Proximity switches come in two-wire and three-wire versions. Three-wire proximity switches are further divided into NPN and PNP types, with different wiring configurations. Wiring for a three-wire proximity switch: The red (brown) wire connects to the positive terminal of the power supply; the blue wire connects to the 0V terminal; the yellow (black) wire is the signal wire and should be connected to the load. The other end of the load is connected as follows: For the other end of the load: For an NPN proximity switch, it should be connected to the positive terminal of the power supply; for a PNP proximity switch, it should accept a 0V power supply. How to distinguish between normally open and normally closed proximity switches? What are their functions?
I. How to distinguish proximity switches
1. When the proximity switch is not activated (sensing part is not blocked), it is open; when the proximity switch is activated (sensing part is blocked), it is closed. Normally open means the signal output line is open under normal conditions, with no signal output, and closes only when an object is sensed. This is a normally open three-wire DC24V sensing PNP type: brown connects to +24V, blue connects to 0V. The black and red probes of the digital multimeter's DC voltage range are 0V. Multimeters have 24V normally closed and 24V normally open types. PNP is different. The circuit diagram of a PNP signal proximity switch is as follows: As can be seen from the diagram, the PNP signal belongs to a pull-down resistor load, therefore its common terminal (i.e., the COM terminal) is negative. So, when acquiring the signal, the line to be connected is the signal line and the negative power supply. Distinguishing method: Use a switching power supply to approach the switching power supply. When there is no object to be measured within the sensing range, use a multimeter to measure the voltage between the signal line and the negative power supply. If the voltage is 0 at this time, then when there is an object to be measured within the sensing range, the voltage is approximately 24V (absolute value) for a normally open PNP signal.
2. When the proximity switch is not activated (the sensing part is not shielded), the proximity switch is closed; when the proximity switch is activated (the sensing part is shielded), it is open. Normally closed means the signal output line is closed under normal conditions and continuously outputs a signal. When it senses an object, it opens and shuts off the signal. This is the difference between a normally closed NPN and a normally closed NPN. The circuit diagram of an NPN signal proximity switch is as follows: As can be seen from the diagram, the NPN signal is a pull-up resistor load, so its common terminal (i.e., the COM terminal) is the positive terminal of the power supply. Therefore, when acquiring the signal, the connection should be between the signal line and the positive terminal of the power supply. Distinguishing method: Use a switching power supply to approach the switching power supply. When there is no object to be measured within the sensing range, use a multimeter to measure the voltage between the signal line and the positive terminal of the power supply. If the voltage is 0 at this time, and there is an object to be measured within the sensing range, the voltage is approximately 24V (absolute value), then the switch is an NPN normally open signal.
II. Normally Open and Normally Closed Functions of Proximity Switches
Because some applications require normally open contacts while others require normally closed contacts—for example, a normally open start button and a normally closed stop button—push-button switches typically have both normally open and normally closed contacts (including both). Unlike our lighting panel switches, the release contact of a push-button switch is not pressed (the button can also remain pressed).
How to distinguish between normally open and normally closed proximity switches? What are their functions? When the power is on, current oscillates through the LC resonant circuit, and then through the resonant circuit. The oscillation generated by the oscillation circuit L2 is fed back to the VT2 base, and VT2 opens. The collector potential of VT2 is high. R5 is connected to the VT3 base. If the low potential of the VT3 base is not connected, the high potential of the VT4 base is not connected, and VT5 is not connected. The collector potential of VT5 is high, which can be regarded as a high-level output. The button does not have a protective function. The lighting panel switch is a circuit breaker, providing overload and short-circuit protection. After being closed, it will remain closed unless someone opens it, provided there is no fault.
