Sensor Basics: What is the difference between PNP and NPN sensor outputs? (Sourcing vs Sinking)

How do I choose PNP vs NPN? What does sourcing or sinking mean for a sensor? This basic discrete sensor output topic throws off experts and novices both.

To start, the sensor output is always tied to a load. A load can be: a light or a LED or a valve or a relay or PLC input or I/O block.

There’s a great automation-insights blog by Henry M. called “An Easy Way to Remember PNP & NPN Sensor Wiring.” Henry explains this in the simplest way as I’ve seen it:

“Just remember the first letter tells you everything you need to know.”
✅ PNP = Switched Positive so P for Positive
✅ NPN = Switched Negative so N for Negative

Sometimes you might hear someone say sourcing vs sinking:
✅ Sourcing (which is PNP) is bringing “power” to the load, sourcing power to it.
✅ Sinking (which is NPN) is bringing “common” to the load, sinking power away from it.

This way you can always answer the question: “Is the sensor bringing positive or negative to the load/light/LED/valve/relay/plc/block?”

So why pick one over the other when selecting a sensor?
✅ PNP sensor outputs tend to be easier to understand for technicians and engineers as the “positive output” indicates “on” and no power indicates “off”. This logically makes more sense.
✅ Another reason is that if an output wire from an NPN sensor got damaged and grounded, this would read to a controller as an “on” signal causing motion or create a hazard unintentionally.
✅ NPN sensors are more common in Asian manufactures.
✅ PNP is more common in Europe and the Americas.

As with any design, there are both options as one style might be more appropriate for the automation sensing application or controls wiring architecture.

Check out the blog link I mentioned.
https://automation-insights.blog/2018...

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