Self Powered Vs Loop Powered Transmitter Current Loop

A Transmitter is used to transmit the field measured value to Controller / Indicators, Recorders or any other input device.
Generally the signal is transmitted in form of DC Current (4 – 20 mA (most used form of feedback current))
Self Powered or 4 Wire Transmitter Current Loop
Simplest form of Transmitter Current Loop is Self Powered or 4 Wire Transmitter Current Loop.
In Self Powered Loop, the transmitter has four terminals :
1) Two terminals for the 4-20 mA signal wires. (Measured Value is transmitted via this terminal)
2) Two more terminals for connecting a power source (Transmitter power requirement is met through this terminal)

A transmitter works as a current source in 4 wire Transmitter Current Loop.


Disadvantages of 4 Wire Transmitter Current Loop :
1) Additional wire requirement
2) Larger electrical conduits requirement due to Increased size of cable.
3) Extra terminal requirement in the marshalling panel or Controller Panel.

Loop Powered or 2 Wire Transmitter Current Loop
In this type of Transmitter Current Loop , Power and Feedback signal (4-20 mA) both are on the same two wires.

Difference between 2 wire and 4 wire Transmitter Current loop :

A 4 wire transmitter requires extra two wires than a 2 wire transmitter.

A 4 wire transmitter requires extra terminals during termination than a 2 wire transmitter.

Unlike a current source as in 4 wire Transmitter Current Loop, A 2 wire transmitter circuit works as a current regulator limiting the feedback current (4-20 mA) in accordance to the measured value.

A 4 wire transmitter acts as a true electrical source , while A 2 wire transmitter acts as an electrical load .

#PLC, #Transmitter, #SelfPoweredTransmitter, #LoopPoweredTransmitter, #Instrumentation




Source – Lessons in Industrial Instrumentation, By : Tony R. Kuphaldt, (V – 0.4)
http://openbookproject.net/books/socr...