A gas thermocouple is a safety device used in older furnaces. Older furnaces depend on a small burner called the pilot light which stays on all the time. When the furnace turns on, the pilot light lights the main burners. If the pilot light goes out accidentally, gas can build up in the house, potentially causing asphyxiation or an explosion. The thermocouple senses if the pilot goes out and automatically turns off the valve.
When one end of a strip of metal is at a higher temperature than the other, it produces an electric current. This phenomena, called the Seebeck effect, works with all metals but different metals produce different voltages. A thermocouple has two different types of metal soldered together in the center. Each strip attaches to a circuit, which measures the current difference between the strips to determine the temperature of the soldered end.
Gas Furnace Thermocouples
The thermocouple is positioned so that the place where the two strips join is in the hottest part of the flame of the pilot light.If the flame is on, it sends a voltage to the control circuit and the gas stays on. When the flame goes out, however, the voltage drops quickly. The control circuit sends a signal to a small motor called the solenoid. The solenoid immediately shuts off the gas valve. The gas will stay off until someone manually resets the circuit and lights the pilot light.