Thermocouples and Valve Actuation

This is one of those things that wikipedia is really great for chasing. There is great information like this that one can chase forever.

Simply put, run current (AC or DC) through a wire and a magnetic field is produced. There are tons of DC coils produced. Don't confuse a coil with a transformer though. That's where AC comes in. Again, chase it on wiki.

I used have lots of fun making holes in the shed wall with a 1/2" x1' rod wrapped with a bunch of turns, then tap the poles on the tractor battery.

It is not possible. The millivolt output from the thermocouple goes to a "controller" that actually sends the voltage and current needed to open the pilot valve.

You all are confusing thermopile ... milivolt...with thermocouple...maybe. They both get the job done and they both generate milivolts..thermopile normally has two wires from the " head " and thermocouple has the capillary to a " button "...

Sorry guest, but you are the one confused. Please read post 1. The term thermocouple is used incorrectly to refer to bulb and capillary.

I beg your pardon... a "thermocouple" has two wire leaving a junction. Period.

Thank you for all the responses to my thermopile problem. I am not confusing thermocouple with the thermopile, which for someone who does not know is a series of thermocouples. In my problem I am reffering to a combination gas valve and standing pilot where a thermocouple heated by the pilot flame generates current to keep the pilot valve open. Every book says that it generates DC voltage (milivolts) and I simply cannot understand how DC current can create magnetic field.

Hi,

The simple answer is that although the thermocouple does develop only say 35-40 millivolts this is achieved with an extremely low internal resistance. It is connected to an equally low resistance coil located in the gas valve assembly.The resultant current creates a magnetic field in the solenoid which is sufficient to hold the spring loaded switch parts together, once they have been brought together by the force applied by a thumb to the trip plunger. As you realise this mechanism is used to interrupt the 240V supply to the main gas valve, until the two parts are held together by the emf developed by thermocouple. A simple and yet elegant solution.....if only all engineering could be so cost effective.

The answer supplied about a thermometric fluid version is correct for some completely non-electric gas burners typically found on portable natural convection atmospheric heaters found on bottled gas kit.

As far as the generation of a magnetic field is concerned all conductors carrying a current possess a magnetic field the only difference between the alternating current one and the direct current one is that the former has a field that reverses in direction with the changes in the sine wave whilst the latter is a constant.

Keep faith with Ohms Law and all will be well!