A brief discussion on the wiring method between PLC and proximity switch

Since the input of PLC is divided into two types: NPN input and PNP input, the proximity switch is also divided into two types: NPN and PNP. Only when the type of proximity switch is consistent with the input type of PLC, when an object approaches, the PLC will have an input signal. Generally, the default type of proximity switches of Japanese and Korean series is NPN type, and the default type of proximity switches of European and American series is PNP type. The proximity switch is different from the PLC. Whether it is Japanese, Korean, European and American series, each proximity switch has two types of NPN and PNP. PLC cannot have NPN type and PNP type. This is because the polarity of PLC can be changed through wiring, but once the proximity switch is fixed, it cannot be changed.

This is a common proximity switch, which is cylindrical. Generally speaking, the thicker its diameter, the longer the detection distance.

Depending on the detected object, proximity switches are divided into inductive and capacitive types. Inductive proximity switches use the Hall effect to detect changes in the magnetic field. Since only metals can cause changes in the magnetic field, inductive proximity switches mainly detect metals, mainly steel. Capacitive proximity switches are essentially the plates of a capacitor. When the detected object approaches, the dielectric constant of the entire capacitor changes, so it is considered that an object is approaching. Therefore, capacitive proximity switches can detect both metals and non-metals. However, we generally use inductive proximity switches to detect metals, so everyone assumes that capacitive proximity switches are used to detect non-metals. For example, ceramics, paper, wood, etc.

It is the actual application of proximity switch in industrial field. It is usually fixed by bracket. When the detected object is close, its indicator light will light up, indicating that the proximity switch has output. Similarly, if the wiring is correct, the PLC will have input signal. How to connect the proximity switch to the input point of PLC? In fact, it is similar to the push button switch. Generally, the proximity switch has three wires, brown, blue and black. Brown and blue are power supply, usually brown is 24V and blue is 0V. Black is the signal line, which is connected to the input of PLC, and this signal is exactly what we need.

Figure 2 PLC Hardware Manual
As shown in Figure 2, this is the PLC hardware manual. If we want to connect the signal of the proximity switch to the input point I0, which is the yellow fluorescent pen part, what should we do? In fact, it is very simple. Look at the red circle part. The PLC has provided us with output power. We only need to connect the three wires of the proximity switch in sequence.

Actual wiring of proximity switch

As shown in Figure 3, this is the actual wiring of the PLC. The yellow fluorescent pen marks the three wires of the proximity switch. The brown and blue wires are connected to the 24V and 0V of the PLC respectively, and the black wire is connected to I0. When an object reaches the sensing distance of the proximity switch, the I0 of the PLC will have an input, and we can process it in the program. It should be noted that 0V and COM0 are short-circuited in the figure, so this connection method is suitable for PNP type proximity switches. However, if the NPN type proximity switch is connected in this way, there will be no signal. The wiring of the proximity switch is only related to the NPN and PNP types and has nothing to do with inductive and capacitive.