AM Detector, A Real Hot Performer

[pwdrive]

http://www.youtube.com/watch?v=jXRtLhuYcIo

And here's a multivib. (towards the end of the video)

http://www.youtube.com/watch?v=WNl0sFViaxs

Lawrence.

[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).

[pwdrive]

Brings a new meaning to the hot carrier diode..

Lawrence.

[ppppenguin]

The cost of a running a permanent pilot is more than you might expect. I haven't got the figures to hand but ISTR it was of the order of £50 per year at current gas prices. I realise that while the main burner is running, it contributes towards useful heating and may contribute a small amoutn at other times but it's still a waste.

[Yorkie]

The cost of a running a permanent pilot is more than you might expect. I haven't got the figures to hand but ISTR it was of the order of £50 per year at current gas prices. I realise that while the main burner is running, it contributes towards useful heating and may contribute a small amoutn at other times but it's still a waste.

Yes, it's easily £50 a year and it's wasteful. It contributes little in the way of useful heat because it simply sets up a convection current in the boiler heat exchanger, the heat rises and introduces cold air into the boiler combustion chamber via the flue. Also, many boilers (mine included) are in locations where any adventitious heat given off by the pilot is in an area not occupied - utility room, garage, roof space etc.

Under Gordon Brown's short-lived 'greenwashing' £400 grant scheme to upgrade older boilers to a modern condensing boiler, if your boiler had a permanent pilot it qualified, as did ours. Before the old boiler was removed I asked the guy to test its efficiency, which was measured at 58% so for every £100 we spent on gas, £48 was going out into fresh air, plus the amount wasted by the pilot. The Worcester-Bosch Greenstar that replaced it was 96% when tested. We don't economise on heating - 20C when we're out and about, and we put it up to 22C when we come home. (£200 'old fogey' winter fuel allowance goes a long way towards our winter heating costs).

I find it surreal that back in the mid 1960s when gas central heating began to become popular, at BG we used to design systems to provide 15C in living rooms, (60F) and 13C (55F) in bedrooms, halls etc. Do what?!! Customers used to be more bothered that it would be 'too hot and stuffy' rather than not hot enough. ('"we only want it in the background"). Back then we lived in Nottingham, which had five open air unheated lidos which were open from March till October! How times change - the lidos are all long gone of course.

[pwdrive]

60F was plenty back then when eating dripping butties....

Lawrence.

[AlanBeckett]

It was usual, as in two of our houses of that era, not to bother heating the upstairs at all.

Alan

[Joe]

My boiler is oil and flame failure is detected by a photocell.

My bedroom has a TRV but the radiators in the other two are turned off. The front is south-facing so gets plenty of sun and the back has the chimney breast in. Both get some heat rising from below as well.

- Joe

[Nick]

Base URL for flame triode: http://www.sparkbangbuzz.com/flame-amp/flameamp.htm

[pwdrive]

Not sure when the phenomenon was first observed but Lee De Forest was aware of it I think.

Lawrence.