4-20 mA Current Loop

The 4-20 mA current loop remains one of the most dominant types of analog output in the industry today.

In this article, I will examine the history of the 4-20 mA loop, its wide use in fixed gas detection, and its advantages and disadvantages.

What is a 4-20 mA current loop?

The4-20 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 industry automation, most mechanical devices were controlled by a pneumatic signal.

These systems were costly, bulky, and difficult to repair. The control signal that was used back then was 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. I often ask why 4-20 mA and not 0-20 mA. The simple answer is that there was a problem with the dead zero.

What is a dead zero issue?

A dead zero is when you start the lowest signal with 0 mA, and the controller cannot differentiate if the 0 mA is because the sensor detects 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 0mA when an open circuit is in the loop. This will have a huge impact on 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 4 mA, and if there is an open circuit, it will send a 0 mA signal; the problem is solved.

Why 4 mA?

We said above that to solve the dead zero issue, the value needed to start at greater than zero. The next question is why 4 mA and not another value? Here is the answer.

Electronic chips require at least 3mA to work.

Electronic chips were introduced to move from mechanical controllers to electronic ones; those chips require a minimum of 3 mA of current to function, so a margin of 4 mA is taken as a reference.

The 20% bias

The original control signal was 3-15 psi, the 20% of 15 is 3, and the 20% of 20mA is 4mA.

Why 20mA?

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 only 30 mA of current. 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 control signal. Since most instruments at the time were using this signal, the new signal needed to follow the same pattern.

Lineality 

The current signal being lineal makes designing and implementing the control system using the 4-20 mA signal easier.

Easy to design

Most industrial transmitters are powered with 24V, and since the signal obeys Ohms law V=IR, it is easier to design devices connected to the 4-20 mA loop.

Simple calculations

A signal ranging from 4-20mA makes it very easy to calculate the expected values. Here are the estimated current values if we have a sensor that detects the 0 to 100 range.

0-4 mA

25-8 mA

50-12 mA

75-16 mA

100-20 mA

It is that simple.

Simple conversion to 1-5V

Converting the signal to a digital signal is necessary for other elements of industry automation to interpret it.

Most ADC (Analog to Digital Converters) use voltage to convert the signal. Using the precision 250ohm resistor makes it easier to convert the analog signal to a digital one, using the ohms law V=IR.

Advantages of 4-20 mA current loop

Worldwide industry standard

Since it is easier to implement and design control loops with 4-20 mA signal, it is widely used in many industrial automation industry.

Easy to connect and configure

The 4-20 mA loop is easy to design, configure, and wire. It does not require extensive training to wire or configure, so 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, it is less sensitive to electronic noise than voltage.

Fault detection using live zero

Since the signal starts at 4 mA, it is straightforward 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 using a simple $10 multimeter, reducing the diagnostic time and fault detection cost.

Disadvantages of the 4-20 loop

There are few disadvantages of 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 crosstalk to the parallel cables; this can be solved by using twisted wire cables.

One pair of cables can only carry one process.

This is huge. When you design a control loop using a 4-20mA signal, you must know that one loop can only control one process variable.

So, if you have many loops, you will need more cables, increasing the installation cost and eventually making the fault diagnostic more complicated.

Conclusion

We looked at the famous 4-20mA current loop, the history of the 4-20 mA loop, the reason it is widely used in fixed gas detection, 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.

 

Leave a Reply