PLC Output Points Keep Burning Out? ULO GROUP Interposing Relays Help Protect PLC Outputs
Frequent PLC output failures are not always caused by programming issues; hardware wiring can also be a contributing factor. Directly driving contactors with transistor outputs and failing to install flyback diodes across DC coils can generate back EMF, potentially damaging PLC outputs. Proper use of interposing relays and flyback diodes helps reduce the risk of output failure.
Direct Drive, Risk of Output Damage
When a PLC transistor output directly drives a contactor, the contactor coil acts as an inductive load. When the coil is de-energized, it generates back EMF, producing a voltage spike that may damage the PLC output circuitry.
Missing Flyback Diodes, Voltage Spikes
For DC coil loads, the absence of a flyback diode allows back EMF generated during de-energization to remain insufficiently suppressed, potentially damaging the PLC output. Installing a flyback diode in reverse parallel across the coil provides a current path for the inductive current, helping reduce voltage spikes.
Interposing Relays, Output Isolation
For inductive loads such as contactors, an interposing relay is recommended between the PLC output and the load. The PLC controls the interposing relay, whose contacts then switch the contactor. This separates the control circuit from the load circuit and helps reduce the impact of electrical transients from the load side on the PLC output.
By combining interposing relay isolation with flyback diode protection, the effects of back EMF from inductive loads on PLC outputs can be reduced, helping prevent output failures and improve the reliability of industrial control systems.
From control signal isolation to inductive load protection, ULO GROUP remains focused on the practical needs of industrial applications, providing reliable relay products and connectivity solutions to support safer and more dependable industrial control.
—— ULO GROUP