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What is the difference between NPN and PNP sensors?

What is the main difference between an NPN and a PNP sensors?

Product Line:
Electronic Sensors and Machine Cabling

All Electronic Sensors

PNP and NPN sensors are 3 wire DC devices. The difference is in the type of transistor used in the sensor. The PNP device has 2 control power leads, one connected to positive, the other to negative. The third wire, connects to the load. The remaining lead of the load connects to the negative pole of the power supply. On an NPN device, also is a three wire device with power leads connected to the positive and negative poles of the power supply. The third wire here, also connects to the load. However, on an NPN device, the remaining lead from the load must be connected to the positive pole of the power supply. Where a sensor is wired into a PLC or computer input card, the card serves as the load and will be connected to either the positive or negative pole of the power supply and will determine the type of sensor required to function correctly. 
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