NPN / PNP Sensor Output
It defines which side an industrial sensor drives its output line towards. An NPN (sinking) type pulls the line to chassis when its output is active, while a PNP (sourcing) type raises it to the supply voltage. Whichever the driver's input structure expects is the type that must be chosen; the wrong type either never triggers the input or makes it look permanently triggered.
In three-wire proximity sensors two wires are supply and one is signal. What makes the difference is where the output transistor inside the sensor connects the signal line.
- NPN: When idle the line is high thanks to a pull-up resistor; when the sensor detects, it pulls the line to chassis and the input sees 0.
- PNP: When idle the line is at chassis through a pull-down; when the sensor detects, it takes the line up to the supply voltage and the input sees 1.
If the driver input works with a pull-up it is compatible with an NPN sensor; if it works with a pull-down a PNP sensor is required. With the wrong pairing the sensor LED blinks on and off but the driver sees no change at all, which leads you to blame the fault on the sensor or the cable.
For a quick test in the field, measure the voltage between the sensor output and chassis: if the voltage falls at the moment of detection it is NPN, if it rises it is PNP. On AXI drivers the type expected by the sensor inputs is stated in the connection diagram; verify the diagram before wiring.
Context where this term is used: DC Motor Driver Selection Guide