Interface circuit having improved isolation among signals for use with a variable speed electrically commutated fan motor

摘要

A low voltage terminal board interfaces a fan motor with a field thermostat. The board includes a power supply providing a 24 volt DC power source for driving an electronic air cleaner relay and fan motor control lines and a 5 volt DC power source for providing optical isolation and timing logic. Several heater stage signals are connected to a diode interlock network so that no matter which signal is generated the first electric stage heater is energized. A first optically coupled isolator receives a 240 volt AC input whenever the first stage electric heater is energized with the isolator output controlling energization of a heat or medium speed operation of the fan motor. Second and third optically coupled isolators receive a 24 volt AC input indicative of continuous fan operation and compressor operation respectively with the outputs controlling energization of low (on/off) fan speed operation and high speed fan operation respectively. A timing circuit is interconnected between the third optically coupled isolator and the high speed fan terminal to provide a delayed off period of fan operation. Diode interconnections ensure that the on/off terminal is energized anytime the high speed terminal or the first stage electric heater is energized and anytime the on/off line is energized the electronic air cleaner relay is energized.