outputs failing

I have a PLC 5/40E with 1771-OB outputs cards. The problem im having is the output card keeps having single outputs fail this has been going on for a while now (a total of 6 ) The previous people were just going on to the next open output. I have gone through the wiring multiple times and cant find Anything wrong (site has been running since the 80s) The outputs that fail are for a valve and a run CMD. for a motor. The valve outputs have failed more times than the run CMD. Just wondering if any one has had this happen and what was it/what did you do. if you would like more info let me know thanks for the help

Both valve solenoids and motor contactors are inductive loads, which generate a reverse voltage spike when they are turned off and the magnetic field collapses.

While this is most damaging to relay contacts, it can also damage solid state devices.

With DC solenoids, the most common way to address the problem is called a "flyback diode", which traps the stored energy in the solenoid and allows it to be dissipated (relatively) slowly after the output has been turned off.

Do you know if your solenoids and motor starters are equipped with any sort of surge suppressor or flyback diode ?

Yes we have the diodes installed to protect the outputs from that but the equipment its self i am not sure

I'm sure you probably thought of this already, but it can be a good idea to have your PLC turn on an external relay for high induction loads. This way, if the external relay, which usually has a much higher current rating, goes out, it is a quick $10 repair instead of replacing the the PLC output card.

It also wouldn't hurt to verify that the diodes Ken Roach mentioned are good. They're one of those things that doesn't always give a visual indication of having failed.

I've taken to exclusively using These Card Relays . Love them. Just about every output that powers a device goes through one of them.

Thanks for the help will try all of this out and possibly change the relays out to the same style of what rdrast mentioned

Since the unit has been in since the 80's, here are my thoughts.

1. the valve coild itself is going bad and taking more current to operate than a new valve.

2. since this is a solid state card, the surge current to operate the device may be to much for the output to carry.

regards,
james