What you'll learn
Drive a 24 VDC pneumatic solenoid through an interposing relay. The PLC output only energises the relay coil; the relay contact switches the field load. Add a flyback diode at the solenoid to clamp the inductive voltage spike when the valve is switched off.
Lab time: ~15 minutes.
Lesson briefing
PLC Relay Output to Solenoid Valve
Drive a 24 VDC pneumatic solenoid through an interposing relay. The PLC output only energises the relay coil; the relay contact switches the field load. Add a flyback diode at the solenoid to clamp the inductive voltage spike when the valve is switched off.
The device in the real world
PLC relay or transistor outputs are small. A pneumatic solenoid valve coil can draw several times more current than one output point should be asked to carry, so panel builders place a small interposing relay between them. The PLC energises a relay coil, and the relay contact switches the valve supply. Across every DC coil you will also find a flyback diode, a small component that gives the coil's stored energy a harmless path when the current is cut off.
Why it is wired this way
The interposing relay protects the PLC and allows separate load protection. A flyback diode is reverse biased in normal operation: cathode to +24 V and anode to 0 V. Reversing it creates a short; omitting it exposes the contact and electronics to a high transient.
Worked example
Suppose output Y0 turns on a 24 V valve that draws 0.4 A. The PLC output only drives the relay coil, about 20 mA. When the program sets Y0, K1 closes between terminals 11 and 14 and puts 24 V across the valve. When the program clears Y0, the valve coil tries to keep current flowing and its terminal voltage spikes to many times 24 V for an instant. A diode wired across the coil, cathode to the positive side, conducts that spike back through the coil and dies away. Without it, the arcing contact wears out early and the noise can reset a PLC. In normal running the diode is reverse biased and does nothing, which is how you can tell you wired it correctly.
Common mistakes
- Fitting the diode backwards. It then conducts every time the valve is on and shorts the supply.
- Driving the valve straight from the PLC output and skipping the relay.
- Wiring the relay coil return to +24 V so the relay never has a voltage across it.
- Putting the diode across the relay contact instead of across the load coil.
What you can now do
You can drive an inductive load safely through an interposing relay, and place the flyback diode correctly with the cathode on the positive side.
Commissioning standard
Check the diode polarity against its stripe before power, then switch the output a few times and listen for a clean click with no chatter.
Hints
Wire PLC Y0 to relay A1 and return A2 to 0 V.
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This lesson uses 8 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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