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4–20 mA current loops for beginners

Why industry sends measurements as 4–20 mA, how a 2-wire loop is powered, how to convert mA to engineering units and how NAMUR NE 43 signals faults.

Why current and not voltage

A current loop sends the same current through every part of a series circuit, so long cables and connection resistance do not change the reading. The receiver measures the current, usually as the voltage across a known resistor such as 250 Ω, which turns 4–20 mA into 1–5 V.

The zero point is 4 mA, not 0 mA. This "live zero" means a broken wire (0 mA) can be told apart from a real minimum value.

Scaling

Converting a current to a process value is a straight line: value = LRV + (I − 4) / 16 × (URV − LRV). For a level transmitter ranged 0–5 m, 12 mA is the middle: 2.5 m. The reverse is I = 4 + 16 × (value − LRV) / (URV − LRV).

Two-wire loop power

A 2-wire transmitter takes its own power from the loop. A 24 V DC supply drives current through the transmitter and the input resistor; the transmitter regulates that current. The voltage left at the transmitter at 20 mA must stay above its minimum operating voltage from the data sheet, after subtracting the drop across the input and the cable.

Example: 24 V supply, 250 Ω input and 20 Ω of cable at 20 mA leaves 24 − 0.02 × 270 = 18.6 V at the transmitter.

Fault signalling (NAMUR NE 43)

Many transmitters follow the NAMUR NE 43 recommendation: currents at or below about 3.6 mA or at or above about 21 mA indicate a device fault rather than a measurement. A PLC can use those limits to raise a "transmitter fault" alarm and move the control to a safe action, as the pump station example in PlantFlow 3D does.

Try it yourself

Open PLC & Process, then set the LT-101 fault to "open loop". Watch the PLC detect the fault and switch to the backup level switch. Use Tools → 4–20 mA to practise conversions.

Open the PlantFlow 3D simulator →

Safety: this guide explains principles for learning. Real installations must follow the applicable codes, the equipment manufacturer's instructions and the approval of a competent, qualified person. Isolate, lock off and prove dead before touching any circuit.