Wiring 6 — 2-Wire 4-20 mA Loop-Powered Analog Input

2-Wire 4-20 mA Loop-Powered Analog Input

Industrial process instruments — pressure transmitters, temperature sensors, flow meters — communicate their measurements using a 4-20 mA current loop. Unlike a voltage signal, a current loop is immune to the voltage drop on long cable runs. A 50-metre cable with 5 Ω loop resistance drops 0.1 V at 20 mA — negligible. A 0–10 V signal over the same cable would lose 1 V and introduce a 10 % measurement error. Current rules in industrial wiring.

The 4 mA live zero

The signal range starts at 4 mA, not 0 mA. This live zero is one of the most important diagnostic features in analogue instrumentation:

• 0 mA — broken wire, unpowered transmitter, or shorted loop. It is always a fault, never a valid process reading.
• 4 mA — transmitter healthy, process variable at zero (e.g. 0 bar, 0 °C offset, or 0 % level, depending on the sensor range).
• 20 mA — process variable at full scale.

A controller can therefore detect a broken wire purely in software, without a dedicated fault contact.

2-wire vs 4-wire transmitters

A 4-wire transmitter uses a dedicated pair of wires for power and a separate pair for the signal. These are used when the transmitter needs more power than the loop alone can supply, or when the transmitter also communicates digitally (e.g. HART or Modbus).

A 2-wire (loop-powered) transmitter is simpler and more common. It uses the same two wires for both power and signal. The transmitter is wired in series in the current loop and modulates its own current draw between 4 mA and 20 mA to encode the process variable. The loop supply current flows through the transmitter electronics — the transmitter is powered by the loop itself.

Polarity and the loop path

The current-loop path in this lesson is:

PSU +24V → terminal block → transmitter + → transmitter − → PLC AIW0+ → PLC AIW0− → terminal block → PSU 0V

Polarity matters on every node in that series chain:

• The transmitter's + terminal connects to the positive side of the loop (coming from +24V).
• The transmitter's − terminal connects to the PLC analog input + terminal. This is counterintuitive: the current exits the transmitter through its − terminal and enters the PLC card at AIW0+. The PLC card contains an internal burden resistor (typically 250 Ω); the ADC measures the voltage drop across it (1–5 V at 4–20 mA). The + and − labels on the PLC card refer to the current-flow convention through the burden resistor, not to a local voltage source.
• Many transmitters include reverse-polarity protection, but reversing the wiring is never intentional — if the transmitter is not reverse-protected it will be damaged.

Analog input card power

The PLCAnalogInputCard has its own +24V and 0V power terminals for the card's logic supply. Do not confuse these with the loop supply terminals. The card must be powered from the distribution bus before the analog input channels will function.

Single-point shield grounding

Instrument cables are shielded to reject electromagnetic interference (EMI) from variable-frequency drives, relays, and other noise sources nearby. The shield must be connected to earth at one end only — the PLC (panel) end. If both ends are grounded, a small voltage difference between the two earth points drives a circulating current through the shield, creating a ground loop that injects noise directly into the measurement. Connecting the shield to the PLC card's dedicated SHIELD terminal ensures a single-point bond via the card's internal PE connection. The field end of the cable shield stays floating (unterminated).