Wiring 4 — PLC Digital Output to Contactor (via Interposing Relay)

PLC Digital Output to Contactor (via Interposing Relay)

A PLC digital output is a small-signal transistor or relay. Its current rating is typically 0.1–0.5 A — enough to drive a pilot lamp or a small sensor, but nowhere near sufficient to pull in a contactor coil that demands 0.5–2 A. Connecting a contactor coil directly to a PLC transistor output will degrade or destroy the output stage over time.

The interposing relay

An interposing relay (sometimes called a coupling relay or pilot relay) solves this mismatch. The PLC output energises the relay coil — a low-current load the PLC can handle comfortably. The relay's normally-open (NO) contact, which can switch significantly higher currents, then passes 24 VDC to the contactor coil. The interposing relay also provides galvanic isolation between the PLC's logic circuit and the contactor's field circuit.

Relay contact terminal numbering follows IEC 60947-5-1: terminal 11 is the common (input); 12 is the normally-closed (NC) output; 14 is the normally-open (NO) output. In this lesson the 24 V supply enters 11, and the contactor coil is wired to 14 so the contactor only picks up when the relay is energised.

The flyback diode

When a relay coil is de-energised, the collapsing magnetic field generates a brief but large voltage spike (inductive kick). Without protection this spike — often hundreds of volts — appears directly across the PLC output transistor, shortening its life with every cycle.

A flyback diode wired in reverse-parallel across the relay coil provides a low-impedance path to absorb the spike. Orientation is critical: the cathode connects to the positive side of the coil (A1, the +24 V side); the anode connects to the negative side (A2, the 0 V side). Reverse this and the diode becomes a short circuit across the 24 V supply — the PSU current-limits and nothing works.

Contactor terminal naming: L vs T

The contactor's three-pole main contacts are labelled by convention: L1, L2, L3 are the line-side (supply) terminals; T1, T2, T3 are the load-side (output) terminals. Mains three-phase power enters the L terminals; the motor winding connections leave from the T terminals. When the coil is de-energised the main contacts are open and no current flows to the motor.

Motor PE bonding

Every motor frame must be bonded to the protective-earth (PE) busbar. In the event of an insulation fault, a properly bonded frame allows fault current to flow and trips the upstream protection device. Without the PE bond a fault can energise the motor casing at line voltage — a lethal hazard.