harry wrote:
On Saturday, 14 March 2020 13:44:13 UTC, Tim+ wrote:
John Kenyon wrote:
On 14/03/2020 07:51, harry wrote:
On Friday, 13 March 2020 17:40:19 UTC, Tim+ wrote:
Anyone know how these work and why my tumble dryer has a long central
electrode in the flame path with an adjacent earth pin that is angled so
that theres a small gap between the end of the insulated electrode and the
end of the earth pin.

I ask as all the replacement ones I see for sale only have a single
insulated electrode and no earth one. Im assuming that the electronics
for these must be a bit different and a single electrode type wont work in
mine.

Its a White Knight gas tumble dryer. Only 30 years old now so Id like to
get a few more years out of it but Crosslee dont stock spares for this
machine any more and I havent seen anything similar on line.

The present earth pin has suffered some erosion/corrosion and its not
always sensing the flame.

Hopefully, heres a picture. https://photos.app.goo.gl/HGzbwdk6Lw1zMNAb9


The resistance path in a flame is very low.
The probe is measuring the absence or not of the flame by measuring resistance.

Quite often the probe is just a an ordinary spark electrode used for this purpose.


The key thing about a flame sensing electrode+flame is that it acts as a
diode and will only conduct current in one direction.
If it conducted in both directions it wouldn't be possible to discern
between a normal "flame present" or a "flame missing and the electrodes
shorted"


How close does the gap need to be to the flame for this effect to work? The
electrodes are several inches from the burner. I cant view the burner in
operation though to confirm how far the flame extends. I dont think its
far though.

It has to be in the flame. Obviously.

They are usually found in blown gas burners (commercial/industrial)


Well in that case I suspect its not using the diode properties. Im pretty
sure its well out of the flame.

I think when it next plays up Ill just make a new earth point next to the
tip of the present electrode and dispense with the eroded one, possibly
just a long slim nut and bolt through the burner casing.

Tim



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