AB Micrologix 1200

hi Guys

I have a micrologics 1200 that has been it service for 4 years on a piece of equipment. This morning we had complaints of a motor contactor chattering.
One of my technicians checked it out and found a relay output was not closing. The indicator light was indicating the output was on.
Assuming the output went bad he went in to the program and re assigned the output to another address. all worked well for about 4 hours and that output seemed to fail as well. To get the machine running I re addressed the output again to another open output on a card. All worked well however I don't expect it to last.
Digging a little deeper I found he swapped out the contactor earlier in the morning( I am assuming the new contact is good,power supply seems good and nothing seems to be overloading. I can force the outputs and the lights indicate they are closed but they both show open on the plc contacts.
I have seen outputs go bad usually when loaded heavy or on coils but this seems strange . any ideas ??
Thanks

Welcome to the forum.

if you look at the specifications on any relay or plc relay output card, the specs will tell you the typical life expectancy of the relay.

Load is one factor, another factor is how often the relay cycles.

If this is a high cycle output, then you would use a triac output.

High current outputs would go through a interconnecting relay
depending on the load rating. Look at the amps making and breaking rating.

WARNING !
Triac outputs are electronic devices and are turned off when enough load is connected to the output. Otherwise, the output is on EVEN if the plc output is off.

regards,
james

You probably also want to look at what the inrush current is for the contactor and what the output of the PLC will tolerate. If you’re on the edge, it will shorten the life of the output. Having said that, I would almost always use an interposing relay. A little more cost upfront but adds a lot of flexibility and makes life much simpler when things fail. Instead of having to reprogram the PLC all you would need to do is replace a relay. Just a thought.

+1 on the interposing relay. I never, ever, ever drive anything straight from an output unless it's a pilot light, and even then only if it's in the same panel as the output card.

One thing to check too - is the contactor coil definitely the correct voltage? I had a panel a couple of years back where I tested the panel, everything checked out OK, then it went to the customer and they complained that the safety contactors didn't come in. I assured them that I had seen them come in 10-15 times with my own two eyes, but when I got to site, sure enough the wouldn't come in. Turns out they were 24VAC contactors being driven by 24VDC, and further bench testing revealed that many 24VAC contactors will pull in on 24VDC - for a few hours at least, long enough to test the panel and put it on the truck to the customer. That did leave me a little red faced