The essence of the PLC output terminal is a “switch” that controls the power supply of external circuits through the on/off state of internal relays, transistors, or thyristors. The different types of PLC output modules (relay type, transistor type, thyristor type) determine the types of components they can drive—this is the “underlying logic” of wiring. Understanding this will prevent mistakes in component selection and wiring in the future.
5 Common Types of Components Connected to PLC Outputs, Along with Application Scenarios
Based on the high-frequency demands in industrial scenarios, the wiring of PLC outputs mainly revolves around three categories of needs: “power drive”, “signal indication”, and “execution actions”. The following five types of components cover over 90% of application scenarios:
Components that can be connected to PLC outputs:
PLC-4> Inverter -> Motor
PLC -> Contactor -> Motor
PLC -> Relay -> Contactor
PLC -> Servo Driver -> Servo Motor
PLC -> Stepper Driver -> Stepper Motor
PLC -> Solid State Relay -> Heating Element
PLC -> Solenoid Valve -> Cylinder
PLC -> Indicator Light
1
Relay/Intermediate Relay: “Bridge” Component
This is the most commonly used “partner” for PLC output terminals. Since the rated current of PLC output contacts is usually small (for example, the rated current of a single channel in relay-type modules is often 2A-5A), they cannot directly drive high-power devices. In this case, an intermediate relay is needed as a “transit”—the PLC controls the on/off state of the intermediate relay coil, and the high-capacity contacts of the intermediate relay then drive contactors, solenoid valves, and other devices.
Application scenarios: Almost all industrial equipment relies on this, such as the start and stop of motors in machine tools and the control of conveyor belts in production lines, which require intermediate relays to achieve “small current controlling large current”.
2
Contactor Coil: Driving High-Power Motors
The contactor is the core component for controlling high-power loads such as motors and heaters, and its coil needs to be controlled by the PLC output terminal. Note: The voltage of the contactor coil must match the PLC output circuit power supply (for example, a 220V coil connects to a 220VAC output circuit, and a 24V coil connects to a 24VDC output circuit). Additionally, a flyback diode (for DC coils) or an RC snubber (for AC coils) must be connected in parallel across the contactor coil to prevent damage to the PLC output module from back EMF when the coil is de-energized.
Application scenarios: Start and stop control of fans, water pumps, and conveyor belt motors. For example, in a factory workshop, the PLC output signal controls the contactor to engage, thereby driving the motor.
3
Solenoid Valve Coil: Controlling Fluid Flow
In pneumatic and hydraulic systems, solenoid valves are common loads for PLC output terminals. The solenoid valve coils also have voltage ratings (such as 24VDC, 220VAC). When wiring, two points must be noted: first, ensure the coil voltage matches the PLC output power supply; second, DC solenoid valve coils must have a flyback diode connected in parallel (with correct polarity), otherwise, back EMF can instantly damage the PLC transistor output.
Application scenarios: Cylinder extension and retraction, hydraulic cylinder actions, and pneumatic gripper opening and closing. For example, in an automated assembly line, the PLC output signal controls the solenoid valve to open and close, pushing the cylinder to grasp parts.
4
Indicator Lights (LED/Signal Lights): Status Visualization
The operational status of equipment needs to be “visible”. Connecting indicator lights to the PLC output terminal is the most direct way. Indicator lights have low power (usually a few watts or even less) and do not require intermediate relays; they can be directly connected to the PLC output terminal. Common types include power indicator lights, operation indicator lights, and fault indicator lights—for example, when the PLC outputs a “device running” signal, the corresponding green LED lights up; when it outputs a “fault” signal, the red LED lights up.
Application scenarios: Status display on control cabinet panels and equipment operation interfaces. For example, when the inverter is running, the PLC output signal lights up the “inverter running” indicator light.
5
Small Actuators: Directly Driving Low-Power Devices
For very low-power actuators (such as small electromagnetic locks, micro solenoid valves, buzzers), if their rated current is within the allowable range of the PLC output module (usually single channel ≤5A), they can be directly driven by the PLC output terminal. For example, in access control systems, when the PLC receives an “open door” signal, the output terminal energizes, unlocking the electromagnetic lock; similarly, in the event of equipment failure, the PLC output signal drives the buzzer to alarm.
Application scenarios: Access control, small alarm devices, and micro pneumatic components. For example, in a laboratory’s small fume hood, the PLC output signal controls a micro solenoid valve to open the exhaust channel.
Before actual wiring, be sure to disconnect the main power supply and use a multimeter to check for zero; when wiring, follow the order of “first strong current, then weak current; first main circuit, then control circuit”; after completion, do not rush to power on. First, check the tightness of terminals and whether the insulation of wires is damaged, then use a multimeter to measure the continuity of the output circuit. These seemingly tedious steps are crucial to avoid equipment damage and extend the lifespan of the PLC.