Since PLC inputs are divided into two types: NPN and PNP, proximity switches are also divided into NPN and PNP types. Only when the proximity switch type and the PLC input type are connected in the same way will the PLC send an input signal when an object approaches. Generally, Japanese and Korean proximity switches default to NPN type, while European and American proximity switches default to PNP type. Unlike PLCs, proximity switches, regardless of whether they are Japanese/Korean or European/American, can have both NPN and PNP types. PLCs cannot have both NPN and PNP types because the polarity of a PLC can be changed through wiring, while the polarity of a proximity switch, once fixed, cannot be changed.
Figure 1 Proximity switch
As shown in Figure 1, this is a common proximity switch. It is cylindrical, and generally speaking, the larger its diameter, the farther the detection distance.
Depending on the object being detected, proximity switches are divided into inductive and capacitive types. Inductive proximity switches utilize the Hall effect to detect changes in magnetic fields. Since only metals can cause changes in magnetic fields, inductive proximity switches primarily detect metals, mainly steel. Capacitive proximity switches are essentially the plates of a capacitor. When an object approaches, the dielectric constant of the entire capacitor changes, thus indicating the presence of an object. Therefore, capacitive proximity switches can detect both metals and non-metals. However, inductive proximity switches are generally used to detect metals, so capacitive proximity switches are commonly assumed to detect non-metals, such as ceramics, paper, and wood.
Figure 2. Field application of proximity switch
Figure 2 shows a practical application of a proximity switch in an industrial setting. It's typically mounted on a bracket. When an object approaches, its indicator light illuminates, indicating that the proximity switch has output. Similarly, if the wiring is correct, the PLC will receive an input signal. How is the proximity switch connected to the PLC's input point? It's similar to a push-button switch. A proximity switch usually has three wires: brown, blue, and black. Brown and blue are the power supply wires; typically, brown is 24V, and blue is 0V. The black wire is the signal wire, connected to the PLC's input, which is exactly the signal we need.
Figure 2 PLC Hardware Manual
As shown in Figure 2, this is the PLC hardware manual. How can we connect the proximity switch signal to input point I0, which is the part highlighted in yellow? It's actually very simple. Look at the part circled in red. The PLC has already provided the output power for us. We just need to connect the three wires of the proximity switch in sequence.
Figure 3 shows the actual wiring diagram of the proximity switch.
As shown in Figure 3, this is the actual wiring of the PLC. The yellow highlighter marks the three wires of the proximity switch. The brown and blue wires are connected to the PLC's 24V and 0V terminals respectively, and the black wire is connected to I0. When an object approaches the sensing distance of the proximity switch, the PLC's I0 terminal will receive an input, which can then be processed in the program. It's important to note that 0V and COM0 are shorted in the diagram; therefore, this wiring method is suitable for PNP type proximity switches. However, NPN type proximity switches will not receive any signal when wired this way. The wiring of the proximity switch is only related to the NPN or PNP type and has nothing to do with inductive or capacitive characteristics.
Another point to note is that the proximity switch should be directly connected to the PLC input, without adding any relays or other conversion devices in between.
