The difference between the two wirings is that in the PNP type wiring, the relay is always connected to 0V, and +24V is switched by the sensor. Let’s have a look at a relay-type circuit, where a relay is operated directly by a proximity sensor. a PNP sensor can be either N/O or N/C as can an NPN be either N/O or N/C) This is dependent only on the application. Two of the wires are used to provide the sensor with power to operate, while the other wire is the output from the sensor.Īt this point, t’s worth mentioning that being a PNP or NPN type sensor, does not imply if the sensor output is normally open (N/O) or normally closed (N/C). They use PNP or NPN type transistors to switch the output when an object is detected. The most popular type of proximity sensor is the 3-wire type. PNP vs NPN for 3 wire connection of a sensorĪlmost all the industrial proximity sensors are solid-state devices, meaning that they do not have moving parts inside. But, in the NPN sensor wiring, the load is always connected to Positive, and the Negative is switched when an object is detected. The Positive is switched when the proximity sensor detects an object. In the PNP sensor wiring, the load is always connected to Negative. Therefore, they must always be connected to a 24V power source to function. To easily remember the wiring of a 3-wire DC proximity sensor, we can use the following analogy: A Way to Remember NPN and PNP Sensor Wiring PNP proximity sensors are also known as ‘ sourcing’ sensors. When connected to a PLC input, it detects this as a logic HIGH signal. When an object enters the detecting range of the sensor, the output of the sensor is connected with +24V. PNP proximity sensors provide an active HIGH output. This type of sensor is also known as ‘ sinking’ sensor. This means that when an object enters the detecting range of the sensor, the output of the sensor is connected with the ground. NPN proximity sensors provide an active LOW output. This article aims to provide an outline of the two main output types and a guide on how to connect a proximity sensor to a PLC. Selecting the correct type of proximity sensor for a particular application can ensure the correct operation of a system. There are two main types of proximity sensors based on their output type: NPN and PNP. There are 2-wire and 3-wire proximity sensors, and 3-wire proximity sensors are more popular. This should not trigger the NPN input, since it is looking for a GND connection to activate, but again there may be some unexpected reverse current flow from the sensor and into the NPN input circuitry that is not intended.Proximity sensors are used to detect objects without making physical contact. When the PNP sensor turns on, it will connect +24VDC to the NPN input. This current flow might be high enough to falsely trigger the NPN input. Depending on the sensor’s internal wiring, there might be some current flow from the NPN input to the sensor, for example there may be reverse current that “leaks” through a protection diode or an LED in the sensor. If connected to a PNP sensor, the NPN input will be floating (open) when the PNP sensor is off (not triggered). I have never seen a sensor with mixed PNP and NPN outputs, but perhaps some specialty types exist on the market.Īn NPN input is looking for the input to be connected to GND to respond. That is, if the sensor is PNP for the Black wire, it is also PNP for the White wire. The sinking / sourcing logic is the same as for the Black wire. Hello George, interesting questions! The White wire is for a second output, typically Normally Closed.
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