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The Structure and Principle of Reed Switch

A magnetic sensor is a device that can sense a magnetic field and its variation and convert it into an electrical signal output. We can artificially place permanent magnets and use the magnetic fields generated by them as carriers of various information, so that magnetic sensors can undertake the tasks of detecting, storing, converting, reproducing and monitoring various magnetic fields and information in magnetic fields. In today's information society, magnetic sensors for cylinders have become an indispensable basic component in the pneumatic industry and information industry. At present, people have developed magnetic sensors using various physical, chemical and biological effects, and have been widely used in various aspects of scientific research, production and social life.


The magnetic air cylinder reed switch is a magnetic sensor with a reed switch as the core sensing element. It is composed of a shell, a reed switch, a PBC board, a power cord and so on.


The reed switch was invented in 1936 by Walter B. Ellwood of Bell Telephone Laboratories. The overlapping magnetizable reeds at the two endpoints are sealed in glass fiber-reinforced plastic. There is a small interval (about a few microns) in the overlapping part, and the contacts are plated with high-strength metal, usually rhodium or ruthenium, which greatly improves the switching times of the reed and the service life of the product. When the reed switch senses an external magnetic field, the two magnetic reeds close together, and conduction forms a closed circuit. Conversely, once the magnet moves away from the sensor magnetic reed switch, the reed will break to its original position. The glass tube is filled with high-purity inert gas, such as nitrogen. The reed is sealed in an inert gas and does not come into contact with the external environment, which greatly reduces the oxidation and carbonization caused by the sparks generated during the opening and closing of the reed contacts, and isolates it from flammable gases and coal dust, achieving The explosion-proof effect improves the safety factor of the product, and at the same time prevents the erosion of the contact points by impurities such as external steam and dust, thereby prolonging the service life.

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*Using an external coil


Place the reed switch on the center axis of the coil (the center axis has the strongest magnetic field)


Under the action of the magnetic force lines of the magnetic field, the two reeds are magnetized into different polarities. The different magnetic poles attract each other to make the two reeds contact each other, the circuit forms a closed loop, and the LED light is on. On the contrary, when the external magnetic force disappears, after the suction force between the reeds disappears, the reeds are separated due to their own elasticity, the circuit is thus disconnected, and the LED light is turned off.

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All reeds need to be tempered. In order to eliminate residual magnetism, when the magnetic field subsides, the magnetic field on the reeds also disappears. If any residual magnetic force exists on the reed, the characteristics of the switch will change. During the manufacturing process, proper processing and annealing are very important.


One of the important characteristics is the sensitivity, which determines the magnetic force required to drive the reed switch to work. Sensitivity is expressed using the AT value, which is the product of the number of turns of the coil and the current of the coil. The conventional pull-in value range of the reed switch is 10~60AT. The smaller the AT value, the more sensitive the response of the reed switch. So the smaller the reed switch, the more sensitive it is to the magnetic field.


According to the characteristics of the reed switch, it is compact in structure and light in weight, can be installed in extremely limited space, and is suitable for micro-equipment.


  Due to the different internal structure, the reed switch is also divided into normally open type and normally closed type.

  Normally open contact

A contact that is in an open state under normal conditions (not energized) is called a normally open contact.


  Normally closed contact

A contact that is in a closed state under normal conditions (no electricity, no current flowing) is called a normally closed contact.


Normally open - NO (normal open) is usually disconnected, that is, the coil is disconnected when it is not powered.


Normally closed - NC (normal close) is usually closed, that is, closed when the coil is not powered.


Normally closed dry reed switch: the normally open piece and the normally closed piece are fixed, and the switchable reed can be switched between the normally open piece and the normally closed piece.


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When there is no influence of the magnetic field; due to its own elasticity, the switchable reed is in contact with the normally closed reed, and the circuit is closed at this time.


When the external magnetic field approaches; the switchable reed is affected by the magnetic field and will approach and contact the normally open reed, and the circuit is disconnected at this time.


  Therefore, as a component that uses magnetic field signals to control line switches, it can also be used as a sensor, counting, limit, etc.


  Precautions for the use of dry reed switches:


*The glass tube is easily broken


·External impact during installation and transportation can easily cause the glass tube to break


Prolonged exposure to high temperature may cause the glass tube to crack due to thermal expansion


· Improper handling may affect the use of the magnetic reed switch


* Difficult to withstand high voltage and high current


·The contacts and reeds are quite small, so it is difficult for the reed switch to withstand high voltage or high current. When the current is too large, the reed will lose its elasticity due to overheating. That is, the switching capacity is small, the contacts are prone to jitter and the contact resistance is large.

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