The 4-20mA current loop is one of the most widely used signaling methods in industrial automation. It is commonly used to transmit sensor values such as temperature, pressure, flow and level over long distances with high reliability.
Unlike voltage-based signals, the 4-20mA loop uses current to represent a measured value, making it highly resistant to electrical noise and signal loss.
What Is a 4-20mA Current Loop?
A 4-20mA loop is an analog signaling method where:
- 4mA represents the minimum value
- 20mA represents the maximum value
For example:
- 4mA = 0°C
- 20mA = 100°C
Values in between are proportional.
Why 4mA Instead of 0mA?
Using 4mA as the minimum has an important advantage:
- 0mA indicates a fault such as a broken wire
This makes fault detection easy and reliable.
How the Loop Works
A basic current loop includes:
- Power supply
- Transmitter (sensor device)
- Receiver (PLC or controller)
- Wiring forming a closed loop
The transmitter controls the current flowing through the loop based on the measured value.
Two-Wire vs Four-Wire Transmitters
There are two common types of transmitters.
Two-wire:
- Powered by the loop itself
- Simple wiring
Four-wire:
- Has a separate power supply
- Can drive stronger signals
Advantages of Current Signaling
- Very resistant to electrical noise
- Works over long distances
- Less affected by voltage drop in cables
- Simple fault detection
Typical Applications
- Industrial sensors
- Process control systems
- PLC inputs
- Building automation
Converting 4-20mA to Voltage
To read a current loop with a microcontroller, a resistor is used.
Example:
- 250Ω resistor
- 4mA = 1V
- 20mA = 5V
This converts the current into a measurable voltage.
Common Problems
- Wrong resistor value
- Open loop wiring
- Insufficient supply voltage
- Grounding issues
4-20mA vs 0-10V
| Feature | 4-20mA | 0-10V |
|---|---|---|
| Noise immunity | High | Lower |
| Distance | Long | Shorter |
| Fault detection | Yes | No |
When to Use 4-20mA
- Long cable distances
- Noisy industrial environments
- Reliable sensor transmission
Conclusion
The 4-20mA current loop remains one of the most reliable analog signaling methods in industrial systems. Its resistance to noise and ability to detect faults make it ideal for critical measurement and control applications.