Need Advice on Wiring PLC System with Two Different PSUs

[PLC_MAGNUS]

I'm currently working on a PLC setup and could use some advice on the best way to manage my power supply units (PSUs). Here's the configuration:

PSU A: 24V DC, 2.5A

PSU B: 24V DC, 10A

1x CPU and 3x I/O Modules

Currently, I'm powering the CPU and the I/O modules using PSU A, with "L+" connected to 24V DC and "M" to 0V DC. My plan is to power the sensors using 24V+ from PSU B, as I'm concerned about overloading PSU A with too many amps.

However, this raises a question about how to complete the circuit for the input cards on the CPU and I/O modules. If I use 24V+ from PSU B for the sensors, it seems necessary to also use 24V- from PSU B for these input cards. This leads to a situation where I have to differentiate between the negative terminals from the two separate PSUs.

To streamline this, I'm considering wiring together the negative terminals from both PSU A and PSU B to create a common reference point. Currently, both PSUs are connected to the same earth.

Would this be a viable solution? Are there any potential issues with wiring the negative terminals of two different PSUs together in this manner? I'd appreciate any insights or suggestions you might have.

 

[RClem]

I've used common negative reference points in the past with multiple PSUs without issue.


Rich

 

[BryanG]

On my test rig I currently have four different Siemens S7-1200 PLCs, some 230VAC supplied, some 24VDC, three different Power Supplies, plus the four power supply outputs that each PLC has. All of my M terminals are linked back to a common point. The only comment I would have for you is to add some overcurrent protection to the 10Amp supply. That could be fuses, but I now go with these:
https://shop.murrelektronik.com/en/...binet/Intelligent-Power-Distribution/Modules/
there are other makes and types.
One other thing, I always earth/ground the linked M commons, there have been many heated discussions here about to ground or not to ground.

 

[JeremyAdair87]

RClem nailed this one. You just have to have a common 0V.

I wouldn't typically worry about "pulling too much from a PSU" if you put too much load on them, they just stop working. It doesn't typically damage anything in my experience. It just ends up taking a while to troubleshoot it, because its not a common error that we get to see very often.

I did have the opportunity to debug this once. We had 10 amp PSUs in several panels. One of the electricians pulled 24v from a power supply in a completely different panel. The meter read 0v and 24v. You could force the output signal going to the drive, and disconnected you would see 24v. Connected you would see nothing. I connected the commons at the drive to make the system run. A few days later after my lower level guys couldn't figure out, I started turning off every power supply on the machine. We turned off all the 24v and the sensor, and that wire at the drive were still powered with 24 and so were 5 conveyor sensors along the line. I traced the conduit back to the wrong panel, turned off it's PSU, and bingo. I asked the electricians, why they were trying to power equipment with the wrong supply, and had them re-run it.

Just be aware that if anyone disconnects the common on those two supplies, the devices on separate supplies will not communicate all, or will randomly float and communicate sometimes. I have seen some stuff killed by floating like that, but it was 120v inputs on an old isolation transformer in a washdown environment.. it was a dangerous design with "estops" going to PLC inputs. They didn't want the line stopping, when an estop was full of water. I got out of there as soon as the headhunter found something else.