The main function of many industrial sensors is to act as an electronic switch. If a sensor has switching output, this will be pictured on a connection diagram in different ways depending on the type of sensor you’re working with.
Inductive, capacitive, or magnetic sensors
For these three types of sensors, the switching output is usually shown inside the box that represents the sensor, and appears as an open or closed contact. This is the normal state of the switch, which is when no activating target is in the sensor’s detection field. Connection diagrams for 3- or 4-wire sensors will also show the required wiring configuration for their type of output:
Current-sourcing (PNP) outputs are internally connected to the power supply voltage, and so should be wired externally to a load that has a connection to DC ground.
Current-sinking (NPN) outputs are internally connected to DC ground, and so should be wired externally to a load that has a connection to the power supply voltage.
Use with a programmable logic controller (PLC).
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.
Two types of input cards exist, those that ‘sink’ current (also known as positive logic) and those that ‘source’ current (also known as negative logic). It is worth mentioning, that whilst the terms sinking / sourcing and positive / negative logic are well known in some industries, they are not always commonly used terms. It is therefore important to identify the type of sensor to be used with the PLC card based on the PLC manufacturer’s documentation and / or wiring diagrams.
Most common in Europe is the ‘sinking’ type of input, these will be used with the PNP sensor as shown below. Less common nowadays are input cards that ‘source’, these were popular in Asia and require the NPN type of sensor in order to operate correctly. Many modern PLC input cards can be configured and wired to be either ‘sinking’ or ‘sourcing’ although it will usually necessitate all inputs on a particular input card being configured the same.