US 5,397,970 AGranted

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

AssigneeTexas Instruments Incorporated

InventorsMitchell R. Rowlette, Darryel L. Price, Craig M. Nold

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Abstract

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.

An interface circuit for a variable speed electrically commutated fan motor comprising a low voltage terminal board having a plurality of terminals, a first terminal for a 24 volt AC compressor energization signal, a second for a 24 volt continuous fan energization signal, and at least one terminal for a 24 volt AC electric heat stage energization signal, a power supply for providing a 24 volt DC source and a 5 volt DC source,

a connector having an input from the electric heat stage energization signal and having a 240 volt AC line output, the 240 volt AC line being energized whenever the electric heat stage is energized,
a first optically coupled isolator having an input coupled to the 240 volt AC line output of the connector, the output of the first isolator being connected to the 5 volt DC source and the base of a transistor the main electrodes of which are coupled to the 24 volt DC source and to a medium speed terminal of the motor so that when the first optically coupled isolator is energized it connects the 5 volt DC source to the base of the transistor turning on the transistor and energizing the medium speed terminal,
a second optically coupled isolator having an input coupled to the continuous fan energization signal line, the output of the second isolator being connected to the 5 volt DC source and the base of a second transistor the main electrodes of which are coupled to the 24 volt DC source and to a low speed, on/off terminal of the motor so that when the second optically coupled isolator is energized it connects the 5 volt DC source to the base of the second transistor turning on the second transistor and energizing the low speed terminal, and
a third optically coupled isolator having an input coupled to the compressor energization signal line, the output of the isolator being connected to the 5 volt DC source and being coupled to the base of a third transistor the main electrodes of which are coupled to the 24 volt DC source and to a high speed terminal of the motor so that when the third optically coupled isolator is energized it causes the third transistor to turn on energizing the high speed terminal.

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Abstract

Claims12