How does a thermocouple generate voltage, and which two common thermocouple types and their typical use ranges?

• A. A thermocouple generates voltage through the Seebeck effect between dissimilar metals; Type K (-200 to 1350 C) and Type J (-40 to 750 C).
• B. A thermocouple generates voltage by resistance change in a single metal; Type K and Type J refer to resistance.
• C. A thermocouple is a light sensor using semiconductor junctions.
• D. A thermocouple generates a digital signal directly.

Answer: (A)

A thermocouple generates voltage from the Seebeck effect: when two dissimilar metals are joined, a small electromotive force appears if the two junctions are at different temperatures. The voltage is produced by the temperature difference between the hot junction that senses the temperature and a reference (cold) junction, and it depends on the specific metal pair used. This voltage is typically in the millivolt range and requires cold-junction compensation to turn into a temperature reading.

Two widely used types are Type K and Type J. Type K uses Chromel and Alumel and covers a broad range from about -200 C up to 1350 C, making it very versatile for many environments. Type J uses Iron and Constantan and spans roughly -40 C to 750 C, which is well-suited for lower to mid-range temperatures and often chosen for cost or specific material considerations.