The 4-20 mA current loop remains one of the most dominant types of analog output in the industry today.
I have been working with wiring industrial transmitters for some time now and one thing I found out is that most people can not wire them properly because they fail to distinguish between different types of 4-20 mA current loops.
There are 4 types of 4-20 mA current loop, where the two-wire loop version is by far the most common.
Although the wiring can be a little bit different the working principle is the same, understanding how each one is wired can be fundamental to wiring them.
What are the 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 for the creation of mA signals which is dependent on the controller or receiving device (i.e. PLC) to which each field device is connected.
Loop (2-Wire)
This is one of the most common 4-20 mA forms, you just need two wires for power and communication between the field device and the controller.
The controller provides the power to the loop and the 4-20 mA signal flows from the field device to the controller through the common.
The main advantage of the 2-wire loop 4-20 mA signal is that it is easier to wire and it will require two wires hence it will lower the installation cost.
The disadvantage of the 2 wire 4-20 mA loop is that have two wires, so if the signal wire is broken there will be no power on the field device ( they use the same cable for power and signal).
3 wire 4-20 mA loop (Source)
The 3 wire 4-20 mA loop uses three wires to connect the field device with the controller, here the signal has it is own wire, so you have one wire for the +, one wire for the -, and one wire for the signal.
The two wires (the + and the -) are used to power the field device while the signal wire is used to carry the field device signal to the controller, the most important thing to note here is that the current moves from the field device to the controller.
The main advantage for the 3 wires 4-20 mA loop source is that the signal and the power wires are separated, so in case the power wire is disconnected the field device can still be on.
The main disadvantage of this type of 4-20 mA signal is that it uses 3 wires, so more cable is used for wiring hence the cost of installation goes up.
3 wire 4-20 mA loop (Sink)
This is almost the same as the three wires source type. The 3 wire 4-20 mA loop uses three wires to connect the field device with the controller, here the signal has it is own wire, so you have one wire for the +, one wire for the -, and one wire for the signal.
The two wires (the + and the -) are used to power the field device while the signal wire is used to carry the field device signal to the controller, the main difference between the 3 wires sink and 3 wires source is that in the 3 wires sink configuration the current signal moves from the controller to the field device.
The main advantage for the 3 wires 4-20 mA loop sink is that the signal and the power wires are separated, so in case of that the power wire is disconnected the field device can still be on.
The main disadvantage of this type of 4-20 mA signal is that it uses 3 wires, so more cable is used for wiring hence the cost of installation goes up.
Isolated (4-Wire)
The four wires 4-20 mA current loop, is my least favorite, it works almost like the 2-wires loop, but the main difference is that in 4 wires you need to power sources, in this case, the field device will need its power supply.
The current signal will be flowing from the field device to the controller and the loop is powered by the controller as in a 2 wires form.
The main advantage of the 4 wires 4-20 mA loop is that the field device and the controller use different power sources, so if the controller power source goes offline the field device will keep working.
The main disadvantage is that you will need two power sources, the power sources are not cheap, and this will increase the cost of installation.
How do you know which type of 4-20 mA loop you need to wire?
All field devices come with user guides, and in each user guide, you should be able to see the wiring diagram.
If in the user manual, you can not figure out which type of 4-20 mA your device or controller has please contact the manufacturer of your device and they should be able to tell you how to wire it.
Conclusion
That is it, those are Types of 4-20 mA Current Loop, depending on the type the flow of current and the wiring can change a little, if you have one of those and you need some help please post your question below and we will get back to you.
