run circuit limiting circuitry during the compressor starting period for about one second. The run current limiting circuitry includes two voltage comparators arranged in parallel to monitor positive and negative parts of the AC current flow through the compressor. Voltage reference circuitry provides a supply voltage and a reference voltage. The reference voltage circuitry includes an AND gate which regulates the supply voltage. The voltage reference circuitry also includes a zener diode to serve as a precise reference for the reference voltage. The high voltage protection circuitry, the under voltage protection circuitry and the run current limiting circuitry share the same reference voltage.