Recurring failure of 1769- P4 , power supply unit

rejoe.koshy · Jun 4, 2023

Hi folks,
I have plc rack consisting of an 1769- L32E controller and 6 I/Os wherein I had a power supply issue 3 months ago and had replaced it post which all was well.

But suddenly it seems to have conked off again. The unit keeps recycling power to the entire rack.I have had this system working for over 8 years without any trouble but the repetitive failure of the unit is causing a lot of concern.
Any clues as to whats causing this?..please help

JTCat · Jun 4, 2023

rejoe.koshy said:
Hi folks,
I have plc rack consisting of an 1769- L32E controller and 6 I/Os wherein I had a power supply issue 3 months ago and had replaced it post which all was well.

But suddenly it seems to have conked off again. The unit keeps recycling power to the entire rack.I have had this system working for over 8 years without any trouble but the repetitive failure of the unit is causing a lot of concern.
Any clues as to whats causing this?..please help

Hows the incoming power looking? Is it spiking or excessively noisy? Are you powered directly from a breaker panel or is there an isolating transformer providing power?

Also, since the supply issues, there have been a lot of 2nd hand and reman PSU's being sold by suppliers. We've found a couple of remans in the field that failed prematurely. Very low quality capacitors and stuff inside of them. Are your replacement PSU's new or refurbished?

rejoe.koshy · Jun 4, 2023

JTCat said:
Hows the incoming power looking? Is it spiking or excessively noisy? Are you powered directly from a breaker panel or is there an isolating transformer providing power?

Also, since the supply issues, there have been a lot of 2nd hand and reman PSU's being sold by suppliers. We've found a couple of remans in the field that failed prematurely. Very low quality capacitors and stuff inside of them. Are your replacement PSU's new or refurbished?

The power is fed from an UPS but still checked to look for spikes/fluctuations but found nothing of that sort. The unit was a new one but was lying in store house for 18 months .

JTCat · Jun 4, 2023

rejoe.koshy said:
The power is fed from an UPS but still checked to look for spikes/fluctuations but found nothing of that sort. The unit was a new one but was lying in store house for 18 months .

I do not know the capacitor lifetime for these PS units. It's difficult to know for sure without opening up the PSU and seeing which components failed. If it keeps rebooting it sure sounds like a capacitor problem. Are you or any of your peers good with board-level components? Inspection would be valuable in determining the root cause of the issue.

BryanG · Jun 4, 2023

I have had something similar and it wasn't the power supply that was at fault. Also feeding a PLC. As the PLC initially booted everything was fine, then it dropped out and started back at the beginning. We eventually figured that there was a short on one of the outputs, so it was all fine until after the boot finished and the PLC turned that output on. Once the output was ON it overloaded the power supply, the power supply automatic protection circuit kicked in and switched itself off, which powered down the PLC and opened the contact that had the short. And the power supply came back to life because it was no longer shorted.

I_Automation · Jun 4, 2023

If it was a SLC the first thing I would be replacing is the analog IO cards in the rack.

The cards IO work but somehow when they start to go bad they take down the rack power, and can cause the SLC CPU RAM to be wiped and the processor go to DEFAULT.

I haven't encountered this YET on a CLX, but am figuring I will - and this case sounds like it fits.

rejoe.koshy · Jun 4, 2023

BryanG said:
I have had something similar and it wasn't the power supply that was at fault. Also feeding a PLC. As the PLC initially booted everything was fine, then it dropped out and started back at the beginning. We eventually figured that there was a short on one of the outputs, so it was all fine until after the boot finished and the PLC turned that output on. Once the output was ON it overloaded the power supply, the power supply automatic protection circuit kicked in and switched itself off, which powered down the PLC and opened the contact that had the short. And the power supply came back to life because it was no longer shorted.

The outputs are fed by 24 V from the smps
which is a different circuit altogether. Will a short field output affect 240V power supply to the PSU of the PLC or the rack power supply for that matter?

I_Automation · Jun 4, 2023

rejoe.koshy said:
The outputs are fed by 24 V from the smps
which is a different circuit altogether. Will a short field output affect 240V power supply to the PSU of the PLC or the rack power supply for that matter?

A card problem wouldn't effect the 240 input side of the power supply, but the output side, which provides 24V, and in the other families, also 12 and 5V, maybe even 3.3V for the CPU.

It only takes a problem down the rack in one of those voltages for the power supply to shut down to save itself. It doesn't just shut down the 24 or 5V and leave the others on.

There also could be a problem in the backplane of the rack itself. I have had to replace a CLX rack before.

Recurring failure of 1769- P4 , power supply unit

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