Control Wiring - Sinking and Sourcing, NPN-PNP Devices and PLC Inputs

We’re going to talk about sinking, sourcing, PNP, NPN, and how to determine which type of sensor you need depending on your PLC input.

In the video to the right we explain what a Sinking and Sourcing PLC input is, the fact that manufacturers are not consistent with the use of these terms, how they equate to NPN and PNP sensors, and most importantly how to look in the PLC user manual and determine how to wire your control circuit based off of the manufacturer's wiring diagram.

Next Steps

Go to the Control and Power Wiring Lesson series to select your next lesson.  There are also many other Lesson Series on PLC Programming and Industrial Automation.



One of the things that people working with plc's get confused on is the use of NPN and PNP devices and connections. Thanks to Terry Woods, a member of the forum at  for providing a very graphical and detailed explanation of NPN/PNP.

Sinking and Sourcing

Electron Current Flow

Conventional Current Flow

Open Circuit

Conventional Open Circuit

PLC Input

Sinking and Sourcing PLC Input

NPN Not Pointing In

Sourcing Field Device

Sourcing PLC Input

NPN Sinking Output

PNP Sourcing Output

PNP Sinking Output

NPN Sourcing Output

Sourcing and Sinking Circuits