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4–20 mA Current Loop — Industrial Analog Signaling

4–20 mA is the universal language of industrial process measurement. Every pressure transmitter, temperature sensor, flow meter, and level gauge in a process plant almost certainly speaks 4–20 mA. It has been the dominant field instrumentation standard since the 1960s and is not going away.

It is not a digital protocol. It has no addresses, no frames, no clock. It is an analog current signal — but its simplicity and robustness are exactly why it outlasted every digital challenger for field sensing.

The Fundamental Concept

Unlike voltage signals (0–5 V, 0–10 V), a 4–20 mA loop transmits information as a current. Current, unlike voltage, does not change as it travels through wire resistance. A sensor transmitting 12 mA at the start of a 300-metre cable delivers exactly 12 mA at the control room end — regardless of cable resistance.

flowchart LR classDef src fill:#dbeafe,stroke:#2563eb,color:#1e3a5f classDef wire fill:#dcfce7,stroke:#16a34a,color:#14532d classDef rcv fill:#fee2e2,stroke:#dc2626,color:#7f1d1d PSU["24 V DC\nPower Supply"]:::src TX["Field Transmitter\n(pressure, temp, flow...)\nControls current\n4 mA → 0%\n20 mA → 100%"]:::src WIRE["2-wire cable\n(up to 1000 m+)"]:::wire PLC["PLC / DCS\nReceiver\n250 Ω shunt resistor\n4 mA → 1 V\n20 mA → 5 V\nRead by ADC"]:::rcv PSU --> TX --> WIRE --> PLC --> PSU

The transmitter, power supply, and receiver form a series loop — the same current flows through all of them.

The Scale: Why 4 mA and Not 0 mA?

Current Meaning
4 mA 0% of range (e.g., 0 bar, 0°C, empty tank)
12 mA 50% of range
20 mA 100% of range (e.g., 10 bar, 200°C, full tank)
< 3.6 mA Fault — broken wire or failed transmitter
> 21 mA Fault — transmitter overrange or short circuit

The live zero (4 mA instead of 0 mA) is the key design decision:

  1. Wire break detection: A broken wire reads 0 mA — clearly different from a valid 4 mA signal. You always know if the sensor is connected.
  2. Loop power: The 4 mA flowing at zero-output powers the transmitter electronics. No separate power supply cable needed (two-wire transmitters).
flowchart TD classDef ok fill:#dcfce7,stroke:#16a34a,color:#14532d classDef warn fill:#fef9c3,stroke:#ca8a04,color:#713f12 classDef err fill:#fee2e2,stroke:#dc2626,color:#7f1d1d A["< 3.6 mA → FAULT\nBroken wire or dead transmitter"]:::err B["3.6–4.0 mA → Underrange\nSensor below minimum"]:::warn C["4.0–20.0 mA → Valid signal\nLinear: 0% to 100%"]:::ok D["20.0–21.0 mA → Overrange\nSensor above maximum"]:::warn E["> 21 mA → FAULT\nShort circuit or hardware failure"]:::err

Two-Wire vs Four-Wire Transmitters

flowchart LR classDef pwr fill:#fee2e2,stroke:#dc2626,color:#7f1d1d classDef sig fill:#dbeafe,stroke:#2563eb,color:#1e3a5f subgraph TW["Two-Wire (Loop-Powered)"] PSU2["24 V Supply"]:::pwr TX2["Transmitter\n(powered by loop current)"]:::sig RCV2["Receiver\n250 Ω"]:::sig PSU2 --> TX2 --> RCV2 --> PSU2 end subgraph FW["Four-Wire (Separately Powered)"] PSU4["24 V Supply\n(powers transmitter)"]:::pwr TX4["Transmitter\n(has own power)"]:::sig RCV4["Receiver\n250 Ω"]:::sig PSU4 -->|power wires| TX4 TX4 -->|signal wires| RCV4 end
Two-Wire Four-Wire
Wires 2 (power + signal share the loop) 4 (separate power and signal)
Power From the loop current itself (≥4 mA minimum) Separate supply
Complexity Simple, fewer wires More wiring, more flexibility
Current draw limit Must transmit at 4–20 mA range only Can draw more power independently
Typical use Field sensors (pressure, temp) Analyzers, high-power transmitters

Two-wire transmitters are the norm for field instruments. Four-wire is used when the transmitter needs more power than the loop can provide.

Reading the Signal at the PLC

At the control room end, a 250 Ω shunt resistor converts current back to voltage for the ADC:

4 mA × 250 Ω = 1.0 V   → 0% of range
12 mA × 250 Ω = 3.0 V  → 50% of range
20 mA × 250 Ω = 5.0 V  → 100% of range

Modern PLC and DCS analog input cards include this resistor internally. You just connect the wires and configure the channel range.

Converting Current to Engineering Units

The conversion from milliamps to the actual measured value is linear:

Value = (mA − 4) / 16 × (Max − Min) + Min

Example: A pressure transmitter ranged 0–10 bar reads 14.4 mA:

Value = (14.4 − 4) / 16 × (10 − 0) + 0
      = 10.4 / 16 × 10
      = 0.65 × 10
      = 6.5 bar

Why 4–20 mA Has Survived 60 Years

Challenge How 4–20 mA handles it
Long cable runs Current is immune to resistive voltage drop
Electrical noise Current source is high impedance — noise injects voltage, not current
Wire break detection < 3.6 mA = definite fault alarm
Ground loops Series loop — no reference ground needed
Hazardous areas Intrinsically safe barriers limit power easily (current ≤ 20 mA, voltage ≤ 24 V)
Interoperability Any transmitter + any receiver from any manufacturer — universal standard

Voltage signals (0–10 V) lose accuracy when cable resistance creates a voltage divider. Current does not have this problem.

HART — Digital Communication on Top of 4–20 mA

HART (Highway Addressable Remote Transducer) overlays a 1200 baud digital signal on top of the 4–20 mA current loop. The digital signal is a frequency-shift keying (FSK) signal centred around 0 mA (it averages to zero so it does not disturb the DC current value):

flowchart LR classDef analog fill:#dcfce7,stroke:#16a34a,color:#14532d classDef digital fill:#dbeafe,stroke:#2563eb,color:#1e3a5f A["4–20 mA\n(Process value\n— continuous)"]:::analog D["HART digital signal\n(±0.5 mA FSK overlay\n1200 baud)"]:::digital WIRE["Same 2-wire loop"] A -->|"superimposed"| WIRE D -->|"superimposed"| WIRE

With HART you can simultaneously:

  • Read the primary process value (via 4–20 mA as normal)
  • Communicate digitally to retrieve secondary variables, diagnostics, calibration, device tag, serial number

HART is still in millions of installed devices. It is why 4–20 mA instruments are often called "smart transmitters."

4–20 mA vs Digital Fieldbuses

4–20 mA Modbus RTU PROFIBUS IO-Link
Signal type Analog current Digital Digital Digital
Variables per cable 1 (the process value) Many registers Many Many
Wire break detection Yes (built-in) Software timeout Software timeout Yes
Distance 1000+ m 1200 m 1200 m 20 m
Noise immunity Very high High High Medium
Interoperability Universal Good (with profile) Vendor-specific Universal
Installed base Enormous (legacy) Large (industrial) Large (process) Growing (factory)
Complexity Very simple Moderate Complex Moderate

For a single-variable sensor that needs to run 500 m to a control room, 4–20 mA wins every time. For multi-variable sensors in a short-range factory network, digital buses are better.

Where You Will Find 4–20 mA Today

  • Pressure transmitters — every process plant
  • Temperature transmitters — thermocouple/RTD converted to 4–20 mA
  • Flow meters — electromagnetic, vortex, Coriolis output a 4–20 mA signal
  • Level transmitters — hydrostatic pressure, radar, ultrasonic
  • Valve positioners — receive 4–20 mA command, position the valve
  • Variable frequency drives — speed setpoint via 4–20 mA input
  • pH, conductivity analyzers — water treatment plants

Key Takeaway

4–20 mA is an analog current loop standard, not a digital protocol. A 4 mA live zero enables loop-powered two-wire transmitters and built-in wire-break detection. Current is immune to cable resistance and common-mode noise — that is why a signal launched 500 metres away arrives at exactly the same value. HART adds digital communication on the same loop without disturbing the analog signal.

It is the most widely deployed sensor interface in industrial process control, and understanding it is essential for anyone working with PLCs, DCS systems, or field instrumentation.