How a 2-wire transmitter works with 24 volts supply and 4mA?

The 2 wire transmitter, DC power supply and the load (some receiver, an analog input on a PLC/DCS/Controller/recorder/indicator) are wired in series.

The installer ‘gets them in the same path’, an electrical series circuit.

The transmitter uses about 3.5-3.6 mA of current as power to run its own electronics that excite and/or read the sensor, interpret the sensor’s signal and create a scaled output.

A 2 wire transmitter acts like a variable resistor, or transistor, in that it regulates its internal resistance so that the output is scaled 4.0-20.0 mA, proportional to the process variable it is sensing, with the loop current the ranging 4-20mA.

The power supply voltage must be high enough to drive 20mA (21.xmA if upscale failsafe is used/selected) through the load (the input’s resistance, wire resistance, any stray resistance (terminal block).

24Vdc power supplies are the most widely used power supplies, but most 2 wire transmitters can operate with power supplies up to about 32-36 volts DC.

Author - Carl Ellis