Current loops are used in industrial automation to communicate the process measurement information from the transmitter to the controller such as indicators, alarms, PLCs. Different transmitters needed different types of wiring because. there transducers in passive and active types. And there is resistive type transducer which doesn't need the power supply. So based on the wires required to connect the transmitters, there are 4 wire and 2 wire transmitters.
4 Wire transmitter:
Minimum two wires are needed to connect a transmitter to the controller through which the information is passed through. And extra two wires are needed for some other transmitter, which is to connect the power source. So these transmitters are known as 4 wired or self-powered.
More cables are needed to connect the circuit which increases the wire size, counted as a disadvantage.
Some process controllers are not equipped to directly accept milliamp input signals, but rather can only interpret DC voltage signals. In such cases, we must connect a precision resistor across the controller’s input terminals to convert the 4-20 mA transmitter signal into a standardized analogue voltage signal the controller can understand. A voltage signal range of 1 to 5 volts is standard, although some models of controller use different voltage ranges and therefore require different precision resistor values. If the voltage range is 1-5 volts and the current range is 4-20 mA, the precision resistor value must be 250 ohms according to Ohm’s Law
2 Wire transmitter:
In 2 wire connection transmitter doesn't need any power source, transmitter acts as load regulator. A power source is added in line with the transmitter. Transmitter, power source, and controller are connected with in a single loop so that there only need of two wire.
All sensing, scaling, and output conditioning circuitry inside the transmitter must be designed to
operate on less than 4 mA of DC current, and at a modest terminal voltage. In order to create loop
currents exceeding 4 mA – as the transmitter must do in order to span the entire 4 to 20 milliamp
signal range – the transmitter circuitry uses a transistor to shunt (bypass) extra current from one
terminal to the other as needed to make the total current indicative of the process measurement.