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Old February 22nd, 2018, 08:55 AM   #1
United States

locotumbler is offline
Join Date: Jan 2013
Location: Otsego, MN
Posts: 81
Micrologix 1200 sticking relay

I have an issue on a system i've inherited. I had one of my relay outputs sticking on. When the lights off and i'm still getting voltage through it and I tap on the unit itself it releases. I thought initially since the system was around 9 years and over 5 million cycles that it simply failed. I replaced the PLC and had no issues for about 50k cycles, with the way we are running it now is like 3 days, and the relay started doing the same thing. It is running a small solenoid valve. I moved the output to a different unused output but on the same COM supply and had the output itself run an ice cube relay. I was gone yesterday and come in today to find out it started doing the same thing.

the system is running 24vdc through the relay. Any ideas on where I might start a deeper inspection into this? Could it possibly be getting back-fed from somewhere else on that COM?

Thank you in advance.
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Old February 22nd, 2018, 09:22 AM   #2
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g.mccormick is offline
Join Date: Jul 2012
Location: IN
Posts: 565
If it is actually relay output, it's probably due to voltage spike from connected coil. Put in a flyback diode across the solenoid to extinguish voltage spikes.
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Old February 22nd, 2018, 09:46 AM   #3
James Mcquade
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James Mcquade is offline
Join Date: Oct 2007
Location: Tennessee
Posts: 2,364
relay outputs have a lifespan before they fail, look at the specs of your system.
I had the same issue, and eventually had to go to a triac output.
in your case I would go with solid state dc outputs and also do as suggested.

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