DC Solenoids are the worse culprits for electrical surges on your system. When the electrically generated field collapses an opposite polarity voltage is generated. This voltage spike can be high enough to weld the contacts on a PLC output relay.
To protect your PLC output relay, use a diode to ensure that when the solenoid switches off the voltage spike is released through the diode instead of the relay.
The diode should be rated to handle 10 times the voltage that you are switching and enough for the current flow of the circuit.
Parts of the diode:
The cathode of the diode is marked by a band. The electron flow will only occur in one direction.
Install the diode as close as possible in parallel with the solenoid. The cathode should be wired to the positive source of the solenoid. (Dissipate negative polarity voltage spike)
Note: You could also install an interposing device to handle the surge such as a SSR. (Solid State Relay) This is generally more money, space in the panel and wiring.
Note: Allot of solenoids come already with surge suppressing diodes from the manufacturer. If not, you will usually need this information when troubleshooting and discover your welded contacts of the output relay.
If you have any questions or need further information please contact me.
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