How does a thermocouple have enough power to operate a gas valve?

On older boilers (furnaces if you're American), when the heating isn't actually running (eg. the thermostat says the house is warm enough), there's no power to the boiler, so how does the pilot light valve stay open with the tiny voltage (40mV?) and current from the thermocouple?

Reply to
Bruce Farquhar
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To *hold* the valve open only requires a small voltage & current. To

*pull* the valve open would require a larger voltage. That's why you have to "Press & Hold" the manual knob to restart a pilot.

See here for more detail:

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Reply to
Anonymous

I see, thanks. I thought the "press and hold" was just to keep the valve open until the thermocouple warmed up. So I'm providing the effort to open the valve with my thumb. That link states 0.2-0.25A - do you really get that much current off a thermocouple?

Reply to
Bruce Farquhar

And while the voltage may be tiny, the available current isn't so small

- after all, the source impedance is very low. Still not much power, but enough.

Cheers

Reply to
Clive Arthur

Yes, it's a very low impedance source, a metal to different metal contact. 10mV 200mA is 50 milliohms. It's only 2mW, but that's a very small proportion of the pilot flame power.

Cheers

Reply to
Clive Arthur

I see. I didn't realise you got a decent current off them. I guess it's enough to run a little coil to pull against a small spring which I've already opened with my thumb.

Reply to
Bruce Farquhar

Well they used to have a bimetallic strip to hold the pilot light valve open rather than a thermocouple. Or maybe that's when boilers didn't have an electrical connection at all.

Reply to
Max Demian

When I lived in America, I was confused that what Honeywell called a Thermocouple was actually a bulb and capillary tube.

See:

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They do not use the differential metal concept, but rather pressure due to expanding liquid.

Reply to
Davey

The electricity from the mains supply (on an old basic/system boiler/furnace) is nothing to do with the pilot light, it just operates the main gas valve. The mains connection to my boiler is only on when the room stat calls for heat. When the room is warm enough, no electricity is given to the boiler at all.

What do you mean "when boilers didn't have an electrical connection at all"? Surely they need something to tell them to start burning gas?

Reply to
Bruce Farquhar

I'm surprised a decent company like Honeywell would misuse the term "thermocouple".

Whatever I have in my Baxi boiler is a real thermocouple, it looks like one, and it produces 40mV, and it operates the pilot light valve without any mains power.

Reply to
Bruce Farquhar

Plus, the coil has a lot of windings.

Reply to
Andy Burns

Sorry, not so.

Just because it looks like a bulb and capillary, it's not.

See

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Reply to
Anonymous

+1
Reply to
dpb

Fuck off and die Hucker.

Reply to
Mr Pounder Esquire

I'm sure you know this but the Voyager spacecraft are using thermocouples using the heat from decaying plutonium for power all the way out in the cosmos. it may be reducing now but its been one heck of a long time. Brian

Reply to
Brian Gaff

Indeed - a pilot flame is typically around 250W.

Reply to
Andrew Gabriel

A thermocouple produces enough to power a spacecraft?!? Or just for some small electronics?

Reply to
Bruce Farquhar

Get a hobby or contribute something to the group, wanker.

Reply to
Bruce Farquhar

They can generate up to a few hundred watts, enough for the electronics, instruments and transmitter:

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Reply to
Max Demian

They _can_ generate as much as you have space and weight to be able to package... :)

For extra-terrestrial use, the payload limits are the issue.

Reply to
dpb

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