rectification

stevetheplumber · Dec 15, 2013

just want toi run this by you got a water heater in the van that goes to lock out its out and on the bench burner lights but ignitor continues to spark and goes to lock out in about 5 seconds ignition is obviously working so its a flame sensing problem
as there is just a single probe on the sensing side should shorting it to the burner with a screwdriver complete the circuit and hold the gas valve in ? or should shorting it cause the flame to drop out ?actually reading this ive probably answered my own question as it seems to do neither thoughts?

Masood · Dec 15, 2013

I'd say shorting it should complete the circuit, but I remember from the Baxi course that the PCB expects a current in the micro amps range, so if you short with a screwdriver which has a lot less resistance than a thin cable, the PCB might not accept this. The reason for having this logic is that if the sensor is bent and touches earth, it could hold the valve open.

It may be total rubbish but I don't know enough electronics to know.

neily01 · Dec 15, 2013

Flame rec can be ac or dc if its ac the clever little buggers know its shorted so wont work, the flame will only pass a part of the ac wave pattern. If it gets the full wave pattern it wont work which is why they changed from a dc current to an ac current as the probe could be bridged by a bit of debris and think its working safely

Masood · Dec 15, 2013

neily01 said:
Flame rec can be ac or dc if its ac the clever little buggers know its shorted so wont work, the flame will only pass a part of the ac wave pattern. If it gets the full wave pattern it wont work which is why they changed from a dc current to an ac current as the probe could be bridged by a bit of debris and think its working safely

Ah yes, the bit about debris also rings a bell. Basically the trainer was telling as that a logic circuit built in as a safety device to prevent accidental rectification.

Roger Welton · Dec 15, 2013

IIRC if its just a flame ionisation probe then shorting out the probe will work, if it is flame rectification then it wont as it is looking for a change from ac to dc on the return path.