![]() When selecting a sensor to be used with a PLC, it is very important that the sensor matches the type of PLC input card to be used. Use with a programmable logic controller (PLC). PNP sensors tend to be more commonly used. ![]() When used in a traditional relay type control circuit, it is normally possible to use either the PNP or the NPN type of sensor as shown below. The selection of a PNP sensor verses an NPN sensor is determined by the nature of the circuit the device is to be used in. Further learning material regarding PNP and NPN transistors can be found online from several sources. However, for ease of understanding please see attached a page extracted from our Practical Aspects book, showing (in brief) transistor terminology and circuit diagrams. Please note that the subject of this FAQ is specifically related to wiring PNP/NPN outputs for sensors, not to give a detailed understanding of transistor technology. a PNP sensor may be either N/O or N/C as can an NPN be either N/O or N/C. Refer to attached document for simple explanation of the two.Ī key point to observe is that PNP and NPN has nothing to do with whether the sensor is normally open (N/O) or normally closed (N/C), i.e. The difference is a result of the internal circuit design and type of transistors used. Two specific types of 3 wire sensors are available PNP and NPN. ![]() Solid state electronic components such as transistors are used to switch the output of the sensor upon detection of an object. The term solid state refers to the type of components used within the sensor. Most industrial proximity sensors (inductive, capacitive, ultrasonic and photo electric) are solid state.
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