Industrial sensors: NPN and PNP

A large number of industrial sensors are available on the market. These include proximity switches, photoelectric switches and many other more esoteric types.

The vast majority of industrial sensors will comply with what is known as "NPN" or "PNP" interface conventions. The NPN and PNP sensor output acts as an electronic switch.

NPN interface

In an NPN interface, when the sensor turns on (sensing something), a circuit is closed between the output pin of the sensor and circuit common (ground, 0V). When the sensor is off its output is effectively an open circuit.

PNP interface

In a PNP interface, when the sensor turns on (sensing something), a circuit is made between the output pin of the sensor and the positive supply voltage. When the sensor is off its output is effectively an open circuit.

Typically these sensors can carry 25-50mA on their outputs.

Which is best?

The digital input circuits on most SPLat controllers, standard and custom, are NPN compatible, i.e. designed to work with NPN sensors. We make that design choice simply because it costs less to make an NPN input than a PNP input. That is the pro-NPN argument.

The pro-PNP argument is a bit more subtle: If the insulation on the input wire from a sensor to a controller should fray, the most probable failure mode is a short circuit to circuit ground, i.e. to the metal body of the machine. This will register as an activated (ON) input in an NPN circuit. In certain designs this might represent a safety hazard (say if it were a 'door closed' sensor on a jet plane). Some designers therefore prefer PNP sensors because a short to ground will register as an OFF input.