What is 4–20 mA? How does a current loop work and how do you measure it?
Most pressure, temperature, level and flow sensors in a factory send their reading to the PLC as a current, not a voltage: somewhere between 4 and 20 milliamps. A signal that survives hundreds of metres of cable and announces itself when a wire breaks.
What does it do? In a 4–20 mA loop the 24 V supply, the transmitter, the field cable and the PLC input are all in series. A 2-wire transmitter takes its power from the loop too, and sets the current it draws according to what it measures: the bottom of the range is 4 mA, the top 20 mA. Since the current is the same everywhere in a series circuit, cable resistance and length don't change the reading, and it shrugs off noise. A 250 Ω resistor at the PLC input turns the current into 1–5 V. Making zero 4 mA (a 'live zero') does two jobs: the transmitter powers itself from that current, and 0 mA (or < 3.6 mA) is recognised as 'wire broken / sensor fault'. With 0–10 V signals a broken wire looks just like 'value zero'.
Tap the dashed parts for what they do, how to measure them and how they usually fail. Numbered points are scope measurements.
Phase 1/4: 4 mA (0%)
Even with the measured value at the bottom (0 bar), the transmitter draws 4 mA from the loop. This 'live zero' lets the transmitter power its own electronics from the loop and lets the PLC tell 0 bar apart from a broken wire. Across 250 Ω: 4 mA × 250 Ω = 1 V.
1. Loop current
Moves between 4 and 20 mA as the process value changes. It's the same at every point in the loop.
On the scope
Multimeter4–20 mA
🔍 How to spot it on a board
•Two cable entries on the sensor head and a '4…20 mA, 10…30 V DC' label (2-wire transmitter)
•An 'AI' (analog input) card on the PLC, with I+ / I− or '0/4–20 mA' at its terminals
•A 24 V DC supply in the panel, sometimes with separately fused terminals for each loop
🔧 How to measure it
•Current is measured in series: lift one core from its terminal and insert the meter on mA DC (probe in the mA jack).
•Without opening the loop: an mA clamp, or the voltage across the 250 Ω at the PLC input (I = V / 250).
•Work out the value: I = 4 + 16 × (reading / range). On a 0–10 bar transmitter, 5 bar → 12 mA.
•Voltage at the transmitter terminals: even at 20 mA it must stay above the transmitter's minimum (usually 10–12 V).
⚠ Common faults
•0 mA: a broken wire, loose terminal, blown supply fuse or a transmitter wired with reversed polarity.
•Around 3.6 mA or 21–22 mA: the transmitter is reporting its own fault (NAMUR NE43); check the sensor.
•Between 4 and 20 but wrong: transmitter calibration or PLC scaling (range, units) is off; leakage in a damp junction box also shifts the reading.
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