In the field of industrial automation, the most disparate types of sensors for various physical quantities are used.
Digital output sensors, that is, sensors whose logic output can only take on two states (typically “object present” and “not present”) are divided, classifying them according to the electrical type of their outputs, into two main categories: PNP and NPN.
The differences between the two types concern the method of connecting the load to be controlled.
NPN SENSORS:
The sensor output is normally floating (open collector), therefore it can be considered virtually isolated and free of voltage. When active, the sensor output is brought to ground by the electronics inside the sensor itself. One end of the load is permanently connected to the positive supply, while the other end, through the sensor, is connected to ground when the sensor output is activated.
PNP SENSORS:
The sensor output is normally floating (open collector), therefore it can be considered virtually isolated and free of voltage. When active, the sensor output is brought to the positive power supply by the electronics inside the sensor itself. One end of the load is permanently connected to ground, while the other end, through the sensor, is connected to the positive supply when the sensor output is activated.
The load is usually the photocoupled input of a PLC, however it is also possible to connect the sensor to the sensor, LEDs and other types of loads, taking care to check that the current absorbed by the load does not exceed the maximum current that can be supplied by the sensor.
The colors for coding the sensor wiring normally follow the following convention:
- BLUE wire -> ground
- BROWN wire -> + power supply
- BLACK wire -> sensor output
NORMAL STATE OF THE OUTPUTS
Another fundamental parameter for choosing a sensor is the state of its output in a normal situation, that is, with the sensor not activated. In other words, when there are no objects to be detected in front of the sensor.
- N.O. (normally open) output: the sensor output is not activated when the sensor is not triggered, it is activated when an object enters the sensor detection field
- N.C. (normally closed) output: the sensor output is activated when the sensor is not triggered, it is deactivated when an object enters the sensor detection field
By combining the type of outputs with their rest state, there are a total of four possible configurations:
- NPN – N.O. : NPN output, normally open
- NPN – N.C. : NPN output, normally closed
- PNP – N.O. : PNP output, normally open
- PNP – N.C. : PNP output, normally closed