2 24VDC power supplies

cskaggs2

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Join Date
Jul 2009
Location
Seguin, Texas
Posts
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I have a machine that has two 24VDC power supplies. One power supply is for a vision system (recommended to be isolated power supply by the vision system supplier). The 2nd power supply is for the PLC and all other I/O.

As it was originally wired by the machine builder, the vision system was powered by one power supply, but the I/O on the vision system was powered by another power supply. This gave a voltage difference of 24V between the power common for the vision system and the I/O common. This is specifically cautioned against in the manual for the vision system.

When I tried to power the I/O for the vision system from the same power supply that powers the rest of the vision system, my input to the PLC from the vision system would not work. The only way I could get things working for now was to power the PLC, vision system, and other I/O's all off the same power supply.

I know it can be done with separate 24VDC power supplies. I think it has to do something with the grounds. Can someone help explain this for me?

Thanks,
Chris
 
To use 2 power supplies and keep them separate you would need to use the output from the vision system to power a relay, then have the 24V for the PLC pass through the contacts of that relay. This is known as using an interposing relay.

Your other option is to ground both DC commons creating a common reference point. This however has other pitfalls and would not leave you with an isolated power supply.
 
I might be missing something here. When you tried to use the Vision Power supply to energize the PLC Inputs did you tie the common of that power supply to the common for that section of I/O. On most I/O cards you can break Inputs into banks based on the common wire.

An Example would be that Inputs 0 to 4 used Comm 1 wire this to the visision power supply. and inputs 5 to 8 use comm 2 wire this to the PLC power supply.

Depending on the input card you can make a bank sinking or sourcing depending on how you wire up the common.

Remeber though once you wire the card this way inputs 0 thru 4 could ONLY be used for the vision system in my example.
 
I have a very simple PLC. It only has one common for its inputs. Looks like I will have to go the interposing relay solution for this one.
Thanks,
Chris
 
Depending on the Visision System you may need to check the dwell on that input. I have seen on certain systems the output coming from the vision was so fast the PLC did not see it. Adding a relay will mean this dwell needs to be even longer.
 
I would use a common ground, ie connect the 0 volts for each supply together. They are still separate supplies they just have a common reference. The only 'pitfall' being that one supply cant float in relation to the other.
Personally I always ground the 0 volts of 24V power supplies.
 

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