A temperature transmitter draws current from a remote dc power supply in proportion to its sensor input. The actual signal is transmitted as a change in the power supply current.
Specifically, a thermocouple input transmitter will draw 4 mA of current from a dc power supply when measuring the lowest temperature of the process. Then, as the temperature rises, the thermocouple transmitter will draw proportionally more current, until it reaches 20 mA. This 20 mA signal corresponds to the thermocoupleâs highest sensed temperature. The transmitterâs internal signal-conditioning circuitry (powered by a portion of the 4-20 mA current) determines the temperature range that the output current signal will represent.
Physically, only two copper wires are necessary to connect the temperature transmitter output signal in a series circuit with the remote power supply and the process equipment. This is made possible since the signal and the power supply line are combined (one circuit serves a dual function).