NPN & PNP sensors

NPN outputs pull down to the negative supply rail and drive loads that are connected between the output and a positive supply. PNP outputs pull up to the positive rail, and drive loads connected between the output and the negative supply rail. If you have a lot of loads (like a PLC input card) hat has all its inputs commoned to the negative rail, you use PNP outputs. If the loads are all commoned to the positive supply rail, you use NPN outputs.

You can also think of it as a normally open or close configuration. Normally logic 0 or 1, active low or high. Depending on how you interface your sensor. There many other ways of looking at this. The NPN PNP terms basically come from the transistor that is at the output of the sensor, acting like a switch. For further information, you may like to search for topic relating to using transistor as a switch.

-- Best Regards, Lim Siong Boon

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