I have a 24 VDC PLC. It is powered by a power supply that takes in 12 V and turns it into 24 V. This 12 V is coming from a car battery. The inputs for this PLC is 12 V.

Here is the smart relay: It is a phoenix Contact PLC model PLC-V8C/PT-24DC/SAM. PLC Tech Data

Here is the power supply: MINI-PS-12-24DC/24DC/1 also from Phoenix Contact enter image description here

I have a separate 12 V system (which is also run by the same 12 V car battery) which has a switch in it. The positive side of this switch I have going to an input on the PLC. This does not produce 12 V on the PLC input. The only reason I know of is that the grounds are not connected.

I have tried connecting both grounds, one from the 12 V side and then the one coming out of the 24 V power supply. This is pulling my 12 V input down to 1.75 V for some reason.

How should this be wired to the PLC can read the 12 V input?

enter image description here

  • \$\begingroup\$ More information about the 12V/24V boost-converter (make, model, datasheet)? How much current can the "12V board" source? \$\endgroup\$ Feb 21, 2019 at 20:17
  • \$\begingroup\$ Can you specify the PLC you are using? \$\endgroup\$
    – Ron Beyer
    Feb 21, 2019 at 21:06
  • \$\begingroup\$ @RonBeyer I added links to what I am using. \$\endgroup\$
    – mScientist
    Feb 22, 2019 at 2:43
  • \$\begingroup\$ The smart relay link is broken. Link to the datasheet, not the overview page or, even better, put enough info into your question so that we don't have to follow a link. \$\endgroup\$
    – Transistor
    Feb 22, 2019 at 7:39
  • \$\begingroup\$ I added more information \$\endgroup\$
    – mScientist
    Mar 1, 2019 at 4:07

1 Answer 1



simulate this circuit – Schematic created using CircuitLab

Figure 1. Relay isolation between two systems.

You have been vague about the type if inputs on your PLC - current sourcing or sinking - so you may have a little work to do adapting this solution.


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