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When it comes to solid state devices, such as PLC transistor outputs and sensors, the concept of PNP and NPN configurations can be easily confused. The difference between these two configurations is simply the direction in which current flows.When a PNP PLC output is on, positive current flows from the output terminal on the PLC, to the output device and then to ground to complete the circuit. PNP outputs can be thought of as a switch placed between the positive terminal of a battery and a light bulb with the other side of the light bulb connected to the negative terminal of the battery. With the PNP configuration, the output device (the light bulb in the analogy) is always provided a path to ground (the negative side of the terminal in the analogy), and is turned on and off when the PLC output provides a path or blocks the path for current to flow from the positive voltage to the output terminal. Since the PNP output is the source of the output device’s current, PNP outputs are also known as “sourcing” outputs.
Figure 1-PNP Switch and PLC Circuit Equivalents
When an NPN PLC output is on, positive current flows through the output device and then to the output terminal on the PLC, where it returns to ground to complete the circuit. NPN outputs can be thought of as a switch connected between the negative terminal of a battery and a lightbulb with the other side of the lightbulb connected to the positive terminal of the battery. With the NPN configuration, the output device is always provided a positive voltage and is turned on an off when the PLC output provides a path or blocks the path for current to flow from the output terminal to ground. Since the NPN output “sinks” the output device’s current to ground, NPN outputs are also known as “sinking” outputs.
Figure 2-NPN Switch and PLC Circuit Equivalents
For sensors, PNP and NPN configurations are similar to PLC outputs. When the sensor detects an object, the sensor allows current to flow from positive voltage to the output wire, or from the output wire to ground, depending on the configuration of the sensor. For a PNP sensor, when an object is detected, current flows from positive voltage to the output wire which is connected to an output device which must have its other side connected to ground. When an NPN sensor detects an object, current it allowed to flow from the output wire to ground, so the output device must be connected on one end to positive voltage, and the output wire on the other end.
Even though the concepts of NPN and PNP configurations can get confusing, you can always remember which signal is being “switched” by looking at which letter appears more in the name: PNP “switches” positive voltage because “P”, or positive appears twice, and NPN “switches” the ground signal because “N”, or negative, appears twice.