What are 2-Wire and 4-Wire Transmitter Output Loops? 

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In this video, we’re going to take a close look at 2-wire and 4-wire transmitters and discuss where they are used and why. We’re also going explore wiring configurations and how transmitters are connected to a PLC.
Before we get started, you might want to review our video called PLC Analog Inputs and Signals (realpars.com/p....
A transmitter is an instrument that converts the small signal from a sensor to a signal which represents the variable being measured.
Transmitters come in all different shapes and sizes and connect with several types of sensors.
The transmitter output signal representing the variable being measured can be voltage or current.
A transmitter analog output loop contains the transmitter, power supply, and the receiving device which could be a PLC or DCS.
Just like any other instrument, a transmitter needs a power supply to operate. But, is it a 2-wire or a 4-wire transmitter?
The actual wiring connection between the transmitter and the power supply depends upon which type it is.
A 4-wire transmitter has 2 wires connected to a power supply, and 2 signal wires connected to the PLC.
The power supply can be AC or DC depending upon the vendor and model.
As its name states, a 2-wire transmitter has only 2 wires.
In a 2-wire current loop, the transmitter, DC power supply, and PLC are connected in series.
Not only are the 2 wires providing power for the transmitter, but they are also the signal lines!
Earlier we said that the transmitter output can be either current or voltage. Current is by far the most common transmitter output signal for many reasons.
One reason is if the wires between the transmitter and the PLC are very long, there could be significant voltage losses across these wires.
Regardless of the length of wire in a current loop, basic electrical theory tells us that the current is the same in the circuit regardless of where that current is measured.
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