The 4-20 mA current loop remains one of the most dominant types of analog output in the industry today.
This article will examine the history of the 4-20 mA current loop, why it is widely used in industrial automation, and its advantages and disadvantages.
What is a 4-20 mA current loop?
The 4-20 mA current loop especially refers to the wire connecting the sensor to a receiver that receives the 4-20 mA signal and then returns to the transmitter.
The history of 4-20 mA current loop
At the beginning of industrial automation, most mechanical devices were controlled by a pneumatic signal, these systems were costly, bulkier, and difficult to repair. The control signal that was used back then was the 3-15 psi.
With the huge development of electronics in the 1950s, electronic devices became cheaper, and eventually, the old pneumatic 3-15 psi systems were replaced by the analog controllers that used the 4-20 mA.
Why 4-20, and why not 0-20 mA?
Now we know that the control signal picked was 4-20mA. The question I get a lot is why 4-20mA and not 0-20 mA? The simple answer is that there was a problem with the dead zero.
The dead zero issue
What is a dead zero issue?
A dead zero is that if you start the lowest signal with 0mA, the controller will not be able to differentiate if the 0mA is because the sensor is detecting the lowest signal value or there is an open circuit.
If you have an H2S sensor that detects 0 to 100 ppm, it will show 0mA when there is 0 ppm of H2S, and also it will show 0mA when there is an open circuit in the loop. This will have a huge impact on the process control.
How do you solve a dead zero issue?
The solution was simple, start with a number above zero, in the same example if the sensor reads zero it will send 4mA, and if there is an open circuit it will send a 0mA signal, and the problem is solved.
Why 4 mA?
We said above that in order to solve the dead zero issue, there was a need to start the value at a value greater than zero, the next question why 4ma and not another value? Here is the answer.
Electronic chips require at least 3 mA to work
In order to move from mechanical controllers to electronic ones, electronic chips were introduced, those chips require a minimum of 3 mA of current to function so, a margin of 4 mA is taken as reference.
The 20% bias
The original control signal was 3-15 psi, the 20% of 15 is 3, and the 20% of 20 mA is 4 mA.
Why 20 mA?
There are three reasons why 20mA was picked:
The human heart can withstand up to 30mA
20 mA is used as the maximum because the human heart can withstand up to 30 mA of current only. So, from a safety point of view, 20 mA is chosen.
1:5 rule
The 4-20mA was designed to replace the old 3-15 psi, since most instruments at the time were using this control signal, there was a need to design the new signal that would follow the same pattern.
Lineality
With the current signal being lineal it is easier to design and implement the control system using the 4-20 mA signal.
Easy to design
Most industrial transmitters are powered with 24V, and since the signal obeys Ohms law V=IR, it makes it easier to design devices that can be connected to the 4-20 mA loop.
Simple calculations
Having a signal that ranges from 4-20mA makes it very easy to calculate the expected values.
If we have a sensor that detects the 0 to 100 range here are the estimated current values.
0-4 mA
25-8 mA
50-12 mA
75-16 mA
100-20 mA
It is that simple.
Simple conversion to 1-5V
In order for other elements of industrial automation to interpret the signal, there is a need to convert it to a digital signal.
Most ADC (Analog to Digital Converters) use voltage to convert the signal. Using the precision 250ohm resistor makes it easier to convert the process analog signal to digital one, by using the ohms law V=IR.
Types of 4-20 mA current loop
There are 4 types of 4-20 mA current loop, where the two-wire loop version is by far the most common.
There is a three wire 4-20 mA source, 3 wire 4-20 mA sink, and four wire 4-20 mA variants that are similar in their fundamental working principle.
I explain the difference between them in this article here.
Advantages of 4-20 mA current loop
Here are some of the advantages of using the 4-20 mA current loop for process control and automation.
Worldwide industry standard
Since it is easier to implement and design control loops with a 4-20 mA signal, it is widely used in many industrial automation applications.
Easy to connect and configure
The 4-20 mA loop is easy to design, configure, and wire. You do not need a lot of training to wire or configure it, hence it is used in most applications.
Less sensitive to electronic noise
Electronic noise can affect the information the cables are carrying since the signal is transported as a current, and is less sensitive to electronic noises than voltage.
Fault detection using live zero
Since the signal starts at 4 mA it is very easy to know if there is a fault in the loop, if we receive 0 mA, we know there is a fault somewhere.
You can use a simple multimeter to detect a fault
Since the loop will carry current, you can measure the current by using a simple $10 multimeter. This will reduce the diagnostic time and fault detection cost.
Disadvantages of the 4-20 loop
There are a few disadvantages to using the 4-20 mA loop. For me, these two are the main ones.
The current may introduce a magnetic field
The current may introduce magnetic fields and cross-talk to the parallel cables, and this can be solved by using the twisted wire cable.
One pair of cables can only carry one process
This is huge. When you design a control loop using a 4-20mA signal, you need to know that one loop can only one process variable, so if you have many loops, you will need more cables, and this will increase the cost of installation and eventually, it will make the fault diagnostic more complicated.
Conclusion
We took a look at the famous 4-20mA current loop, we looked at the history of the 4-20 mA loop, the reason why it is widely used in industrial automation, and its advantages and disadvantages.
If you have anything to add to this or a question, please leave your comment below. Thank you for reading.