Protecting Outputs

[Guest dunbarbe]

I have question regarding protection for outputs. I have found that every now and then an output (relay) will stop working. If I have another output, I just swap and change the code and fix it that way. However, is there a way of protecting the outputs, say with a zener diode? This has happened about 4 or 5 times in the last couple of years. Or is this something common with relay outputs? I'm relatively new to the world of PLC's and any feedback would be greatly appreciated. Temperature is nominal, dust may be an issue (unlikely). I think its a 1746 AB Relay Output card. The outputs have controlled various devices, the latest being the coil of a standard 11 pin relay.

[Andy_P 7]

The first thing to check is if the rated load of the output relay is sufficient for the devices you are controlling. If you are driving a DC device then the usual basic way to supress arcing across contacts is to wire a diode in reverse across the connections of the device. This is standard practice for any kind of DC load like relay and solenoid coils. Another option is to have the PLC output drive another small relay which in turn switches power to your actual device. Even if your small relay then fails, you only have to replace this and not your output card. Obviously, when choosing relays for this job, loading considerations have to be taken into account. Hope this helps. Andy.

[Hawkmoon42 0]

where I work they some time go for over kill. Rather than use your standard teminal strip we always use fused teminals for outputs. It may be a little bit more expensive but how much is a AB output card!

[Guest]

Greetings dunbarbe, ... and beside the excellent advice you're already received, you can find the "official" answer in the: SLC Modular Style Hardware Manual there is a specific note on page 2-30 that mentions diode 1N4004 ... once you confirm your particular hardware, that sounds like the thing you need ... best regards, Ron

[Guest dunbarbe]

Great advice everyone. Thanks for the help.