Distinguish between PNP and NPN sensors and their wiring to a 24V DC PLC input.

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Multiple Choice

Distinguish between PNP and NPN sensors and their wiring to a 24V DC PLC input.

In DC sensor wiring, you’re looking at whether the output sources current from the supply or sinks current to ground. A PNP sensor sources current to the load: when it’s active, its output provides +24 V to the load, so current flows from the sensor output toward ground. An NPN sensor sinks current to ground: when active, it pulls the load down to 0 V, so current flows from +24 V through the load into the sensor to ground. When wiring to a 24 V DC PLC input, you match the sensor type to the PLC input’s configuration. A PNP (source) sensor is paired with a PLC input that is designed to receive current from a positive supply (a sinking input), so the input gets energized when the sensor feeds it with +24 V. An NPN (sink) sensor is paired with a PLC input that is designed to be fed from a positive supply and then pulled down (a sourcing input), so the sensor can pull the input to ground when active. The essential idea is the direction of current flow: PNP outputs +24 V to the load, NPN pulls the load to ground, and the wiring must align with the PLC input’s expected current path. Dismissing AC as a requirement and recognizing that the two types are not interchangeable helps avoid incorrect connections.

In DC sensor wiring, you’re looking at whether the output sources current from the supply or sinks current to ground. A PNP sensor sources current to the load: when it’s active, its output provides +24 V to the load, so current flows from the sensor output toward ground. An NPN sensor sinks current to ground: when active, it pulls the load down to 0 V, so current flows from +24 V through the load into the sensor to ground.

When wiring to a 24 V DC PLC input, you match the sensor type to the PLC input’s configuration. A PNP (source) sensor is paired with a PLC input that is designed to receive current from a positive supply (a sinking input), so the input gets energized when the sensor feeds it with +24 V. An NPN (sink) sensor is paired with a PLC input that is designed to be fed from a positive supply and then pulled down (a sourcing input), so the sensor can pull the input to ground when active.

The essential idea is the direction of current flow: PNP outputs +24 V to the load, NPN pulls the load to ground, and the wiring must align with the PLC input’s expected current path. Dismissing AC as a requirement and recognizing that the two types are not interchangeable helps avoid incorrect connections.

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