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Sourcing and Sinking PLC I/O using a sensor

Hi I need help as I'm just not getting this..


Sinking - Power externally supplied. Current passes through load, then into sensor and then to ground.

Sourcing - Sensor uses supply voltage to pass current. Sensor drives current out, through the load on to ground.


I really can't picture how this works even using google images!

Does sinking have a constant supply and sourcing doesn't?


Sorry it just isn't making sense to me.
  • Former Community Member
    0 Former Community Member
    Hi Steven,

    It’s not quite as you described.


    Sourcing, has a power source to the plc card.

    Think of the plc card acting as a relay it switches power to the actuator.

    The power then does the job of energising the actuator and returns to the power source, which may or may not be grounded.

    Akin to a light  switch in the live conductor.


    Now sinking is the same, but the relay is in the neutral line, & the plc is in this case connected to the opposite sude of the piwe supply.


    Next we have inputs, but let’s see if I have explained outputs ok first.
  • Ok sourcing output I understand fine. It makes sense in that it supplies power to relay, to the actuator and back to supply again. 

    What I don't understand is the sinking; why would you have a need for the plc to be fed from the return?
  • Former Community Member
    0 Former Community Member
    OK, lets talk about a specific example, if I can explain that right, then perhaps you can them make the leap to the generic.


    Take a 24V DC supplied contactor coil, let the PLC be the on/off switch.

    Ignoring for a moment protective conductors and the details of the power supply wiring to afford circuit protection, we'll just look at the conventional current flow for the function of the devices.  Also we will let the PLC wiring for the processing function be excluded, we will simply look at the current path for driving the output.


    For a sourcing output, the +Ve of the 24V DC is wired from the power supply (via appropriate circuit protection) to the PLC output card common +Ve connection.

    The internals of the PLC then switch this +Ve under software control to the specific output terminal connected to the contactor coil.

    This is then wired to say the A1 of the contactor.

    The -Ve of the power supply is taken to the A2 of the contactor.

    Thus, when the PLC supplies current the contactor coil energises.

    A sourcing output.


    Now for a sinking output, the +Ve of the 24V DC is wired from the power supply (via appropriate circuit protection) to the contactor coil A1.

    The A2 of the contactor is wired to the specific output terminal which is to control the contactor.

    The internals of the PLC then switch this +Ve feed (which runs via the contactor coil) to the internal -Ve within the PLC, thus one side of the coil is connected to the power supply +Ve, the other side is connected to the power supply -Ve, thus the coil energises.

    The -Ve comes from the -Ve side of the power supply being connected to the common -Ve connection on the PLC output card.


    Now, it may or may not be possible to use the same card for both configurations, but, normally, the card configurations would be different because it all boils down to PnP & NpN junctions. but that is not really relevant yet.


    Does the above descriptions of conventional current flow for sourcing and sinking to drive outputs now make sense?


    If you get that, then for an AC supply just swap the +Ve for the unearthed side of the AC supply and -Ve for the earthed side and it works the same, the actual voltage is irrelevant, and is decided by the circuit components, the function is identical.

  • Hi Paul.


    I had to find a simple image on Google and look at it while reading your explanation. I admit it took me a few reads but yes I understand them now. Thank-you for your time in writing all of that, i really appreciate it. It takes me a little while to understand things but I got there lol.
  • Former Community Member
    0 Former Community Member
    Great Stephen,

    That covers outputs, inputs can also be sourcing and sinking, do you understand that, or not?
  • Hi Paul,


    I haven't looked at inputs yet to be honest as we haven't covered them in college. Only outputs and that was a brief skim, hence why I had to do further research. If you wouldn't mind explaining inputs also, in the same fashion, I would be grateful.


    Thanks
  • Former Community Member
    0 Former Community Member
    Hi Stephen,


    OK, inputs are the same but the opposite way around! ;)


    Now if we use the same analogy of a 24V DC supply system.  Ignore ground/earth, because whilst systems are earthed, it is not for functional reasons so much.

    Please realise that I am using conventional discrete components to explain the way this kind of system works, this is an analogy, rather than a real life system, just for you to grasp the concept, if I am explaining it well enough!


    For a sourcing input card.

    This time the actuator is the external sensor, let's use a limit switch, it could even be a manual on/off switch, as simple as a light switch on the wall.

    The +24V DC goes to the common +ve connection on the input card.

    Simplifying this somewhat, imagine that there is a relay on the input card.

    The +24V will go to the A1 terminal on the relay on the input card.

    The A2 terminal will go to the connection that is the designated input.

    This designated input is then wired out to one terminal of the switch.

    The other side of the switch is then wired to the 24V DC supply -Ve (0V).

    Thus, when the switch is closed current will flow from the +24 through the A1, coil, A2, wiring to switch, through the closed switch, to the 0V (DC-Ve) terminal of the power supply.

    This will allow the relay on the board to close and thus the contacts of this imaginary relay to close and provide a signal internally within the PLC.
    Sourcing PLC Input Module,
    plc input modules




    For a sinking input card.

    Again, the actuator is the external sensor, again, let's use a limit switch, it could even be a manual on/off switch, as simple as a light switch on the wall.

    The +24V DC goes to one side of the switch.

    The other side of the switch is then wired to the input terminal on the PLC card.

    Again, simplifying this somewhat, imagine that there is a relay on the input card.

    The +24V from the card input, will go to the A1 terminal on the relay on the input card.

    The A2 terminal will go to the common -Ve/0V DC connection on the card.

    This common -Ve is then wired back to the -Ve of the 24V DC supply.

    Thus, when the switch is closed current will flow from the +24 through the closed switch contacts the A1, coil, A2, back to the -Ve common of the card, to the 0V (DC-Ve) terminal of the power supply.

    This will allow the relay on the board to close and thus the contacts of this imaginary relay to close and provide a signal internally within the PLC.

    Sinking PLC Input Module
    plc input modules 


    Diagrams above from:
    https://library.automationdirect.com/3-wire-sinking-sourcing-devices-plc-input-modules/


    And, no, I didn't copy the text and concepts from anywhere, it's all from memory, I used to work for a PLC OEM! ;)

    There is some more on sinking and sourcing here:
    https://library.automationdirect.com/sinking-sourcing-concepts/

    I've not checked it's validity though, I would expect it to be reliable and correct.

     


  • Ok brilliant. Thanks for your help. I understood all of that no problem
  • Former Community Member
    0 Former Community Member
    Respected Paul,


    Your explanation is outstanding stay blessed keep in touch take care sir.


    Regards

    Paul Immanuel