4 - 20 mA : What is 4-20 mA ? Let's Understand it in Simple Way

Industrial Automation • Instrumentation

Understanding 4–20 mA Current Loop

A simple, practical explanation of one of the most important analog signal standards used in industrial measurement and control.

If you are learning industrial instrumentation, PLC, SCADA, DCS, or process control, you will frequently encounter the term 4–20 mA. But what does it actually mean?

What is 4–20 mA?

4–20 mA is an analog electrical signaling standard commonly used to transmit measurement information from a field instrument to a controller such as a PLC, DCS, or other control system.

In simple words: the instrument converts a physical measurement such as temperature, pressure, flow, or level into an electrical current between 4 mA and 20 mA.

The important idea is that the current represents the measured value.

A Simple Example

Suppose a pressure transmitter is configured for:

Pressure	Output Current
0 bar	4 mA
25 bar	8 mA
50 bar	12 mA
75 bar	16 mA
100 bar	20 mA

Therefore, when the PLC receives 12 mA, it can determine that the measured pressure is 50 bar for this particular transmitter configuration.

Why does it start at 4 mA?

This is one of the most important points to understand. The range is 4–20 mA, not 0–20 mA.

Typical 4–20 mA Signal Range

4 mA 0% of measurement

20 mA 100% of measurement

The 4 mA point is often called the live zero. It allows the control system to distinguish between a valid zero measurement and a broken signal circuit.

Example: If the transmitter should normally produce 4 mA at zero pressure, a reading close to 0 mA can indicate a problem such as an open circuit or loss of power.

How to Calculate the Measurement

If the relationship is linear, the measured value can be calculated from the 4–20 mA signal.

Measurement = [(I − 4) / 16] × Span

I = measured current in mA
Span = Maximum value − Minimum value

Example: 0–100 bar Transmitter

Suppose the transmitter range is 0–100 bar and the measured current is 12 mA.

[(12 − 4) / 16] × 100

= (8 / 16) × 100

= 50 bar

Why is Current Used Instead of Voltage?

Industrial plants often have long cable runs and electrically noisy environments. Current signaling offers several practical advantages.

📏

Long-Distance Transmission

A properly designed current loop can transmit a signal over relatively long distances without the signal being directly affected by normal cable resistance in the same way as a voltage signal.

⚡

Better Noise Immunity

Current-loop signaling is well suited to industrial environments where motors, contactors, drives, and other equipment can create electrical interference.

🔎

Easy Fault Detection

Because the normal signal starts at 4 mA, a very low current can indicate a wiring, power, or instrument problem.

🔧

Widely Used Standard

4–20 mA is widely encountered in process instrumentation, transmitters, PLC analog input modules, DCS systems, and SCADA applications.

Where is 4–20 mA Used?

You will commonly find 4–20 mA signals in measurement and control applications such as:

🌡 Temperature

Temperature transmitters convert temperature measurements into a corresponding 4–20 mA signal.

💧 Level

Level transmitters can communicate tank or vessel level to a PLC or DCS using the current loop.

📈 Pressure

Pressure transmitters frequently use 4–20 mA to communicate pressure measurements.

🌊 Flow

Flow transmitters can represent a flow range using the same 4–20 mA principle.

4–20 mA: Key Points to Remember

Signal	Typical Meaning
4 mA	0% of the configured measurement range
12 mA	50% of the configured measurement range
20 mA	100% of the configured measurement range
~0 mA	May indicate a wiring, power, or instrument fault

🎓 Exam & Interview Tip

Remember this simple relationship:

4 mA → 0%    |    12 mA → 50%    |    20 mA → 100%

If you understand these three points, you can solve many basic 4–20 mA instrumentation questions quickly.

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Final Takeaway

The 4–20 mA current loop is a simple but powerful method of transferring analog measurement information in industrial automation.

Instead of sending the physical measurement directly, the transmitter represents the measurement as a current between 4 mA and 20 mA. The receiving PLC, DCS, or controller converts that current back into a useful engineering value such as bar, °C, %, or m³/h.

Remember: 4 mA represents the lower end of the configured range, 20 mA represents the upper end, and the values in between represent proportional measurements.