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.

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.