Chinese name: reed switch mbth: reed switch patent number. : 2264746 Another name: reed switch application: definition, structure, principle, characteristics, advantages and disadvantages, structure, reed contact, glass seal, working characteristics, magnet, excitation form, selection, performance, application and reed relay of household appliances, automobiles, communication, industry and medical treatment. Dry reed sensor, defined as 1936 Walter B. elwood of Bell Telephone Laboratory invented the magnetic reed switch and applied for a patent in the United States on June 27th with the patent number of 2264746. The basic type of reed switch is to seal two pieces of reed in a glass tube. Although the two pieces overlap, there is a small gap between them. When an external magnetic field is applied, two magnetic reeds will contact and then conduct electricity. Once the magnet is pulled away, the reed switch will return to its original position. The structure of magnetic reed switch is composed of two magnetic reeds (usually composed of iron and nickel)? Sealed in a glass tube. ? Two magnetic reeds overlap, but there is a small gap between them. An appropriate external magnetic field will bring the two magnetic springs into contact. ? The contacts on the two reeds are plated with hard metals, usually rhodium and ruthenium, which greatly improves the life of switching time. Nitrogen or some equivalent inert gas is usually injected into the glass tube, and some magnetic reed switches will vacuum the interior to improve the switching voltage performance. The working principle of magnetic reed switch is very simple. Two magnetized reeds (usually made of iron and nickel) are sealed in a glass tube by overlapping at the ends, and there is a small gap (only a few microns) between the two reeds. The contacts on the two reeds are plated with hard metals, usually rhodium and ruthenium, which greatly improves the switching time and product life. The glass tube is filled with high-purity inert gas (such as nitrogen). In order to improve its high-voltage performance, some reed switches will make the interior vacuum. The reed acts as a magnetic flux conductor. When it is not operated, the two reeds are not in contact; When the magnetic field generated by the permanent magnet or electromagnetic coil passes through, the applied magnetic field makes the two reeds have different polarities near the end position. When the magnetic force exceeds the elastic force of the reeds themselves, the two reeds will attract each other and make the circuit conductive. When the magnetic field weakens or disappears, the dry reed is released due to its own elasticity, and the contact surface will be separated, which will break the circuit. The basic structure and components of A-type (normally open) magnetic reed switch The basic structure and components of A-type (normally open) dry reed switch are as mentioned above: A-type (normally open (N.O.) or single pole single throw (SPST) dry reed switch. Type B stands for normally closed switch, and Type C (SPDT) stands for switch with reed, normally open blade and normally closed blade (see the figure below). Basic structure of C-type (SPDT) three reed dry reed switch Structure of C-type (SPDT) three reed magnetic reed switch
When there is no magnetic field, the switchable reed is in contact with the normally closed piece. When a magnetic field with sufficient strength is generated, the reed will move to the normally open piece, and both the normally open piece and the normally closed piece are fixed. The two fixed parts and the swingable switch reed are ferromagnetic parts, but the contact surface of the normally closed dry reed is welded to the dry reed by non-magnetic metal. When placed in a magnetic field, the normally open and normally closed fixed pieces on both sides have the same polarity, and the polarity is opposite to that of the swingable reed. The non-magnetic metal at the normally closed end will isolate the magnetic flux, so when the magnetic force between the normally open end and the swing reed is large enough, the swing reed will contact with the normally open piece and close. There are usually two ways to attract the dry reed of reed switch: using permanent magnets.
As shown in the following figure, under the magnetic field generated by the permanent magnet, the basic operation of the reed switch is that the two reeds have opposite polarities, and sufficient suction force is generated between them to make them contact each other.
The principle of reed switch 1 uses an external coil, as shown on the right: put the magnetic reed switch on the central axis of the coil, where the magnetic field is the strongest, and the polarities of the two dry reeds are opposite, so that enough suction is generated between the two reeds to make them contact each other. When the permanent magnet is close to the dry reed switch, the two reeds will be magnetized into different polarities and can attract each other. When the magnetic field is large enough, the two reeds will generate enough gravity to contact each other. Reed switch has been tempered to eliminate the remanence, so when the magnetic field recedes, the magnetic field on the reed switch disappears immediately. If there is residual magnetic force on the dry spring, the characteristics of the dry spring switch will change. In the manufacturing process, suitable manufacturing technology and annealing treatment are very important. Principle of reed switch 2 According to this characteristic of reed switch, reed switch can be made into a switching element with very small size and volume, with very fast switching speed and very good reliability. The orientation and direction of the permanent magnet determine the time and times of opening and closing the switch. Features and advantages The contact is sealed in the glass tube with inert gas, which is not affected by the external atmosphere. The action answers quickly. The action system and circuit are located on the same axis, which is suitable for high frequency transmission applications. Small size and light weight. The contact has excellent corrosion resistance and wear resistance, long service life and stable switching action. Combined with permanent magnets, contactless switches can be formed economically and simply. Disadvantages The contacts and reeds of the magnetic reed switch are quite small and delicate, which is not suitable for dealing with reed induction switches caused by large voltage or current. Reed switch provides the typical rated voltage and current of the switch. It is only necessary to switch the high current between the relay circuit that operates the relay coil and the reed switch. There are many troubleshooting programs. The faulty reed switch needs to be detected by special instruments (such as AT tester, insulation withstand voltage tester, internal resistance tester, etc.). ). Not suitable for product design with small error range: the AT value range is large, so it is impossible to ensure the consistency of the AT value of batch products from the cost point of view, and the supporting magnets are different. The processing loss of magnetic reed switch is large. The magnetic reed switch is encapsulated in glass, which is easy to be damaged during transportation and processing, affecting the product and life. Good electrical connection of reed contacts is achieved by plating a thick layer of non-magnetic precious metal on the contact part of two reeds. Silver with low resistivity is more suitable as electroplating material than gold with corrosion resistance. And wet reed switch uses mercury, and the contacts of wet reed switch must be installed in pairs. The reed contact of the magnetic reed switch is constructed. The two spring wires are made of nickel/iron (ferronickel) alloy (52% nickel). The affected magnetic spring wires passing through the magnetic field must be ferromagnetic. Three most popular material properties, easy-to-anneal ferromagnetism: iron, cobalt and nickel. The tips of the two reed contacts are plated or sputtered with rhodium, ruthenium or iridium. Reed switch reed contact structure contact material rhodium contact
Rhodium plated contacts are the most commonly used. This kind of contact has very stable working characteristics and long working life from low load to heavy load because rhodium has high melting point and high hardness. mercury contact
Mercury contact magnetic reed switch has the characteristic of no jump operation, so it does not need additional jump suppression circuit. They have the characteristics of high power switching ability, low and stable contact impedance characteristics and long working life. They can also be used for high inrush current switches. Ruthenium contact
Ruthenium is harder than rhodium. Rhodium plated contacts have good mechanical wear and heat loss characteristics, but they are only used for small load switches. Due to these characteristics of ruthenium, Hua En Electronics has successfully developed ruthenium oxide coated rhodium or platinum double plated contacts. These double-plated contacts have excellent switching characteristics from low load to high load. Glass sealing is used for the outer packaging of glass tubes, and its coefficient of thermal expansion (TCE) is completely matched with NiFe alloy. At both ends of the glass tube, it is heated and the glass melts, forming an airtight seal covering both ends. In the process of glass sealing, the glass cavity is usually filled with inert gas (usually nitrogen) or cavity, which may produce vacuum and vacuum. This vacuum usually supports high voltage switching (over 1000 volts). The glass sealing performance of reed switch (PI) is that the reed switch opens the contact of reed switch and opens the contact of reed switch (Do). Most companies measure ampere turns (AT), and some companies use millitesla (mT) as the unit of magnetic measurement. The most common way to excite reed switches by magnet excitation is to use magnets. Typical excitation forms are shown in the following figure: horizontal excitation reed switch horizontal excitation reed switch vertical excitation reed switch horizontal excitation rotary excitation.
Selection of rotating excitation of reed switch Generally speaking, the selection of reed switch (reed switch) magnet needs to consider different application factors, such as working temperature, demagnetization effect, magnetic field strength, environmental characteristics, movement and use. The highest energy product of NdFeB NdFeB is very high remanence coercivity, and the price is relatively low. Compared with samarium and cobalt, some brands can be used up to 200°c, and hydrogen bonding is not recommended. However, the high magnetic energy product of SmCo samarium cobalt wire is not suitable for high performance applications. The most expensive magnet can be used at temperatures as high as 300°C to facilitate cutting-it should not be used as a structure. Al-Ni-Co Al-Ni-Co is cheaper than rare earth magnets. The maximum operating temperature is 550°C, and the lowest temperature system. Compared with other types with low coercivity and high induction level, ferrite has poor brittle thermal stability and is the cheapest. All types can be used at temperatures up to 300°C and need grinding to meet strict tolerance requirements. Magnetic reed switch with high corrosion resistance is a unique technology, and its completely sealed characteristics make it applicable to almost all environments. Although the structure of magnetic reed switch is simple, its manufacturing process involves many processes. Over the years, the size of magnetic reed switches has grown from about 50 mm (2 inches) to as thin as 6 mm (0.24 inches). The appearance of these small sizes enables it to be applied to more fields, especially to meet the requirements of radio frequency and fast time domain. The performance of the reed switch is as follows: it can switch up to 10,000 volts and up to 5 amps, or the load is as low as 1 0 nanovolts without signal loss, or the load is as low as1milliampere without signal loss, or the load is as high as 6 MHz. However, there is only a small signal loss, the insulation between contacts is10Ω, and the contact resistance is usually 50 milliohms (mΩ), which can provide bistable function without consuming any energy or power under normal open circuit conditions. The working time range is 100μs to 300μs, which can be used in the extreme temperature range of-55 C to 200°C, and can work in various environments. Including air, water, vacuum, oil, fuel and dusty environment, it can bear the impact force as high as 200Gs, the vibration from 50Hz to 2000Hz, the magnetic induction intensity as high as 30g·s (gauss) and the service life is long. Under the working condition of 5 V 10 mA, the magnetic reed switch can be operated for one billion times in the fields of household appliances, automobiles, communications, industry, medical care, security and so on. The application of reed switch in reed switch relay and reed switch sensor is briefly described. Reed relay is to place a reed switch around the coil, so that the coil through which current passes is equivalent to the magnetic field generated by the permanent magnet. Place a coaxial rod around the switch to switch the high frequency signal to 20GHz. Since the reed switch has no wearing parts, the contacts can switch low-level signals. Reed switch relay is widely used in the whole testing field. Test system, matrix, RF, modem, alarm, very suitable for reed switch relay;
1, high cycle count
2. High voltage application
3. Low-current, low-voltage switch reed sensors use magnetic reed switches, and reed sensors can sense various actions using permanent magnets. Dry reed switches absorb zero current in the open state, which makes them an ideal choice for energy-saving equipment applications, even if air, plastic and metal are separated, magnets are also suitable. Magnets and reed switches are usually separated by physical enclosures or other obstacles. Dry spring sensor is used to detect movement, count, detect liquid level height, measure liquid level and implant equipment in harsh switching environment.