Furnace control apparatus

摘要

An electric control for gas furnaces which controls a two speed main blower fan and an induction draft fan based on inputs from a room thermostat, a high limit and an ignition control including a gas valve. The control has a circuit board having a power supply for providing 24 volts DC current source to drive DC relays and a 5 volt DC power source to power a microprocessor. 24 volt AC input signals are coupled to the input ports of the microprocessor through current limiting resistors and to AC ground through pull down resistors. AC ground is also connected to the IRQ port of the microprocessor. The output ports of the microprocessor are connected to a relay driver which in turn is connected to the relays. Several breakaway tabs in the board provide optional features such as eliminating a normally provided draft delay timing function. Test pads are provided on the board so that the board can be tested during manufacture. An optional feature is shown comprising an LED which can be used to indicate the status of the system. Another optional feature incorporates a zener diode and resistor coupled to each input port to increase input thresholds. This feature is provided for use with power stealing electronic thermostats. The control calibrates itself on a continuing periodic basis to read the AC inputs synchronously at the peak of their wave and switches the relays asynchronously based on the real time clock of the microprocessor.