What you'll learn
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.
Lab time: ~12 minutes.
Lesson briefing
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.
Hints
Start with the 24 V and 0 V distribution buses. Wire psu-1.+V → tb-24v.t0 and psu-1.0V → tb-0v.t0, then supply the PLC from t1 on each bus. The PSU PE bond goes to gnd-bar.pe0.
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This lesson uses 11 placed components on the lab canvas. Components are vendor-neutral (no proprietary trademarks); the wiring rules apply to any equivalent industrial part.
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