1756-OB32 output keeps on

[khyap96]

Hi all,

IÂ’m having issue on 1756-OB32 where one of the channel (CH14) keep on has 24vdc, causing my solenoid keep on energising without command. ThereÂ’s no indication light on the module, thereÂ’s no command from plc, but the channel output is on (24vdc). If swapped a new DO card, initially everything went well but after few weeks the same things happen again at the same channel.

Can anyone please advise?
Has anyone met the same issue before?

Regards
Sally

 

[parky]

My initial thought is that what ever is on that channel has been drawing too much current or there are spikes on the supply, if this is something like a contactor or solenoid either it is drawing too much current or when it de-energises a large spike is destroying the transistor or the opto isolator on the bord.
Spikes can be reduced by fitting a diode across the solenoid/contactor or what ever it is, alternatively fit an interposing relay if it does not need a fast operation.

 

[JeremyM]

+1 on interposing.

You might try cross-referencing the IO in your Logix project to see if anything is writing to it.

 

[plvlce]

Sounds like a textbook case of voltage spike from solenoid turn-off destroying the output. See here for a more in-depth article on the topic.

As parky says, your basic options are to install suppression across the load and/or add an interposing relay to take the hit from the surge.

 

[ASF]

Also, check the 0VDC connection to the module.

We had an issue once with a bunch of faulty interposing relays. We used a link bar to join all the A2 (0VDC) terminals, and then used that link bar to bring off several wires to supply all the 0VDC terminals on the output cards themselves.

The relays had a manufacturing defect that meant the link bar occasionally stopped conducting to the relay screw, which meant occasionally some of the output cards would lose their 0VDC reference. The result of that was that all of the outputs associated with that 0VDC reference would come on.

There were a lot of DOL motors on that output card. Some of them were reversing (fortunately with an electrical interlock post-interposing relay, but it didn't stop a whole lot of "chattering" forward-reverse-repeat operation). The term "clusterf***" got used several times.

 

[joseph_e2]

Also, check the 0VDC connection to the module.

We had an issue once with a bunch of faulty interposing relays. We used a link bar to join all the A2 (0VDC) terminals, and then used that link bar to bring off several wires to supply all the 0VDC terminals on the output cards themselves.

The relays had a manufacturing defect that meant the link bar occasionally stopped conducting to the relay screw, which meant occasionally some of the output cards would lose their 0VDC reference. The result of that was that all of the outputs associated with that 0VDC reference would come on.

There were a lot of DOL motors on that output card. Some of them were reversing (fortunately with an electrical interlock post-interposing relay, but it didn't stop a whole lot of "chattering" forward-reverse-repeat operation). The term "clusterf***" got used several times.

Can you share what relays you used? We're doing something similar...

 

[TheWaterboy]

You asked this in the AB forum as well and you got some answers there. Have you tried those? Pretty sure you spiked the output and need to buffer it.

 

[khyap96]

Hi all,
Thank you all for the explanation. It actually took me awhile to understand all these because I’m not a EE background.

Hi Jeremy,
I’m very sure nothing is writing the DO.

Hi parky, plvlce,
It looks like this is the guess from most of the people, that there’s spike during de-energised. We actually has SPD installed in between the DO module and SOV. Does this actually help the spike from load?

Hi Joseph,
the 0vdc is connected to the load. For the card, it only has +24vdc connected.

Hi Waterboy,
The answer from AB forum more or less is similar to the answer in this forum.

Hi all,
Have attached the drawing of the project for your reference.

 

[khyap96]

https://drive.google.com/file/d/1cv3bc1QdzD0bGrcFA4qmg97xWEIfvwgd/view?usp=drivesdk

Sorry, this is the link. I’m not sure why previously was not attached

 

[ASF]

Can you share what relays you used? We're doing something similar...

I believe they were some variant of the Omron G2RV. We've probably installed 250,000 of those over the years and they've been absolutely bulletproof. Then (I presume) someone with more certificates on their wall than days of practical experience decided to value engineer the design, and overnight we had to find an alternative because putting them into a panel was asking for your own reputation to be trashed when a machine drove something beyond the hardware limits until an overload tripped.

From what I understand they have revisited the redesign and resolved it now, but I hope whoever made that call got told not to make any more smart decisions until they'd at least learned how to use a screwdriver.

 

[khyap96]

Hi guys,

Is it possible that power short at the solenoid terminal (+24vdc to body and/or 0vdc to body) damaged the channel?

 

[joseph_e2]

Also, refer to the wiring diagram on page 116:
https://literature.rockwellautomation.com/idc/groups/literature/documents/um/1756-um058_-en-p.pdf
You should have 24VDC on pins 18 and 36 and 0VDC on pins 17 and 35 of the RTB.

 

[parky]

Yes it is possible, a short will almost certainly destroy a transistor, I always put fuses in transistor outputs in case of a short like cables or perhaps the coil shorting, the fuse ideally should be fast blow but high enough ampage to allow for inruch plus a bit more.

 

[James Mcquade]

TheWaterboy,

what is the site address for the AB forum?
regards,
james