controller is for gas furnaces and ours started to intermittently not
function in 2020. I decided to buy a used spare and investigated
how to test it.
Test setup on my bench.
test setup is shown above. At the top is the meter on the gas
valve output. Going clockwise from there, you see a timer for measuring the
delays, on the bottom is a neon night light on the igniter output, and
then on the bottom left is the lamp on the draft inducer fan output.
Above the controller you see the flame simulator consisting of a
diode and 1 Meg resistor.
To get the controller to not throw an
error code, I had to jumper closed the "Roll Out" switch and the "High
Limit" switch. That tests those circuits to make sure they work. Then once I closed the thermostat contacts to call
for heat (causing power to flow the Inducer Motor output), I had to jumper the
"Pressure Switch". This indicates good vacuum in the exhaust duct
and that the Inducer Motor is running. Closing the Pressure Switch early causes an error. Then after about 40
seconds, the igniter is powered up. After another delay of about
10 seconds, the Igniter is powered and then the controller looks for
a good flame. If that is not detected, the Gas Valve is powered
down. By putting the flame simulator in circuit at the correct time, the Gas Valve is held open.
Video of the furnace controller being tested on my workbench.
Flame sensor One interesting aspect of this controller is the flame sensor, which uses flame rectification
to operate. The hardware is a simple metal probe that is hit by
the flame, and an AC voltage is applied to it. The presence of a
flame causes current to flow in one way from the sensor to chassis, and depresses
the positive peak voltage.
Waveform on the flame sensor when a flame is present.
no flame is present, the voltage on the sensor swings from -115Vp to
+115Vp. However due to the output impedance of the circuit and
the flame, the swing lowers to -127V to +83V. The sensor is the
only part of the controller that needs 120Vac to operate.
I found a circuit that simulates the sensor
in the form of a diode in series with a 1 MegOhm resistor (cathode to
flame sensor lead). With this in place the positive peak swings
to about +63Vp, showing that the output impedance of the flame sensor
circuit is about 1 MegOhm.
- Test of purchased spare furnace controller. Main controller has
worked fine over the past few weeks and I will be watching this