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Snap-in Module Power Supply

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  • MVP 2014

The snap-in I/O modules are strict about supply voltage, and it needs to be 24V.  For example, with digital inputs the switching thresholds are based on 24V, so 12V applied to a 24V input will not be enough to switch it on.  The threshold is usually around 17V (this is specified in the datasheets for each module).  There are issues with digital outputs as well, but I won't blabber on and on.


As Eyal mentioned you can use the V120 and select a model that has 12V capability (usually controlled by jumper settings inside the unit).


Alternatively, if you want to stay with the larger PLC then you can use expansion I/O on 12V.  You need the special "-L" versions, look at the listing of I/O modules on the Unitronics website for those.


The only issue I have had with the 12V operation is the lack of analogue outputs.  In the M91/V120 families, none of the models with analogue outputs have a 12V option, they are 24V only.  When looking at the "-L" I/O modules, there is also an absence of analogue output models.


The final option is to use a 12V-24V step-up convertor and run your automation logic on 24V.  Note if you have relay outputs on a 24V module, you can still use them to switch 12V directly, but you would need 24V power for the relay coils.

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