The difference between PNP and NPN sensor



These types of sensors are only found in digital sensor category. The digital sensor is that sensor which gives 24V/0V DC output.

PNP and NPN sensors
 
Let's consider an example of three wire sensor. It has brown (24Vdc) wire, blue (0Vdc) wire and black wire. Black wire that is wire which gives us output of sensor. If we receive 24Vdc from black wire that sensor is PNP type and if we receive 0Vdc from black wire that sensor is NPN sensor.

Wiring with PLC

Now the question arises, how to connect sensor with PLC? 

1. PNP Wiring 
If your sensor is PNP, you must go for source input PLC and commen terminal of PLC must be connected to 0Vdc.

2.NPN wiring 
If you choose NPN sensor, you must go for sink type PLC and commen terminal of PLC must be connected to 24Vdc.

Why do we give supply opposite of sensor output to commen terminal of PLC?
In this case an optocoupler plays its role. You will find such type of thing inside PLC built in. Optocoupler is a combination of LED and transistor. Whenever LED glows PLC starts functioning itself. One terminal of LED is connected to commen and other is connected to Input module of PLC. In order to glow LED we have supply it 24Vdc ahe 0Vdc. That's why we provide supply to commen terminal of PLC that is opposite of sensor output.
a. PNP sensor - 0v dc to commen terminal 
b. NPN sensor-24v dc to commen terminal 

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