Answer / carl_ellis

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.

Carl Ellis
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