AM Detector, A Real Hot Performer

[Yorkie]

The phenomenon of flame rectification is used as a flame failure device in modern gas central heating boilers, which have no permanent pilot. Older boilers used a thermocouple, which - when heated by a gas pilot flame - generates a minute amount of current to energise an electro-magnetic valve to allow gas to flow to the main burner. (As anyone who has such a boiler will know, the current isn't sufficient to pull the valve open - it has to be held open manually until the thermocouple generates enough current to hold the valve in the open position. If the permanent pilot light fails, the thermocouple/gas valve is de-energised and in a few seconds, gas is shut off to the main burner.

Modern boilers don't have a wasteful permanent pilot. The sequence of event is that when the boiler calls for heat, gas is released to the pilot, a spark igniter lights the pilot, whereupon a minute AC current passes through the flame, where it is rectified to supply a DC voltage to the flame sensing circuit, which then allows gas to flow to the main burner. Should anything bridge the electrodes, (a flake of metal for example from the heat exchanger) only AC would flow to the flame detection circuit, which would not therefore allow the gas valve to the main burner to open.

Simple really, but effective.

(Gas appliances such as hobs, oven and fires invariably rely on thermocouple flame failure devices as they don't need a permanent pilot, being manually operated each time they're used).