Control Wiring - NPN-PNP Devices and Connections

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