Current-sinking devices are PNP sensors, while current-sourcing devices are NPN sensors. A current-sinking input must be connected to a current-source sensor.
NPN stands for Negative-Positive-Negative. The term "switched" refers to the electrically switched side of the regulated load (relay, tiny indicator, PLC input). Either the load is connected to Negative and the Positive is switched (PNP), or the load is connected to Positive and the Negative is switched (NPN).
During a "on" state, PNP sensors produce a positive signal to your industrial controls input, whereas NPN sensors produce a negative signal. When turned on, NPN, or "sinking" output sensors act in the opposite direction, sinking ground voltage to an input.
A piece of P-type silicon (the base) is sandwiched between two pieces of N-type silicon in an NPN transistor (the collector and emitter). The layers of a PNP transistor are reversed in type. The schematic symbols for NPN and PNP transistors are extremely similar. The direction of the arrow on the emitter is the sole change.
Electrons make up the bulk of charge carriers in an NPN transistor, while holes make up the majority of charge carriers in a PNP transistor. The mobility of electrons is greater than that of holes. NPN transistors are therefore preferred over PNP transistors.
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