Confused by PNP and NPN Wiring in the Samba Manual

[de003]

Hi all

I am relatively new to PLC programming and using a Samba 35 to teach myself. I am familiar with the relevant principles but I am being a touch confused by how PNP and NPN wiring are visualised in the manual. For reference I am looking at the manual at https://www.unitronicsplc.com/wp-content/uploads/2015/12/SMXX-J-T20_R20_DOC17015-A4_02-15.pdf , pages 7 & 8.

The manual as linked above, seems to suggest that a + common is NPN and a - common is PNP.  But every other resource I've read (for example, http://plchowto.com/wiring-plcs/ ) describes them as the opposite.

I have been used to the idea that for NPN, the switch or sensor goes between the 24VDC supply and the PLC input; whereas for PNP, the switch or sensor goes between the Common and the PLC input.

 I'm a bit confused... the manual seems to contradict everything else. Can anyone clarify this for me?

Thanks

 

 

[Simon]

Hi,

I think you have the sensor side back to front.  Below are two diagrams from 2 brands of sensors.  The PNP sensor is connected between +24V and the PLC input (load) and the NPN sensor is connected between 0V and the PLC input.

It's the opposite for outputs.

The terms Sinking and Sourcing often get used, and that can also be confusing without the right reference point.  It's better to think in terms of current flow rather than voltage.  The sinking and sourcing of current is relative to the signal driver.  And for the purists, we are talking conventional current not electron flow 🙂

Sourcing - the signal driver is pushing current

* sourcing input - the driver (sensor) pushes current into the load (PLC input)

* sourcing output - the driver (PLC output) pushes current into the load (solenoid, relay, etc)

Sinking - the signal driver is receiving current

* sinking input - the driver (sensor) receives current from the load (PLC input)

* sinking output - the driver (PLC output) receives current from the load (solenoid, relay, etc)

 

I hope that's helpful, I don't claim to have the only way, or the best way, of explaining this.

[de003]

What you say follows what is in the manual in essence. I am confused by the below image which seems to contradict what you're saying.

I have seen various versions of this graphic in several places.

I'm not saying you, or it, are wrong... but my confusion is that it doesn't seem possible for both to be correct!!

In the below drawing, it seems that where the driver (in this case a momentary switch) pushes current into the load (PLC input), that is referred to as a sinking input.

This is where I'm confused. 

[Simon]

Hi, 

It's all about the reference point.  First let me say, with the outputs, the diagram you post is consistent with how i've summarised it.

Regarding the inputs, the diagram you post is showing everything relative to the PLC, which is opposite to how I have summarised it.

So

for the PNP input, the external device (sensor, switch) is sourcing, and the PLC input is sinking (current flows from the external device into the PLC input)

for the NPN input, the external device is sinking, and the PLC input is sourcing. (current flows from the PLC input into the external device)

so the trick is to understand whether the source/sink terminology refers to the external input device or the PLC input.

In my explanation, I have used the external device as the reference point.

There are many ways to represent this, but the physics is consistent 🙂

[de003]

12 hours ago, Simon said:

for the PNP input, the external device (sensor, switch) is sourcing, and the PLC input is sinking (current flows from the external device into the PLC input)

for the NPN input, the external device is sinking, and the PLC input is sourcing. (current flows from the PLC input into the external device)

so the trick is to understand whether the source/sink terminology refers to the external input device or the PLC input

This now makes total sense.

Thanks muchly.

[Ausman]

Well said, Simon, on an often confusing subject.

cheers, Aus

[Simon]

Thanks, glad it was helpful.

[draganjr]

Good Evening, new to this forum and new to program with Unitronics.

I had a similar question but on the SM70-J-T20.  It states it is defaulted to PNP, but in the above manual there should be a jumper to switch from PNP to NPN, JP1.  I removed the back and did not see a JP1 to switch from PNP to NPN.  Is that even needed?  Can't I just change how the sensor is wired to the PLC?

Thank you

[DanT]

Hi;

 

Check the Install Guide, Page 7  for the Jumper settings.

 

SM35.43.70   R20 and T20  Install guide  - download from the    Tech Docs.

 

DanT