PNP and NPN Sensors | Short Description

We will use an example that will help understand the differences between PNP and NPN.

Controlling a traditional relay with a 24 VDC coil.

PNP - After the sensor is triggered, we get a PLUS signal at the output, e.g. +24 VDC

NPN - After the sensor is triggered, we get a MINUS signal, 0, GND at the output.

Most of the sensors that can be found on the market today (inductive sensors, capacitive sensors, optical, ultrasonic) belong to semiconductor devices, so-called (SOLID STATE). The term semiconductor (SOLID STATE) refers to the type of transistor used to build the sensor. Transistor, or semiconductor elements, is used as a switch and, in the case of the sensor being triggered, for example, when the sensor head is close to the housing, "CLOSES THE CIRCUIT" and causes a signal to appear at the NO normal open output. We can distinguish sensors with 3 or 4 wires of the NPN and PNP type.

-3-wire sensors have a "+" power line, a "GND" ground line and an output signal line in a normally closed OR normally open configuration depending on the sensor configuration.

-4-wire sensors have a "+" power line, a "GND" ground line and TWO output signal lines, both NO normally open and NC normally closed.To understand the operation of both types of sensors, it is important to note that the type of sensor (PNP/NPN) has nothing to do with the type of contact at the output, whether normally closed (NC normal closed) or normally open (NO normal open). This information is important, however, as it informs the user of the state of the output when the sensor is in a low or "inactive" state (i.e. no object detected, logical 0) as well as in a high state (i.e. object detected, logical 1). Controlling the inputs in the PLC controller

The first step is to select the appropriate type of sensor for the type of inputs (INPUT I/P) of the controller. For the PNP type, labeled as "SINK", and for the NPN type as "SOURCE". It is important to standardize the types of inputs as not all manufacturers use the same nomenclature:

-PNP - SINK

-NPN - SOURCE

The type of input must be looked for in the catalog card for the specific PLC used by us. The most popular type of input in Europe is the PNP input. Many PLC controllers support two types of PNP and PNP sensors. However, one type of input is still required to be maintained as used during installation.