How to suppress relay output interference in I/O modules?

Suppressing interference from I/O module relay outputs requires addressing the arcing generated during contact opening and the back electromotive force of inductive loads. A freewheeling diode, such as a fast recovery diode RS1AM, should be connected in parallel across the relay coil to absorb the reverse voltage spikes during turn-off. An RC snubber circuit, typically 100Ω in series with a 0.1μF resistor, should be connected in parallel across the contacts to extinguish the arc and suppress voltage spikes. For AC loads, a varistor such as a 7D series resistor or a bidirectional TVS diode can be used across the contacts. Output cables should use twisted-pair or shielded wires and be kept away from sensitive signal lines.

On the PCB layout, relays should be kept away from analog circuits and communication interfaces, and their drive circuit ground should be treated as a separate noise ground and connected to the main ground via a ferrite bead. If relay switching is frequent, consider using solid-state relays to eliminate the arcing problem of mechanical contacts. Additionally, add a π-type filter to the relay drive power supply to prevent switching noise from being conducted through the power lines. During system installation, ferrite cores can be used on the relay output lines to suppress high-frequency radiation.