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Honeywell furnace flame detector

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Not sure this is the right place...
Does anyone know how a Honeywell flame detector on a gas furnace works?  I had one go flaky today.  There's a single stainless-looking rod near the burner, in an area where flame fills the gap between the rod and the burner.  It is obvious it is some kind of thermionic scheme where it detects the electrical conductivity of the flame.  But, what sort of voltages are used and what currents flow?
I cleaned the rod and moved it a bit closer to the flame and metal parts, and it seemed to work much better then.  Before it got a flickering yellow "flame" LED,
after cleaning, it got a couple blinks and then went to a solid-on LED, so I could clearly tell I had improved the operation.
Thanks for any insight on how this works.  Google searches didn't show any background theory.

I believe the basic idea is that a small AC bias is put on the sensor rod and ionization allows a small DC current to flow.  I don't remember the exact process, but the flame ionization acts as a sort of diode.  The currents involved are single digit or tens of microamps--my HVAC-specific Fluke 116 has a high-impedance 600uA range for this specific purpose.  This is a different device than the thermocouple flame sensors that you see on self-contained (no power) gas control valves.

The phrase to search for is flame rectification sensor.  They apply a current limited AC voltage, something like 50 VAC I think?  Then measure the DC current flow.

Here is a decent description of a commercial system where the voltages and currents are much higher but the principal of operation is the same. Most YouTube/diy technical info is a joke.

Residential sensors produce 5-10 uA from ~110 volts.  Clean with ScotchBrite as sandpaper and similar abrasives may leave silica on the probe which melts and then insulates the tip, rendering it DOA.

Generally speaking, you are applying HV AC to the kanthal rod and looking for the rectification property of the flame. This is thermionic.
Excitation is around 50-100VAC and very low current under 10uA. Older equipment used mains sine-wave and modern gear uses a square-wave driven by MCU as safety codes require a lot of self-test for flame detection circuit.

The rod must not have soot etc. buildup which is easily cleaned off with steel wool. Also, any leakage current to ground due to rust or soot, or shoddy/melted/wet wiring or cracked porcelain can be a problem. Note this leakage does not count as a false-positive for flame present because it's just an ohmic resistance seen and not rectification.

If you're having problems, check the rod is clean with no leakage current path, and the pilot flame is centered on it. Not to the side or lifting up off the burner, which happens if the pilot burner is dirty or backdraft or chimney/negative pressure in the building issue. The flame detector can be working fine but if the flame is poor, flickering, off to the side of the rod etc. due to ash/soot buildup, it will constantly trip out.

If you are very careful and need to spoof flame-present, then use a 1N4007+10MEG series resistor switched in at the moment when the controller needs to see flame, and of course manually light the pilot. I do this testing burner firmware. It's a dangerous procedure.

There's two types, as far as a single rod - flame sensor, or igniter+flame sense function. If it's the combo function, the igniter will put many kV at the flame rod and the flame-sense circuit portion has clamp diodes so it does not get damaged during ignition.


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