What is a 4–20 mA Signal? Working Principle, Wiring & Industrial Applications
There are 2 type of wirings. “Power wiring” and “Control wiring“.
Power wiring is the one Where heavy current flow at a voltage; higher or equal than the control side voltage.
Control wiring is the light wiring where low current flow at a voltage; lighter or equal than the power side voltage. Control wiring actually guides supply to power wiring.
For better understanding, Heating element controlled by temperature controller is shown below.
In this arrangement, Yellow and black colored wiring represents the power wiring. Power wiring’s rated voltage is 230V AC. In power wiring, Heater load current flows while it is also act as a power source for temperature controller.
While Blue and red colored wiring represents the control wiring. Where controller command the control power to SSR further SSR control the power to heater. Controller produces the rated voltage of 12V DC.
In the arrangement above, Controller produces the command either on or either off. That is simple on off control. But in control valve control systems, Valve needs to be regulate from 0% to 100%. These type of control are done by 4-20mA signals. In this post, Next we are going to discuss 4-20mA signals.
What is a 4-20 mA signal ?
“4-20mA is “current loop signal“. This signal ranges from 4mA to 20mA. This 4-20mA signal can be used as an input or can be used as an output. Since it is a type of range, It is more than just on and off. It is like controlling the system from 0 to 100%.
PID controllers and process controllers have provision of 4 to 20 mA signal. According to the set parameters and set target, These controllers by regulating the 4 to 20mA signal, try to achieve the set target.
An image of PID controller P104 is shown below. This PID controller produces mA signal output to control the pneumatic actuator valve. Further this valve control the steam flow or chilled water flow to achieve the target value.
In normal temperature controllers, There are two scenario either it will completely open the valve or it will completely close the valve. A margin need to be adjust around the set value in normal temperature controllers. Normal controllers do not have precise control as it fluctuate around the set value.
In a Process controller, PID is actually controlling the control valve in a proportion of percentage from 0% to 100%. PID produces the milli amps output from 4 to 20mA. At 4mA, valve get 0% open While at 20mA, valve will get 100% opened
Wiring 4-20mA devices
These devices comes in 2 wires, 3 wires and 4 wires.
In 4 wire system, 2 wires are power supply terminals. While remaining 2 wires act like an output terminal. OR 3rd terminal act like a output terminal and 4th terminal act like IO-Link.
In 2 wires system, Only 2 wires act like a closed loop current system.
In 3 wires system, 2 wires are power supply terminals. While remaining single terminal act like an output terminal.
Terminals connections of temperature sensor IFM TA2232
Use of 4-20mA signal !
4 to 20mA signals are generated by electronic circuitry according to the “stored instructions” & “present input signals”. It is either used to control the “electromagnetic field of inductive coil” or as a feedback to another “digital circuitry“.
4 to 20mA signal for Inductive coils
In this, Signal current produced by the device, flows through the coil. According to the current level, Electromagnetic coil produces the magnetic field.
An primary power source such as compressed air at rated air pressure is already connected to the system while this signal current actually controls the air pressure going to the actuator valve.
This closed loop signal try to control the air pressure input at positioner valve. So that valve opening can get controlled from 0 to 100%.
An image of pneumatic positioner valve is shown below :
Pneumatic positioner valve
4 to 20mA signal as a feedback to digital circuitry
In this arrangement, 4-20mA device or instrument feed the 4 to 20mA signal to another digital device. The receiver device further reads the signal according to the stored instructions and produces the output or reflects the correspond reading. This process act like a “feedback“.
An image of flush pressure sensor with display is shown below.Sensor produces the 4 to 20mA output signal.
Flush pressure sensor with display | PI1703 by Ifm
This concept is called “Live Zero.” 0 mA can not be taken as initial value Which can be an error in case of broken wire, power failure and sensor failure.
Advantage of 4-20mA signal
The key advantage is that current remains the same throughout a series circuit, regardless of cable resistance. Apart it offers :
Long-distance transmission
Better noise immunity
Simple fault detection
Industry standard
Easy PLC integration
Common Applications
Temperature transmitters
Pressure transmitters
Flow transmitters
Level transmitters
Control valves
Motor speed control
Common Problems and Troubleshooting
Reading can stuck at 4 mA
Reading above 20 mA
Grounding issues
One signal current can be connected to one system at a time.
Know the process value (Signal current)
Range 4 to 20mA. Where 4mA is minimum while 20mA is maximum.
Range 0 to 200°C. Where 0°C is minimum while 200°C is maximum
At 0°C, signal current will be 4mA
At 200°C, signal current will be 20mA
What will be the current value at 120°C ?
If we are working in the field of instrumentation, It is very important to know about the proportion of process value and signal value. For 4 to 20mA range, there is formula :
Frequently Asked Questions (FAQs) About 4–20 mA Signals
1. What is a 4–20 mA signal?
A 4–20 mA signal is an analog current loop standard widely used in industrial automation to transmit process variables such as pressure, temperature, level, and flow over long distances.
2. Why does the signal start at 4 mA instead of 0 mA?
The 4 mA value represents the “live zero.” It allows the receiving device to distinguish between a valid zero measurement (4 mA) and a fault condition such as a broken wire (0 mA).
3. What does 4 mA represent?
Typically, 4 mA corresponds to the minimum value of the measured process variable (0%, lower range value).
4. What does 20 mA represent?
20 mA represents the maximum value of the measured process variable (100%, upper range value).
5. Why is 4–20 mA preferred over voltage signals?
Current signals are less affected by voltage drops, electrical noise, and cable resistance, making them more reliable for long-distance transmission.
6. How far can a 4–20 mA signal travel?
Depending on the power supply voltage, cable resistance, and loop load, a 4–20 mA signal can typically travel hundreds or even thousands of meters without significant accuracy loss.
7. How do I convert a 4–20 mA signal into engineering units?
Use the formula:
Process Value=16(I−4)×Span+LRV
Where:
I = measured current (mA)
Span = URV − LRV
LRV = Lower Range Value
8. What happens if the current drops below 4 mA?
A current below 4 mA usually indicates a fault condition, such as sensor failure, wiring problems, or transmitter diagnostics.
9. Can multiple devices be connected in a 4–20 mA loop?
Yes, provided the total loop resistance remains within the transmitter’s load capability and the supply voltage is sufficient.
10. What is a two-wire transmitter?
A two-wire transmitter uses the same pair of wires for both power and signal transmission. It draws operating power directly from the 4–20 mA current loop.
11. What is loop resistance in a 4–20 mA circuit?
Loop resistance is the total resistance of cables, indicators, PLC input cards, barriers, and other devices connected in the current loop.
12. How can I measure a 4–20 mA signal with a multimeter?
The multimeter must be placed in series with the loop and set to the DC current measurement range.
13. Can a PLC directly read a 4–20 mA signal?
Yes. Most PLC analog input modules are designed to accept 4–20 mA signals directly.
14. What process instruments commonly use 4–20 mA signals?
Examples include:
Pressure transmitters
Temperature transmitters
Flow transmitters
Level transmitters
Control valve positioners
15. Why is 20 mA chosen as the upper limit?
20 mA provides sufficient signal range for accurate measurement while keeping power consumption low and ensuring safe operation in industrial environments.
