In industrial automation and process control systems, 4-20mA current signals and 1-5V voltage signals are two widely used analog signal standards.
According to IEC industrial signal transmission standards, field instruments typically use 4-20mA DC current signals for long-distance transmission, while control systems often use 1-5V DC voltage signals for signal processing and communication.
This combination is commonly known as:
Current transmission in the field + Voltage reception in the control room.
But why do industrial systems convert a 4-20mA signal into a 1-5V signal?
Let’s take a closer look.
Why Do Field Instruments Use 4-20mA Signals?
Industrial field devices, such as:
- Pressure transmitters
- Flow transmitters
- Level transmitters
- Temperature transmitters
are usually installed far away from the control room.
For long-distance signal transmission, 4-20mA current signals have significant advantages compared with voltage signals.
1. Less Influence from Cable Resistance
When transmitting a voltage signal over a long cable, the resistance of the cable can cause voltage loss and measurement errors.
For example:
- A voltage signal travels through a long cable
- Cable resistance creates a voltage drop
- The receiving device may detect an incorrect value
However, a 4-20mA signal works as a constant current transmission system.
As long as the circuit is not interrupted or branched, the current remains almost unchanged regardless of cable length.
Therefore, 4-20mA signals provide better accuracy for long-distance industrial transmission.
2. The “Live Zero” Function
One important advantage of the 4-20mA standard is the 4mA zero point, also called a “live zero.”
The relationship between measurement value and output signal is:
| Measurement | Output Signal |
|---|---|
| 0% | 4mA |
| 50% | 12mA |
| 100% | 20mA |
Because the minimum signal is not 0mA, the system can easily identify abnormal conditions.
For example:
- 4mA → Normal zero measurement
- 0mA → Possible cable break, power failure, or transmitter fault
This makes troubleshooting easier in industrial applications.
Why Do Control Systems Use 1-5V Signals?
Although 4-20mA is ideal for field transmission, control systems often use voltage signals internally.
The reason is that many devices in a control room need to share the same signal.
Examples:
- PLC analog input modules
- DCS systems
- Digital displays
- Alarm systems
- Controllers
A voltage signal allows multiple devices to receive the same measurement signal more conveniently.
If multiple instruments directly receive a current signal, the input resistance of each device would affect the current loop and may exceed the transmitter’s load capability.
Therefore, voltage signals are more suitable for signal distribution inside control panels.
How Is 4-20mA Converted to 1-5V?
The conversion principle is based on Ohm’s Law:
A standard 250Ω precision resistor is connected in the current loop.
At 4mA:
At 20mA:
Therefore:
4-20mA = 1-5V
The 250Ω resistor converts the current signal into a corresponding voltage signal.
In many industrial systems, this conversion is performed by devices such as:
- Signal converters
- Isolators
- Distribution modules
- PLC input modules
Practical Example: Pressure Transmitter Signal Conversion
A pressure transmitter with a measuring range of:
0-10 bar
outputs:
4-20mA
The PLC receives:
1-5V
The signal conversion is:
| Pressure | 4-20mA Output | Converted Voltage |
|---|---|---|
| 0 bar | 4mA | 1V |
| 5 bar | 12mA | 3V |
| 10 bar | 20mA | 5V |
The PLC can then calculate the pressure value based on the received voltage signal.
4-20mA vs 1-5V: Which Signal Is Better?
Both signals have their own advantages.
| Feature | 4-20mA | 1-5V |
|---|---|---|
| Transmission distance | Excellent | Limited |
| Resistance to interference | High | Lower |
| Cable resistance influence | Very small | Higher |
| Multi-device connection | More difficult | Easier |
| Typical application | Field instruments | Control panels |
In modern industrial automation systems:
4-20mA is mainly used between field instruments and control rooms, while 1-5V is commonly used inside control systems.
Conclusion
The conversion from 4-20mA to 1-5V is not simply a signal format change. It is a practical solution that combines the advantages of both transmission methods.
- 4-20mA provides reliable long-distance transmission with strong noise resistance.
- 1-5V allows convenient signal sharing and processing inside control systems.
- A simple 250Ω resistor enables accurate conversion between the two standards.
Understanding this signal conversion principle is essential for engineers working with industrial instrumentation, PLC systems, and process control applications.