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

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"Clive Arthur" wrote in message
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On 08/12/2018 17:51, Bruce Farquhar wrote:
On Sat, 08 Dec 2018 17:40:57 -0000, wrote:

On 12/8/18 11:41 AM, Bruce Farquhar wrote:
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?

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:
https://en.wikipedia.org/wiki/Thermo...pliance_safety

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?

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
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Clive