industrial//instrumentation//4-20 mA

2026-09-06

A 4-20 mA loop is the traditional analog signal of a field instrument, a current between 4 and 20 mA that represents the measured variable, for example 4 mA for 0 bar and 20 mA for 100 bar. The PLC's analog input converts the current into a digital value, usually by passing it through a precision resistor (250 Ω turns the range into 1 to 5 V) in front of an analog-to-digital converter.

The semantic intelligence is outside the sensor. The instrument sends one magnitude and nothing else; the PLC has to know what 4 mA and 20 mA mean. A smart sensor measures the same quantity and puts the meaning in the opposite place, handing over the value with its own status, diagnostics and identity.

The live zero is the design's best idea. Because the bottom of the scale is 4 mA and not 0, a reading of 0 mA can only mean a cut wire or a dead transmitter, so the loop reports its own failure instead of reporting an empty tank. Transmitters extend the trick by driving the current outside the measuring range on purpose when they detect an internal fault (the NAMUR NE 43 recommendation puts failure signals at or below 3.6 mA and at or above 21 mA), and the PLC treats those values as alarms rather than as measurements. A meter on the loop at the terminals tells the cases apart: no current is a dead loop, a current in range read wrongly is a scaling fault.

Current, not voltage, because it survives long cables and noise: the same current flows through every metre of the loop whatever the resistance of the wire, so the signal runs hundreds of metres across a plant, and two wires can carry both the signal and the instrument's power. The price is one variable per pair of wires.

It is still a measurement chain with its own errors: the scaling configured in the PLC, the tolerance of the resistor, the resolution of the input converter and the drift of the transmitter all end up in the number, and a span set wrong in one place reads as a perfectly plausible process.

It is the baseline every later step is measured against: HART keeps the loop and adds digital data on top; a smart sensor drops the loop for a digital protocol altogether.