The General Model 50A50-110 

Furnace Controller

Model 50A50-110

This 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
Test setup on my bench.

The 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.

Flame sensor
Waveform on the flame sensor when a flame is present.

When 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.

  • 12/20/2020 - Test of purchased spare furnace controller.  Main controller has worked fine over the past few weeks and I will be watching this situation.
  • Hookup diagram inside the cover of the furnace.  Error codes:
    • 1 Flash - System Lockout due to retry
    • 2 Flash - Pressure Switch contacts closed
    • 3 Flash - Pressure Switch contacts open
    • 4 Flash - Open High Limit switch
    • 5 Flash - Open Roll Out switch
    • 6 Flash - Incorrect 120V grounding
    • Continuous flash - Flame sensed with no call for heat
  • Closeup of jumpers I used on the controller to cause it to run.
  • Photo of board.

Back Home

(c) 2020 Edward Cheung, all rights reserved.