Understanding 4-20mA Current Loops in Process Control

Acronym Legend:

mA – Milliamp
PID – Proportional Integral Derivative
SCADA – Supervisory Control and Data Acquisition
PLC – Programmable Logic Controller
DC – Direct Current
AC – Alternating Current
HMI – Human-Machine Interface
GPH – Gallons per Hour
pH – Power of Hydrogen
ORP – Oxidation-Reduction Potential

Introduction

The 4–20mA current loop is a DC signal standard used to transmit analog process information in industrial control systems. It’s widely used because, unlike voltage, current remains constant over long distances, unaffected by resistance in wiring, making it the most reliable way to send process signals from sensors to PLCs, PIDs, or SCADA systems.

You can think of it like water in a pipe: the flow stays the same regardless of the length of pipe, unless there’s a leak, and in this case, a “leak” would be a loss of current due to equipment failure or wire damage.

Background and Evolution

Before electronics, process control ran on pneumatic signals, using air pressure ranging from 3–15 psi. Why 3–15? Below 3 psi, the signal was unreliable. Using 3 psi as a “live zero” helped differentiate between an actual 0 (like power loss or failure) and a low signal. The range also divided easily:

3 psi = 0%
6 psi = 25%
9 psi = 50%
12 psi = 75%
15 psi = 100%

Once electronics became cost-effective in the 1950s, current-based signaling started replacing air. Several current ranges were tested, such as 10–50mA, which suited magnetic amplifiers of the era.

Eventually, 4–20mA became the preferred standard:

It follows the same percentage breakdown as pneumatics.
It corresponds easily to 1–5V analog inputs when paired with a 250Ω resistor.
It offers a live-zero (4mA) to detect system health.
And it’s safer: the human heart can handle up to 30mA, so 20mA keeps things under that threshold.

Ultimately, 4–20mA won over options like 5–25mA because it’s cleaner math (multiples of 2) and stayed within the safe 3–30mA envelope.

Why Current Instead of Voltage?

Voltage drops over long distances and through resistance. Current doesn’t, as long as the loop is closed and properly powered, the signal strength remains consistent. Even with inline resistors (used to prevent equipment damage), current still flows steadily. That’s why current loops are more accurate and dependable than voltage signals for process control.

Components of a 4–20mA Current Loop

1. Power Source

Most loops run on 24VDC, though some older or specialty systems use 12V, 15V, or even 36V. DC is used because it’s constant, unlike AC which cycles and could interfere with signal clarity.

Your power supply must exceed the combined voltage requirements of the transmitter, the voltage drop across the receiver, and the IR drop in the wiring, especially on long runs. Always account for maximum loop current, not just 20mA—some transmitters will signal alarm conditions with higher output.

2. Sensor

This is the device that detects a physical condition—like:

Temperature
Flow rate
Pressure
Humidity
pH
ORP
Tank level
Motor speed
…and many others. It feeds raw data to the transmitter.

3. Transmitter

The transmitter converts the sensor’s raw signal into a 4–20mA signal that a controller or receiver can understand. Think of it as the “translator” between the real world and your control system.

For example:
If your span is 0–300 GPH and your sensor detects 150 GPH, the transmitter outputs 12mA, which is exactly 50%.

Transmitters typically create the voltage drop in the loop, requiring the controller to include a precision resistor to read the signal as voltage if needed (e.g., 1–5V across a 250Ω resistor).

4. Receiver / Controller

At the other end of the loop is your receiver, which could be a:

PLC input card
Panel meter
VFD
SCADA interface
BACTALK or other building automation system

This device reads the 4–20mA signal and:

Displays it locally on an HMI
Logs it in software
Triggers alarms or motor controls based on the value

Example:
If a tank is 40 feet deep and your transducer detects 10 feet, it will send 8mA (25%) to the controller. The controller reads and reflects 10’ on your HMI or stores it for trend tracking.

Span matching is critical—your field device and input card need the same configured span for accurate display and control.

Control Functions Beyond Reading

4–20mA loops don’t just read values, they can control too.

Example:

A tank level reaches 20mA (full).
The PLC reads this and, via ladder logic, triggers a pump to lower the level.
The pump runs at variable speed, if needed, using analog output rather than just on/off (discrete).

Another example is HVAC:

A thermostat set to 71°F might send 15.36mA
If changed to 78°F, it sends 16.48mA
The fan motor (discrete) turns on when the temp exceeds the setpoint.

Analog vs. Discrete Inputs

Analog: Works with 4–20mA signals; reflects continuous values like flow, temp, level, etc.
Discrete: Digital; tells you status; on/off, open/closed, alarm/no alarm.

They work together. For instance:

An analog input detects high level
A discrete output activates a relay to start a motor
The analog output controls how much the valve opens, not just whether it opens

Signal Breakdown Summary

Signal	Percent	Pneumatic (psi)
4mA	0%	3 psi
8mA	25%	6 psi
12mA	50%	9 psi
16mA	75%	12 psi
20mA	100%	15 psi

Conclusion

The 4–20mA current loop remains the gold standard in industrial control for one reason: reliability. It’s low-power, noise-resistant, and straightforward to scale. With proper setup; power, sensor, transmitter, and receiver, you get real-time, accurate control over your systems, whether you’re managing water flow, chemical dosing, HVAC, or plant-wide automation.