Heating, Ventilation, and Air Conditioning (HVAC ) Electrical Practice Test

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What type of current does a typical thermocouple used on a gas furnace generate?

AC electrical current
High voltage DC
Low voltage DC
Pulse current

The correct answer is: Low voltage DC

Thermocouples used in gas furnaces generate low voltage DC current. A thermocouple operates based on the principle of the Seebeck effect, where two dissimilar metals create a voltage when subjected to a temperature difference. This generated voltage, which is directly related to the temperature difference, is typically in the range of a few millivolts, making it classified as low voltage DC current. The application of this principle allows thermocouples to serve as reliable temperature sensors and safety devices in gas furnaces, ensuring that the pilot light remains lit and the furnace operates safely. The low voltage DC produced is sufficient to power small ignition systems or control circuits commonly found in HVAC systems. Given this understanding, it’s clear why the choice identifying low voltage DC is the correct classification for the current generated by a thermocouple in this context.