PLC: Allen bradley

engineeradityasingh · May 16, 2015

Does anyone have suffered problems related to output relay jam( or i will say, shorting of output of plc relay ) with allen bradely PLC's. Because of this, if the input variable changes, out stucks to logic 1. Then we literally have to hit our plc with something till output relay gets back to normal. Any comments/suggetion about this situation??

WillM · May 16, 2015

It can happen with all contacts. What is the output going to, a coil by any chance?

Just move that output to another output or replace the card if full.

Go and become a fitter if you want to hit things

JRB · May 16, 2015

If you have a spare output, you could mount a solenoid driven mallet to slug the output card, or use
a diode across a 24VDC inductive load, or an arc quencher across the 120ACV inductive load.

Aabeck · May 16, 2015

I strip a lot of panels for retrofitting & keep all the old PLC's & cards, and have quite a few AB relay output cards marked that one or two outputs are stuck on & to not use them if I ever need to use that card.

On my panels I make sure to use surge suppressors on anything other than a pilot light & use relays to drive anything I don't think the output card may handle well.

baldone · May 16, 2015

That is the reason I never use PLC relay output modules.
I may add relays to 24 vdc outputs if needed because I would much rather replace a $4 relay than a $300 module.

engineeradityasingh · May 17, 2015

The problem is, i do not have a spare output port, Is there some permanent solution for this if i except changing the PLC?

Ken Roach · May 17, 2015

Generally you only need to change the output module, not the whole controller.

Unless it's a MicroLogix with built-in I/O.

This is not an Allen-Bradley problem, it's a fundamental function on how electromechanical relays work and how you apply and install them.

Aabeck · May 17, 2015

If you're into servicing "No User Serviceable" items, you can replace the relay on the output. This isn't difficult on relay output cards, but if in a brick it will require some disassembly & it's harder to find a compatible replacement relay -depending on the brand.

Then, make sure you use surge suppressors & interposing relays.

Saffa · May 18, 2015

JRB said:
If you have a spare output, you could mount a solenoid driven mallet to slug the output card, or use
a diode across a 24VDC inductive load, or an arc quencher across the 120ACV inductive load.

The first suggestion sounds golden. Could be a market for these in India by the looks of things. The other two, clearly a niche market.

I can't even imagine hitting a PLC. It would be like poking a horse with a sharp stick whilst standing right behind it, you are guaranteed to end up in an uncomfortable position (generally on your ***).

PLC: Allen bradley

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