The purpose and working principle of the detection switch
The detection switch is connected with other circuit elements to detect the parameters of the element or circuit (such as current, voltage, etc.). The detection switch has an inductive function. It is often a variety of inductive switches or proximity switches and requires an external power supply. To work.
The detection switch has various output usage methods according to its purpose:
1. Contact output type
The switching element that uses the contacts of micro switches, limit switches, and relays as outputs, and connects with electromagnetic switches, small motors, solenoids, etc. as the main purpose, can perform switching control of several amperes of current. When connecting with electronic control equipment, pay attention to vibration time and minimum load current.
2. Photoelectric coupling output type
The electrical insulation of the detection circuit is the same as that of the contact output type. It can control the switch of 10～50mA current.
3. DC 3-wire type
a. Voltage output type
During detection, a voltage signal is output to the load. The voltage output type is mainly manufactured for the purpose of connecting electronic control devices composed of transistors or ICs such as electronic counters and contactless relays.
b. Current output type
It can also be called an open, collector output type. As shown in Figure 8, when the output transistor operates, there are an NPN type (current sink) that sinks current and a PNP type (current source) that discharges current. Using a small-capacity power transistor in the output transistor can switch 50-200mA current, and can directly drive loads such as electromagnetic relays, solenoid valves, DC solenoids, and indicator lights.
4. DC 2-wire type
The proximity switch of this method has two wires. Therefore, if you pay attention to the polarity when using it, not only the method of use can be the same as that of the mechanical limit switch, but the wiring is simple, but the following conditions must be paid attention to.
(1) Even if the switch is turned off, it is necessary to supply current to the detection circuit of the proximity switch. For this reason, a small amount of current flows in the load. This current is called leakage current. In the case of leakage current, the “leakage current × load generated at both ends of the load is also the cause of the initial failure. The time of the initial failure depends on the manufacturing method and cannot be generalized. Generally, it usually occurs from one week to 10 after the start of use. Within days.
(2) Incidental failure
Including failures caused by defective semiconductor components, broken wires, short circuits, insufficient capacity of resistors and capacitors, circuit breakage of circuit boards, soldering, etc., but the occurrence rate is extremely low. When the proximity switch often fails, it can be considered as the use environment problem, please consult the manufacturer.
(3) Load short circuit and wiring error
When the load is short-circuited due to incorrect wiring or live work, a large current flows to the detection switch and the output circuit is burned. As a protection countermeasure outside the detection switch, the method of cutting off the fast short-circuit current can be used, and the fuse is used for protection, which not only protects the load from short-circuit, but also protects the ground wire. However, due to the small residual capacity of the output transistor in the switch, the effect of 100[%] cannot be achieved.
(4) Damage caused by interference waves
The damage caused by interference waves is slowly formed, so it is extremely common for damage to occur one month or two or three months after the start of use. Therefore, when damage occurs during this period, the cause can be judged as interference waves. The momentary erroneous operation of the detection switch that occurs when the inductive load is opened and closed is caused by interference waves.
Post time: Mar-30-2021