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Old September 19th, 2018, 05:06 PM   #1
alive15
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Question Low Voltage on PLC Card

Good afternoon, I have an Allen Bradley PLC 1768-L43 with one 1769-IQ32 input card and two 1769-OB16 cards (all three on the right side of the processor), along with some other communication cards and power supply module to the left of the processor.

I have an issue where my outputs only send 18V+ out of the card, not the full 24V, despite have 24V going into +VDC on the card (on the 2nd output card). After replacing the 2nd card, the first card began showing the same issue. We replaced the first card. Everything was good, but then the first card started showing this problem again.

This issue occurs intermittently, confusing our techs. We will recycle power a few times to get it back up and going eventually. One of my techs merely pushed the output card to the left a bit to get it fixed one time, so perhaps a connection issue between the cards?

What I need is a way to check the connection between the cards. Is there any way to accomplish this through the PLC or checking any voltages or ohming any pins out on the card? I really don't want to keep swapping parts out and using trial-and-error method.

My only theory is a bad connection between the first input card and the first output card.

Edit: Thanks

Last edited by alive15; September 19th, 2018 at 05:07 PM. Reason: Thanks,
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Old September 19th, 2018, 05:19 PM   #2
Ken Roach
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Wait... wait... you posted earlier about a Safety Relay where you were measuring lower voltages than expected.

I believe that's part of the cross-fault detection features of a safety relay, and that you aren't really measuring a lower DC voltage but rather an average voltage because there are short pulses on these circuits.

Are your safety relays providing source voltage to these outputs ?

The 1769 data bus is completely isolated from the Input and Output power on the I/O modules. If "pushing the modules together" changed the behavior it's because something is loose in the wiring harnesses.

If these are unrelated systems, you need to keep looking at your DC power supplies and be sure they aren't being over-loaded and pulled down.
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Old September 20th, 2018, 08:42 AM   #3
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Yes, they are providing source voltage to each output card. When the channels are made, the normally open contacts pull in a control relay, which power the output card. So the power going into +VDC on the output card is 24V, but the actual outputs themselves are 18V.

From the literature, Channel 1 should pulse through 14V and channel 2 should pulse 18 V around the loop.

I don't understand your quote: "and that you aren't really measuring a lower DC voltage but rather an average voltage because there are short pulses on these circuits." There should not be a pulse coming out of my outputs, it should be a solid 24V+ from any output that turns on.
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Old September 20th, 2018, 01:54 PM   #4
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You could check the current being drawn from the outputs - these are fairly low current output cards .Also , each output card needs its own 24V and 0V connections from an external 24 VDC supply . There is no connection on the backplane to supply the outputs .
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Old September 21st, 2018, 08:25 AM   #5
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Right, there is no backplace, all the modules are connected together. Right now, I am getting 24V, but I'm sure it will eventually drop to 18V at a complete random time.

I really don't understand the cross-fault detection feature of the relay, I'm unsure why this would even lower my voltage to 18V specifically. You would think it would shut off to 0V.
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Old September 21st, 2018, 03:58 PM   #6
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It could be that some of the load is pulling down the voltage, when this happen try removing the wiring one by one and see if the voltage gets back to 24v
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Old September 21st, 2018, 04:18 PM   #7
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Right, so my outputs go through a fuse into a separate block where all my outputs actually plug in. I removed each fuse one by one, and still saw the low voltage on the output terminal on the PLC card itself.
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Old September 21st, 2018, 05:05 PM   #8
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Are you 100% sure that +VDC at the card is 24vdc at the same time that the outputs are droping to 18v? Did you use 2 multimeters?
If you are sure then I will tight all the wiring of 24VDC including the GND.
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Old September 21st, 2018, 05:16 PM   #9
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No, we did not use two multi-meters, I placed one ground and the other on +VDC and got 24V, then I moved directly to the output terminal and saw 18V+.

The issue comes and goes, so there is probably a wire in the wrong spot somewhere. I have two seperate terminals for my commons, both of which are jumpered together with a wire . With red on common and black on ground, sometimes I even see +5V on one of the commons. However, I'll place my red lead on another common (both are from the same power supply) and will see 0 V.

So we disconnected each common wire from the set of terminals they were plugged into and replugged them back and the problem went away, but then it showed up again the next day. Very very interesting problem here.
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Old September 21st, 2018, 05:34 PM   #10
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I am having problems understanding this . When you measure the voltage to the card are you reading between +VDC and DC COM on the output card to get 24v and then between the output OUT1 for example and Dc Com ON THE CARD and getting 18V , or are you measuring the outputs to 0v at the terminals ? If the latter is the case , when you are getting an 18v reading , see if there is a voltage reading between DC Com on the output card and 0v at the terminals .

Just as a matter of interest , what sort of load are you driving . Solenoid valves , relays , contactors for example ?
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Old September 24th, 2018, 08:31 AM   #11
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Red Lead on +24VDC into the output card. Black lead on either ground or DC COM, I get 24V as expected.

Now, I placed Red lead on any "ON" output between 0 and 15, and black lead on either DC com or ground, and I get 18V. This is for any output on the card that is "ON" through the PLC.

The first output card is for non-hazardous outputs, like basic servo enables, door opening, vfd power, relay resets, air on.

The second output card is for hazardous outputs, such as contactors for 3 motors, gripper open/close, etc.
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Old September 24th, 2018, 09:43 AM   #12
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Just a thought--have you checked that if say 2 or 3 outputs are active on each card that the current to those 4 or 6 outputs added to all the other 24V stuff on your power supply does not exceed the PS rating? Maybe you've sized it correctly but it possibly could be a problem.

Another thing I've done in the past is to force all outputs on one at a time and see if any are pulling you down to 18V. It sounds like you have motor contactors and air solenoids on one card. See if you get 18V by forcing outputs on on just one card, and then same thing on the other. If one that's forced on causes the voltage drop, then it's very simple to find the cause--I.E. loose wire, improperly wired, wrong device, etc. Hope this helps. Good luck
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Old September 24th, 2018, 09:51 AM   #13
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Right, I've done a similar method by removing the fuse from each output one at a time and checking voltage on the very last output. Once we put all the fuses back in, the problem went away for the day, but came back the very next day.

My issue is right here --> With red lead on common and black on ground, I get +5V on one of the commons. However, I'll place my red lead on another common (both commons are from the same power supply) and will get 0V. So does this mean one of my common wires shorted out somewhere?
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Old September 24th, 2018, 04:37 PM   #14
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Do you have the electrical drawings?
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Old September 24th, 2018, 04:56 PM   #15
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Yes I do, but need to double-check with upper management before posting online.

I have this MSR210P Allen Bradley relay I replaced last week as well. It was giving me problems whenever I tried to reset the relay, so I had to recycle power to get it to reset properly. I upgraded from MSR210P to 440R-D22R2 on the 19th, and have not seen the problem since then, even after multiple times of recycling the power.

Usually the 24V issue shows up whenever the power gets cycled. So what would happen, the relay would not reset properly, so tech recycles power, gets the relay going, but do not get 24V from output card, so keeps recycling power until both relay and 24V were good.

I do not understand how changing the relay would have fixed the issue, since we used the same wires from the old obsolete MSR210P into the new 440R, So I'm pretty sure the 24V issue is still there. We replaced the old MSR210P relay with a brand new MSR210P relay, and the problem was still there. But after changing to new style relay, it has not popped up yet.

I don't enjoy this problem as I could not figure out the solution. It's ran for 4 days in total w/o issues, but what is the problem from the start / what is the root cause? This is what I am searching for. Maybe the 24V issue will come up again and I can dig deeper into it. I still don't know why I saw +5V from common to ground at one point, but whatever caused this was giving me the issues.
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