What is a 2-wire, 3- wire, and 4 – wire transmitter, and how are they different from each other?

A transmitter is an electronic device that would convert the process variables such as frequency, pressure, temperature, and flow to a 4-20mA signal.

A transmitter can be described as a combination of the transducer, amplifier, and signal conditioning.

In order to understand the difference in connections, one should first know the components used in a signal transfer loop and their roles.

The below are basic elements of a current loop:

Power supply: Typically a 24VDC power source is used to power the loop.
Receiver: The receiver receives the transmitted signal. In most cases, it is the analog input channel of a PLC.
Transmitter: The transmitter transmits data from a sensor to the receiver.
Wires: To interconnect the power supply, receiver, and transmitter.

2-wire transmitters

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!

A 2-wire 4-20mA signal transmission loop does not require an external power source.
The analog input module should be of source type.
A 2-wire transmitter connection uses only two wires for both power supply and signal transmission.
It is the simplest and most cost-effective configuration.
Typically, it relies on a constant current source to power the transmitter and transmit the signal. The current varies to represent the measured parameter.
Because the same wires are used for power and signal, there can be a voltage drop along the wires, affecting the accuracy of the measurement.
Suitable for applications where simplicity and cost are primary concerns, and where signal accuracy is not critical.

Let’s have a look at how a typical 2-wire current loop is shown on a loop diagram.

As illustrated, the 24-volt power supply is in series with the differential pressure transmitter and the PLC analog input card.

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.

A 4-wire transmitter uses four wires: two for power supply and two for signal transmission.

This configuration offers the highest signal accuracy because there is complete separation between power and signal wires.

It eliminates the voltage drop issue entirely, making it ideal for precise measurements.

Often used in laboratory settings, scientific research, and industrial applications where accuracy is critical.

A 3-wire transmitter uses three wires: two for power supply and one for signal transmission.

It provides better signal accuracy than a 2-wire transmitter because a dedicated wire is used for the signal.

The three-wire configuration helps mitigate the voltage drop issues encountered in 2-wire setups.

Commonly used in applications that require higher accuracy and where cost is less of a concern.