flame sensor, how does it work?

[ghaddox]

how exactly does a flame sensor work? NOT the old MFS but the single wire ones.

[bb]

Do a search for flame sensor.

Here's one to get you started
http://hvac-talk.com/vbb/showthread.php?threadid=45541

[spotts]

if flame is touching it, elect. passes through the carbon in the flame, back to the burner.... Flame sensed.

If electricity doesn't leave that rod due to no flame or corrosion insulating it, flame not sensed.

[Carnak]

The flame rod is small compared to the burner.

Alternating current goes to the rod and it will flow through the flame by "ionization" to the burner head.

Because the current alternates, current is trying to go back from the burner to the rod.

It is easy for the little rod to "hit" the big burner with current however it is hard for the big burner to "hit" the little rod with current.

Because of this, current tends to only travel from the rod to the burner. It is like a half wave rectifier, similar to putting AC current through a diode and only getting pulsating direct current out.

If the flame signal monitored by the gas control was alternating, it would shut off as it would mean there was a short.