Low Voltage on PLC Card

Bear in mind that when energising contactors or other inductive loads , to close them the current draw is 8 to 10 times the holding current , and if the power supply is not big enough they fail to close properly and draw a higher than normal current . I think your next move is to check the current from each output on the 2nd card , then check the PSU size . It certainly sounds like the PSU is undersized .
 
It's a 20A PS. It's ran for two years like this, and I don't believe we upgraded any of the motors or the VFD inside it.

How many amps should I see normally from the outputs? It's a 1769-OB16 I believe.
 
Current per point, max
0.5 A @ 60 °C (140 °F)
1.0 A @ 30 °C (86 °F)

Current per module, max
4.0 A @ 60 °C (140 °F)
8.0 A @ 30 °C (86 °F)
 
Alright thanks, I'll take a look later.

Are you sure about these values? I would think higher temperature means higher current. Is that temperature the air temp. or the wire temp?

Edit: Sorry, 140°F wouldn't be wire temp haha, so probably air temp.
 
A 20A supply is more than enough unless you are using 24 V somewhere else as well as the PLC & outputs. Do you have MCB's betwen the PSU and the output cards ? If so , normal MCB's are not usually rated for DC , so check for voltage drops across the MCB's . If they are double pole , check both poles . This could be where the voltage difference between your 0V,s is originating .
 

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