Electronics > Projects, Designs, and Technical Stuff
Thermocouple / millivolt gas valves - how do they work?
DTJ:
Just out of interest I was reading about thermocouple or millivolt gas valves as used in gas hot water systems and domestic gas space heaters.
Details on how they work is a little elusive and gets buried in all the domestic air conditioning FAQ pages. I looked at a gas heater some time back and the over-temperature sensor was wired to the under temperature (pilot) sensor. I guess they wire them in opposition for that to work.
It seems the thermocouple is more likely a thermopile and the current it generates operates a solenoid coil.
Quoted voltages are anything from 20mV to 800mV.
Any ideas of the current level involved or other details.
mzzj:
There seem to be few voltage levels, this pdf gives some resistance numbers and from there you can assume some power and current numbers available
http://library.coburns.com/specs/CATALOG_Honeywell_Q340A1074-U.pdf
It lists thermopiles and thermocouples separately
Thermopile with 750mV output and ~3ohm resistance would make 250mA short-circuit current or 125mA @ 375mV @3ohm load so you have about 45mW max available for the valve. Suprisingly little for electromechanical valve!
DTJ:
Thanks for the link. They are interesting little devices. I'm impressed the valves can operate on so little power.
I'm surprised how little info there is around on them considering how many are used about the place.
ArthurDent:
A lot of gas stoves use 2 heat generating thermo sources. The thermocouple is in the pilot flame which must be lit for gas to enter and if the flame goes out, this low(20 millivolt) signal goes to zero and the main solenoid valve closes cutting the gas supply off, which is a safety measure. The link below is a video of how it works.
A thermopile is a bunch of series connected thermocouples that generates maybe 800 millivolts and this is also in the pilot flame. The thermopile controls another valve that is in series with a switch type thermostat to turn the main burner on and off to control the temperature output of the device.
Renate:
I too have been amazed at how little electrical power actuates these switches.
I don't know exactly what pressure your valve is using, but my propane uses 11" of water, which is hardly any pressure.
The funny thing is, the electric gas valves in an RV (for fridge, heat, hot water) take about 3W (12V, 48ohm).
On the other hand, my oven has the standard thermopile valve in it.
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