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

To *hold* the valve open only requires a small voltage & current. To

See here for more detail:

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?

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

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

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.

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.

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

See:

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?

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.

Plus, the coil has a lot of windings.

Sorry, not so.

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

See

F*ck off and die Hucker.

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

Indeed - a pilot flame is typically around 250W.

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

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

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

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.