CompactLogix Power Supply Issue

[Brad P 0]

I have a CompactLogix with a 1769-L35E processor with two racks. Rack one has 11 cards and rack two has one card.The problem I am having is the power supply (1769-PA4) goes bad about every 3 to 5 months. Or I am assumming it goes bad. What I see is the power supply LED goes off and on. The cards in the rack are blinking and the processor is faulted and has lost its program. The only way I can get it running is to replace the power supply and download a program into the processor. This has happened atleast six times in the past three years. The last time this happened I installed an extra power supply on the rack. Even though the one power supply is all the rack requires. See attached picture. But I continue to have the same power supply fail. I believe I am losing the program because the processor is going into a hard fault. I have replaced every power supply, I/O card and processor on this rack. Any ideas why I am losing this power supply? Thanks.

[Ken Roach 194]

How fast is the power supply LED turning on and off ? Is it blinking, or flickering, or fading ? It sounds to me like you have an intermittent short to ground or other overload of the power supply, or of whatever circuit or transformer that is feeding the power supply. I can't find any place where the selection, installation, or technical data guides say "do not install more than one power supply per I/O bank", but I have never seen anyone install more than one power supply per I/O bank. I'm surprised that works; the two supplies are back-feeding one another, but maybe they coincidentally work in parallel.

[a062549 5]

I could only assume you've logged the power feeding this device? I had one doing goofy stuff, but not with your consistency, and found it to be getting a high voltage blip and taking it out.

[Brad P 0]

This problem has happened several times before I added the second power supply to the rack. I have never seen anything saying that I can't have two power supplies per rack. Needless to say that didn't help or hurt the problem. The LED on the power supply is fading off and then comes back on. Most of the time it is off. I have tried using a UPS as a power condioner in the past, but that didn't help. When the LED is off I read 120 volts.

[Brad P 0]

This problem has happened several times before I added the second power supply to the rack. I have never seen anything saying that I can't have two power supplies per rack. Needless to say that didn't help or hurt the problem. The LED on the power supply is fading off and then comes back on. Most of the time it is off. I have tried using a UPS as a power condioner in the past, but that didn't help. When the LED is off I read 120 volts.

[Ken Roach 194]

In the CompactLogix Selection Guide, the section that describes the selection and sizing of power supplies says • One 1769 power supply for the controller and local I/O modules • One 1769 power supply for each additional bank of I/O modules It doesn't explicitly say "do not install more than one power supply", but every illustration, photograph, sizing document, and architecture drawing I've ever seen has only one power supply per assembly. I'm not sure what to make of the power supply LED fading off and then coming back on. I would still set up a meter to verify for certain that the incoming power isn't dropping.

[Brad P 0]

I had a meter connected. I never saw a voltage drop when the LED went out. It is always 120 volts AC.

[ASForrest 12]

I know that most of the 1769 cards have a "power supply distance rating" of no more than 6 - i.e., it can only be 6 slots away from the power supply. The CPU has a distance rating of 4. So generally what I see done is CPU-IO-IO-IO-PS-IO-IO-IO-IO-IO-IO. According to your diagram (with the added PS ignored for now), you have 8 cards to the right of your power supply. So I would expect to see problems with the setup on that basis, although my first guess would be I/O cards dropping out, not dying power supplies. But then, perhaps the power supply is being overloaded on the DC side seeing as you're using more modules than normal. From memory, analog cards have quite a high power requirement, and it looks like you have a rack full of them (if your diagram shows the actual cards in your rack). Having said that, I would have still thought there'd be some sort of overload protection... Having added a second power supply, well, I'm with everyone else here in that I've never seen that before on a compact logix rack. I also haven't seen anything to explicitly say you can't do it. But I know that with regular DC power supplies, you can't just whack them in parallel or they'll essentially fight each other and before long one of them will give out. You have to get specific power supplies designed to do this, and I think (although wouldn't bet the farm on it) that one has to be a leader and one a follower. So with two power supplies, based on my (by no means exhaustive) understanding, I wouldn't be all that surprised if one of them dies every few months. I don't know if it's physically possible, given the layout of your panel, but what I'd be doing is shifting the last two or three cards from the top rack to the bottom. As long as you keep the overall order the same (slot the cards in to the left of the existing second rack card), the slot numbers won't change, and then you'll be obeying all the rules for power supply placement. At that point, if you still get power supply failures, you can at least go to Rockwell and ask them for help without having them write it off straight away because you haven't RTFM :)