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Structure And Principle of Magnetic Spring Switch

Magnetic sensors can sense the magnetic field and its variation and convert it into an electrical signal output device. We can artificially place permanent magnets and use the magnetic field generated by them as the carrier of various kinds of information, so that magnetic sensors undertake the task of detecting, storing, converting, reproducing, and monitoring various magnetic fields and information in the magnetic field. In today's information society, magnetic sensors have become essential components in the pneumatic industry and information industry. At present, magnetic sensors for cylinders with various physical, chemical, and biological effects have been developed and widely used in scientific research, production, and social life.

Dry reed tubes were developed in 1936 by Walter Bell of Bell Telephone Laboratories. It was invented by Walter B. Ellwood. Two magnetized reeds with overlapping ends are sealed in FRP. The overlaps are spaced at small intervals (a few microns) and the contacts are plated with a strong metal, usually rhodium or ruthenium, which greatly improves the switching times and service life of the reeds. When the dry reed tube senses the external magnetic field, the two magnetic reeds are closed together, leading to a closed circuit. Conversely, once the magnet moves away from the reed, the reed will break to its original position. The glass tube is filled with high purity inert gas, such as nitrogen. Reeds in the inert gas seal, do not contact with the external environment, greatly reducing the reed contacts in the process of on and off sparks caused by oxidation and carbonization, and separate it from combustible gas and coal dust, achieving the result of explosion-proof, improve the safety factor of the product, to prevent the outside world between steam and dust impurities such as junctions, erosion, so as to prolong service life.

magnetic switch

The reed tube acts as a magnetic conductor. There are usually two ways to make the reed of a reed tube suck together.

Use Permanent Magnets

The magnetic field generated by the permanent magnet produces different polarities at the end position of the reed. When the magnetic force exceeds the elasticity of the reed itself, the heteronym magnetic poles attract each other so that the two reeds absorb and lead the current circuit, and the LED lights up at this time. On the contrary, when the external magnetic force disappears and the suction between the reeds disappears, the reeds are separated due to their elasticity, so the circuit is disconnected and the LED lamp is turned off.

magnetic switch schematic

Use External Coils

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

Under the action of the field lines, the two reeds are magnetized into different polarities. The two reeds contact each other due to the mutual attraction of the heteronymous poles, and the circuit forms a closed loop, and the LED lights up. On the contrary, when the external magnetic force disappears and the suction between the reeds disappears, the reeds are separated due to their elasticity, so the circuit is disconnected and the LED lamp is turned off.

magnetic sensor schematic 

All dry reeds need to be tempered. In order to eliminate residual magnetism, the magnetic field on the dry reeds disappears immediately when the magnetic field disappears. If any residual magnetic force is present on the dry reed, the characteristics of the switch will change. Proper process and annealing treatment are very important in manufacturing.

One important characteristic is sensitivity, which determines the magnetic force required to drive the operation of the reed tube. Sensitivity is expressed using the AT value, which is the product of the number of turns in the coil and the current in the coil. The conventional suction value range for reed tubes is 10 to 60AT. The smaller the AT value is, the more sensitive the reed is. So the smaller the reed, the more sensitive it is to the magnetic field.

According to the characteristics of dry reed tube, compact structure, small weight, can be installed in extremely limited space, suitable for small equipment.

Because of the different internal structures, the magnetic spring switch is also divided into normally open type and normally closed type.

Normally open contacts

Contacts that are in the open state under normal (no power) conditions are normally open contacts.

Normally closed contacts

In the normal (no electricity, no current flow) the closed state of the contact is normally closed contact.

Normally closed -- NC (normal close) is normally closed, that is, the coil is closed without electricity.

Normally closed dry reed tube: normally open sheet and normally closed sheet are fixed, and the switchable reed can switch between connecting normally open sheet and connecting normally closed sheet.

magnetic sensor diagram

When there is no magnetic field; The switchable reed contacts the normally closed reed due to its own elasticity, and the circuit is closed at this time.

When the external magnetic field approaches; Under the influence of the magnetic field, the switchable reed will approach and contact the normally open reed, at which time the circuit is disconnected.



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