A complete technical guide to the 4-20mA current loop — why it is the standard for industrial signal transmission, how it works, how to convert readings, and how to troubleshoot common problems.
Industrial plants need to transmit measurement signals — temperature, pressure, flow, pH, level, humidity — from sensors located in the field to control rooms and PLCs sometimes hundreds of metres away. Early systems used pneumatic signals (3-15 PSI air pressure) or voltage signals (0-10V). Both had significant limitations. Pneumatic signals required air lines. Voltage signals degraded over long cable runs as the wire resistance caused voltage drop.
The 4-20mA current loop solved these problems. A current signal is not affected by cable resistance — as long as the loop voltage is sufficient to drive the current through the wire, the same current flows at both ends of the cable regardless of cable length or resistance. This made 4-20mA ideal for long-distance signal transmission in industrial plants.
A 4-20mA loop consists of three elements — a power supply (typically 24VDC), a transmitter, and a receiver — all connected in series with two wires. The transmitter controls the current flowing in the loop based on the measured process value. At the minimum process value (e.g., 0°C on a 0-100°C temperature transmitter), the transmitter allows exactly 4mA to flow. At the maximum process value (100°C), it allows 20mA to flow. Any value in between is represented by a proportional current between 4 and 20mA.
The receiver reads the current flowing in the loop — typically by measuring the voltage across a precision resistor — and uses a simple linear formula to convert the current back to the process value.
| Current (mA) | % of Range | Example: 0-100°C | Example: 0-10 pH | Example: 0-200 bar |
|---|---|---|---|---|
| 4.0 mA | 0% | 0°C | pH 0 | 0 bar |
| 6.4 mA | 15% | 15°C | pH 1.5 | 30 bar |
| 8.0 mA | 25% | 25°C | pH 2.5 | 50 bar |
| 12.0 mA | 50% | 50°C | pH 5.0 | 100 bar |
| 16.0 mA | 75% | 75°C | pH 7.5 | 150 bar |
| 20.0 mA | 100% | 100°C | pH 10 | 200 bar |
The 4mA minimum — rather than 0mA — is a deliberate design feature that provides two important benefits. First, it enables wire break detection. If a wire breaks or the transmitter loses power, the current in the loop drops to 0mA. Since a valid signal can never be 0mA (the minimum is 4mA), a 0mA reading unambiguously indicates a fault — not a valid zero measurement. This is critical for safety in processes where a zero reading could be confused with a genuine measurement.
Second, the 4mA baseline powers the transmitter in 2-wire configurations. The transmitter electronics draw their operating power from this baseline current, eliminating the need for a separate power supply wire to the field instrument.
| Type | Wires | Power Source | Advantage |
|---|---|---|---|
| 2-wire transmitter | 2 wires only | Loop-powered (from 4mA) | Minimal wiring — simplest installation |
| 4-wire transmitter | 2 power + 2 signal | Separate power supply | More power for complex electronics |
| Signal | Type | Range | Max Cable Length | Noise Immunity |
|---|---|---|---|---|
| 4-20mA | Current | 4 to 20mA DC | 300m+ (practical) | Excellent |
| 0-10V | Voltage | 0 to 10V DC | ~30m | Poor |
| 0-5V | Voltage | 0 to 5V DC | ~20m | Poor |
| 1-5V | Voltage | 1 to 5V DC | ~50m | Moderate |
| HART | Current + digital | 4-20mA + data | 1500m | Excellent |
The conversion formula is simple and linear:
Value = ((mA − 4) ÷ 16) × (Max − Min) + Min
Example: Temperature transmitter range 0–500°C, current reading 12mA:
Value = ((12 − 4) ÷ 16) × (500 − 0) + 0 = (8 ÷ 16) × 500 = 0.5 × 500 = 250°C
Most industrial instruments, PLCs, and data loggers perform this calculation automatically when programmed with the range Low and High values.
When a controller or indicator provides a 4-20mA output for connection to a SCADA, PLC, or data recorder, the output may be isolated or non-isolated. An isolated output means the 4-20mA circuit is electrically separated from the instrument's internal circuitry using an optical or transformer barrier. Isolation prevents ground loops — circulating currents caused by different earth potentials at the transmitter and receiver ends — which cause measurement errors and can damage equipment. For reliable SCADA integration, always specify isolated 4-20mA output.
A standard industrial current loop where 4mA represents the minimum process value and 20mA represents the maximum. Used to transmit measurement data between field instruments and control systems. The preferred signal standard for long-distance transmission in industrial plants.
4mA minimum allows wire break detection (0mA = fault, not zero measurement) and powers 2-wire transmitters from the loop current itself. If the signal were 0-20mA, a broken wire and a zero measurement would be indistinguishable.
4-20mA is a current signal unaffected by cable resistance — suitable for hundreds of metres. 0-10V is a voltage signal that drops with cable resistance — limited to short runs of 20-30m. 4-20mA has better noise immunity in electrically noisy industrial environments.
Use: Value = ((mA − 4) ÷ 16) × (Max − Min) + Min. At 12mA on a 0–100°C transmitter: ((12−4)÷16) × 100 = 50°C. Most instruments calculate this automatically when programmed with the range.
An electrically isolated output where the 4-20mA circuit is separated from the instrument internals using optical or transformer isolation. Prevents ground loops that cause measurement errors when connecting to SCADA or PLC systems. Always specify isolated output for reliable integration.
Countronics pH controllers, conductivity controllers, process indicators, and data loggers all support isolated 4-20mA. Contact us for specifications and pricing.