the 4-20 mA current loop
/ four to twenty milliamp /
The 4-20 mA current loop is the rugged way to send a sensor reading down a long wire in a noisy factory. Instead of representing the measurement as a voltage — which sags over long cables and picks up every bit of electrical hum — it represents the measurement as a current that flows around the loop. The transmitter sets the current: 4 milliamps means the bottom of the range, 20 milliamps means the top, and everything in between is proportional. The receiver simply measures the current to recover the reading.
Why current instead of voltage? A series current is the same everywhere in a loop (Kirchhoff's current law), so it does not matter that the wire has resistance or that the run is hundreds of meters — the 12 mA leaving the sensor is the 12 mA arriving at the controller. There is a second piece of genius: the range starts at 4 mA, not 0. That non-zero 'live zero' does double duty. It powers the transmitter itself (the electronics run on the 4 mA minimum current, so a two-wire sensor needs no separate power), and it lets the system detect a fault: a reading of 0 mA cannot be a real measurement, so a broken wire (which gives 0 mA) is instantly distinguishable from a genuine bottom-of-scale reading (4 mA). The receiver usually drops the current across a precise 250 Ω resistor, turning 4-20 mA into a clean 1-5 V for the converter.
Why this matters: 4-20 mA is the backbone of industrial process control — pressure, flow, level, and temperature transmitters in plants worldwide speak it. It is immune to voltage drop, highly resistant to electrical noise, self-powered over two wires, and fault-detecting by design. The honest tradeoffs: it carries one slow analog value per loop (digital buses like fieldbus carry more), it needs enough loop supply voltage to push the current through all the series resistances, and it is for rugged plant signaling, not for fast or high-resolution measurement.
A pressure transmitter spans 0 to 10 bar as 4 to 20 mA. At 5 bar (mid-range) it pushes 4 + (5/10) times (20 - 4) = 12 mA around the loop. The controller drops that 12 mA across a 250 Ω resistor to get 12 mA times 250 Ω = 3.0 V, sitting neatly in a 1-5 V window — and if the cable breaks, the current falls to 0 mA, flagging a fault rather than reading 0 bar.
Current carries the value unscathed; the 4 mA live zero powers the sensor and flags broken wires.
The 4 mA live zero is not wasted current — it powers a two-wire transmitter and makes a broken wire (0 mA) instantly distinguishable from a true minimum reading (4 mA). The loop also needs enough supply voltage to drive 20 mA through all the series resistance, or the top of the range starves.