In two-wire 4-20mA control loops, we use 2-wire transmitters to convert various process signals to 4-20mA DC for the purpose of transmitting the signal over some distance with little or no loss of signal.
The current loop uses DC power because the magnitude of the current represents the signal level that is being transmitted. If AC power was instead used in the loop, the magnitude of current would be continuously changing, making it difficult to discern the signal level being transmitted.
For 4-20mA current loops with 2-wire transmitters, the most common loop power supply voltage is 24V DC, but there are also instances of 12V, 15V, and 36V DC. Some older systems use higher voltages.
The Receiver itself could be any number of different devices, such as: a panel meter, actuator valve, motor speed control, a PLC (Programmable Logic Controller), or other DCS (Digital Control System).
This is the device used to transmit data from a sensor over the two-wire current loop. There can be only one Transmitter output in any current loop. It acts like a variable resistor with respect to its input signal and is the key to the 4-20mA signal transmission system. The transmitter converts the real world signal, into the control signal necessary to regulate the flow of current in the current loop. The level of loop current is adjusted by the transmitter to be proportional to the actual sensor input signal. An important distinction is that the transmitted signal is not the current in the loop, but rather the sensor signal it represents. The transmitter typically uses 4mA output to represent the calibrated zero input or 0%, and 20mA output to represent a calibrated full-scale input signal or 100%
