PNP vs. NPN

burnhamhouse

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We use PNP digital inputs at my company and we had a sensor that was NPN which we wanted to test. Is there a way to wire the PLC so that it accepts an NPN type input?

Thanks
 
Of course there is a way and its easy. Check how these devices work.

Make a little setup on a bench, without the PLC connected to the device. Just like in school ...

You need a power supply
You need a few resistors.

Wire this gizmo so you don't release the embeded smoke and test it.

See, the device is just like a switch. Changing state from OPEN to CLOSE.

You connect the device + and 0V.
If you start from the PSup. and into a 5K resistor (check the device max. am) then enter you NPN cct at the load connection and out to 0V.

Make it activate. Use your DVM and check where the heck the signal is changing. You'll then find where to connect you PLC.

The ONLY way to see how those darn device work (NPN / PNP) is to play with them on the bench at least once.

Then you'll know that you can connect any sink/source device to any PLC inputs. Wathever there type is... But of course, you need not connect 120 V to this :)
 
Yes it's easy, as Pierre said, all you'll need is a pull-up resistor to convert NPN to PNP. But remember that the sensor will now work OPPOSITE... When the sensor turns ON, the output turn OFF

Download Chapter 2 of any Automation Direct PLC manual. They explain how to do this quite well!

Here's a LINK to one of those "Chapter 2"s

-Eric
 
Eric wrote : "When the sensor turns ON, the output turn OFF"

Yes, and it's how we do it all the time ... when we skiew-up in purchasing ...

But that's only when you connect between load AND 0V ... what would happen if you where to connect the PLC input between + dc and load ?
Load of the device input that is ...

When the gate is closed, you'll get +24 on one side of the PLC input and +24 on the other side... would'nt that be a 0V potential difference? Hence no input...

When the gat would open... short to ground 0V... the PLC input would see +24 and 0V ... Hence = input...

Never done it this way but I thinck it works...

So you could have an NC or an NO logical device, either sinking or sourcing, connected to a PLC input. Only if this input can be wired both ways...
 

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