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 I found out that most people can not wire them properly because they fail to distinguish between different types of 4-20 mA current loops.
There are four types of 4-20 mA current loop, and the two-wire loop version is the most common.
Although the wiring can be slightly 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 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 for creating mA signals, which depends 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, and 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 it has 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 to the controller. Here, the signal has its wire, so you have one wire for the +, one for the -, and one for the signal.
The two wires (the + and the -) power the field device, while the signal wire carries the field device signal to the controller. The most important thing to note 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 installation cost goes up.
3 wire 4-20 mA loop (Sink)
This is almost the same as the three-wire source type. The 3-wire 4-20 mA loop uses three wires to connect the field device to the controller. The signal has its own wire, so you have one wire for the +, one for the -, and one for the signal.
The two wires (the + and the -) power the field device. In contrast, 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 of the 3 wires 4-20 mA loop sink 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 installation cost goes up.
Isolated (4-Wire)
The 4-wire 4-20 mA current loop is my least favorite; it works almost like the 2-wire loop, but the main difference is that in 4 wires, you need two power sources. In this case, the field device will need its power supply.
The current signal will flow from the field device to the controller, and the controller powers the loop as in a 2-wire form.
The main advantage of the four 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, which are not cheap and will increase the installation cost.
How do you know which 4-20 mA loop type 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 get in touch with 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 need help, please post your question below, and we will reply.