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4-Wire Current-Loop Sensor Transmitters

To complement the 2-wire and 3-wire sensor transmitter blogs that Kevin Duke and I have already published on Precision Hub, my next few blog topics will be on 4-wire sensor transmitters. The blogs will explain 4-wire transmitter basics, the circuit structure of 4-wire output stages, and provide examples of each isolation scheme available with 4-wire transmitters.

The 4-wire sensor transmitter is probably the least well-known of the current-loop sensor transmitter circuit types. These transmitters fit market needs for applications that require additional transmitter isolation options that aren’t possible with 2- and 3-wire transmitters. Figure 1 shows a basic representation of a 4-wire sensor transmitter based on the American National Standards Institute (ANSI)/International Society of Automation (ISA)-50.1-1982 standard.

Figure 1

4-wire sensor transmitter simplified block diagram

4-wire sensor transmitter simplified block diagram

Unlike the 2- and 3-wire transmitter representations shown in Figure 2, the 4-wire circuit has separate paths for the power current and signal current. Also, the 4-wire receiver does not share a common return (GND) with the power supply. This allows for several new isolation schemes, including fully isolated, power-isolated and output-isolated transmitters that expand on the input-isolated and non-isolated topologies we described for 2-wire and 3-wire transmitters.

Figure 2

2- and 3-wire sensor transmitter simplified block diagrams

2- and 3-wire sensor transmitter simplified block diagrams

While non-isolated and input-isolated systems exist for 2- and 3-wire sensor transmitters, these isolation schemes are not possible when designing with 4-wire transmitters. This is because non-isolated and input-isolated transmitters do not require isolation between the power supply and the output transmitter and receiver.

I’d like to note that input isolated 3-wire transmitters can typically be connected to 4-wire analog input modules by connecting the negative input (Input-) to GND. Figure 3 shows an example of the transmitter and receiver wiring connections.

Figure 3

Input-isolated 3-wire transmitter connected to a 4-wire analog input module

Input-isolated 3-wire transmitter connected to a 4-wire analog input module

The first and most basic form of a 4-wire transmitter design that I’ll cover is an output-isolated 4-wire transmitter. In an output-isolated 4-wire transmitter, the sensor input and power supply share a common GND, while the output transmitter is powered from an isolated supply derived from the sensor supply. Figure 4 is a simple block diagram of an output-isolated 4-wire transmitter.

Figure 4

Output-isolated 4-wire sensor transmitter block diagram

Output-isolated 4-wire sensor transmitter block diagram

Output-isolated 4-wire transmitters can also be powered from a local power supply instead of being powered from the 4-wire analog input module (see Figure 5). The sensor and power supply still share a common GND while the transmitter is isolated from them.

This 4-wire transmitter scheme is a popular way to reduce the number of wires that must travel long distances back to the input module when there is easily accessible power near the sensor.

Figure 5

Output-isolated 4-wire sensor transmitter with local power supply

Output-isolated 4-wire sensor transmitter with local power supply

Figure 5: Output-isolated 4-wire sensor transmitter with local power supply

In my post next month, I’ll go through the circuit-level design of an output-isolated 4-wire sensor transmitter.

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5 comments on “4-Wire Current-Loop Sensor Transmitters

  1. asdasdged1
    July 19, 2015

    very well explained

  2. kattapa
    July 19, 2015

    mix

  3. sunjeevarali
    July 20, 2015

    nice one

  4. DanyP
    June 23, 2016

    Love it

  5. Olivpsy
    October 22, 2016

    And ? Sorry but i don't understand your reply…

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