Types Of 4-20 mA Current Loop

Most people struggle to wire 4-20 mA sensors because they do not know that different types of 4-20 mA current loop configurations are wired differently.

In this post, I will explain the four types of the 4-20 mA current loop, their advantages and disadvantages, and how to wire them.

Types Of 4-20 mA Current Loop

There are 4 types of mA output signals:

  • Loop (2-Wire)
  • Source (3-Wire)
  • Sink (3-Wire)
  • Isolated (4-Wire)

Each form uses a different reference path to create the mA signals, dependent on the controller or receiving device (e.g., PLC) to which the field device is connected.

2-wire or loop

types of 4-20 mA loop

This is the simplest of the four types of 4-20 mA loops; it contains just two wires, hence the 2-wire loop. One wire will carry the 24V, and another will carry the signal. The latter is also used as a zero (reference).

Advantages of 2-wire

Reduce wiring cost

Since it uses only two wires, you will save money on buying them. Let’s say the distance between the controller and the sensor is 100 ft. With two wires 4-20 mA, you will need 200ft of wire, while with three wires, you will need 300 ft, and with four wires, you will need 400ft.

Easier to wire

It is easier to wire. You just have two wires.

Disadvantage

The signal and the reference are carried using the same wire; if that wire is broken, you lose the field device and the signal.

Source (3-Wire)

This is the most common configuration in the industry. About 90% of field devices are estimated to use 4-20 mA source.

types of 4-20 mA current loop

This 4-20 mA loop configuration will have three wires: one for the 4-20 signal, one for 24 volts, and one for the 0V or reference.

The 4-20mA signal will flow from the field device ( sensor ) to the controller or PLC.

Advantages of 3 wires source 4-20 mA loop

It is commonly used

Most controllers and PLCs are designed to accept three wires source field devices, so if you have one, it may be compatible with your current controller.

It is safer

The source uses 0VDC as a reference, so if someone accidentally touches it and closes the circuit, it is less likely to electrocute them.

It does not interrupt the field device.

Since the zero and signal wires are different, the field device will remain powered if the signal wire (mA) gets disconnected.

Disadvantages of 3 wires source

Higher installation cost compared to 2-wire

You will need one more wire than the two wires in the 4-20 loop, which will increase the installation cost.

Sink (3-Wire)

sink

The 3-wire sink also uses three wires, the main difference between the sink and source being the current flow; in sink configuration, the current will flow from the controller to the field device.

Advantages of 3 wire sink

It uses older technology.

Since the sink technology is older than the source configurations, most older controllers use a sink configuration. This reduces the cost, and you do not need to upgrade the system.

Disadvantages of 3 wire sink

Higher installation cost compared to 2-wire

You will need one more wire than the two wires in the 4-20 loop, which will increase the installation cost.

Four wire 4-20 mA configuration (Isolated)

As its name suggests, this configuration uses four wires to connect the controller to the field device or sensor.

4-20

Advantages of 4 wires configuration

The controller and the field device are isolated.

This is the main advantage of this configuration; having the controller and the field device electrically isolated means that any issues that might happen on one side of the connection will not affect the other side.

If the mA signal wire is broken or the controller fails, the field device will continue working without any issue, which is not possible with the first three configurations.

Disadvantages of 4 wires isolated system

Installation cost

The configuration will need more wires, which is twice the amount needed for the 2-wire system. This will increase the overall cost of the project.

It needs two power sources.

For the system to work properly, you must have two power sources, one to power the controller and another to power the sensor or field device.

This will increase the installation cost and complicate the installation and commissioning process.

I installed the four wires using just one power source, and although the system works without an issue, it is not isolated.

This can be done as long as the power source has enough capacity to power both the controller and the field device.

FAQ: Types Of 4-20 mA Current Loop

How do I know which configuration my sensor has?

You can find this in your sensors’ user guide. Usually, there is a section on wiring, and they should be able to explain which configurations can be wired.

Alternatively, you can call your manufacturer’s tech support. They should be able to tell you the configuration.

Can I change from one configuration to another?

This depends on the device you are wiring. The Honeywell XNX transmitter has a deep switch that can help you change from Sink to source and vice versa, and some devices, like the MDA scientific MIDAS gas detector, can be wired in all configurations.

Key Takeaways: Types Of 4-20 mA Current Loop

There are four types of mA output signals:

  • Loop (2-Wire)
  • Source (3-Wire)
  • Sink (3-Wire)
  • Isolated (4-Wire)

Each form uses a different reference path to create the mA signals, dependent on the controller or receiving device (e.g., PLC) to which the field device is connected.

You can find out which configuration your device has by reading the manufacturer’s user guide or calling them.

 

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