Reed switch, also called reed switch or reed switch, is a magnetically sensitive special switch, invented by Walter Elwood of Bell Telephone Laboratories in 1936. The basic type is to seal two magnetic reeds in a glass tube. Although the two are overlapped, there is a small gap between them. When the external magnetic field will make the two magnetic reeds contact, and then conduct. Once the magnet is pulled away from the switch, the reed switch will return to its original position.
The reed switch is the main component of the reed relay and proximity switch. It usually has two metal reed contacts made of soft magnetic material that are disconnected when there is no magnetism, and some have a third reed as a normally closed contact. These reed contacts are encapsulated in a glass tube filled with inert gas or vacuum. The ends of the reeds encapsulated in parallel in the glass tube overlap with a certain gap or contact each other to form the normally open or normally closed contacts of the switch.
Compared with general mechanical switches, reed switches are simpler in structure, smaller in size, higher in speed, and longer in working life. Compared with electronic switches, it has the characteristics of strong resistance to load impact and high working reliability.
The structure and working principle of reed pipe
The working principle of the reed switch is very simple. The magnetizable reeds (usually composed of iron and nickel) overlapping at the two ends are sealed in a glass tube. The two reeds are overlapped and There is a small gap (only about a few microns) between the two reeds. The contacts on the two reeds are plated with a very hard metal, usually rhodium and ruthenium. This hard metal greatly improves the switching times 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 inside into a vacuum state.
The role of a dry reed is equivalent to a magnetic flux conductor. When it has not been operated, the two reeds are not in contact; when the magnetic field generated by the permanent magnet or electromagnetic coil, the external magnetic field causes the two reeds to produce different polarities near the end points, when the magnetic force exceeds the elastic force of the reed itself When the time, the two reeds will attract and conduct the circuit; when the magnetic field is weakened or disappeared, the dry reed will be released due to its own elasticity, and the contact surfaces will separate to open the circuit.
Basic structure and components of Form A (normally open) reed switch
The above is Form A (Normally Open (NO) or Single Pole Single Throw (SPST) Reed Switch. Form B represents a normally closed switch, Form C (Single Direction Double Throw (SPDT)) represents a switch with a common reed switch. Film, a normally open film and a normally closed film (see picture below).
Form C (Single Pole Double Throw (SPDT) The basic structure of a reed switch with three reeds.
The switchable reed is in contact with the normally closed plate when there is no magnetic field. When a magnetic field of sufficient strength is generated, the reed will move to the normally open plate, and the normally open plate and the normally closed plate are fixed. The two fixed pieces and the swingable reed are both ferromagnetic pieces, but the contact surface part of the normally closed dry reed is welded to the dry reed by non-magnetic metal.
When placed under a magnetic field, the two sides of the normally open and normally closed fixed plates have the same polarity and are opposite to the swingable reed. The non-magnetic metal at the normally closed end will isolate the magnetic flux, so when the normally open end and the swingable spring When the magnetic force between the plates is large enough, the swing reed will contact and close with the normally open plate.
Normally, there are two ways to make the reed switch of the reed switch: 1. Use a permanent magnet; 2. Use an external coil.
The reed switch tube is closed under a magnetic field
The basic operation of the reed switch under the magnetic field generated by the permanent magnet, the two reeds have opposite polarities, and sufficient suction force is generated between the two reeds to contact each other.
The reed switch is closed under a magnetic field
Place the reed switch on the center axis of the coil. The magnetic field is the strongest in this part. The two reeds have opposite polarities, and sufficient suction is generated between the two reeds to contact each other.
When a permanent magnet approaches the reed switch, the two reeds will be magnetized into different polarities that can attract each other. When the magnetic field is large enough, the two reeds can generate enough attractive force to contact each other. The dry reeds are all tempered to eliminate the remanence, so when the magnetic field recedes, the magnetic field on the dry reeds disappears immediately.
Classification of reed pipes
Reed pipes are classified according to different viewpoints as follows:
1. Press the reed switch to classify the atmosphere:
Enclosed type
Open type
2. Classified by contact structure
Type A: It is a reed switch that is often opened and released. The contact is closed only when an external magnetic field is applied, and in the normal state, the contact is kept open and separated.
Type B: It is a frequently closed reed switch. The contacts are only separated when an external magnetic field is applied, and in normal times, they remain closed.
Type C: Transfer switch type.
3. Classified by function
Non-self-holding type: It has the feature that it can act according to the presence or absence of an external magnetic field (mono-stable type).
Self-holding type: Once it works, even if the external magnetic field is removed, it can still maintain the original state (bistable type).
4. Sort by shape
Large: The length of the glass tube is 30-60mm, and the diameter of the glass tube is 3.5-6mm.
Medium-sized: The length of the glass tube is 20-30mm, and the diameter of the glass tube is 2.5-3.5mm.
Small size: The length of the glass tube is 5-20mm, and the diameter of the glass tube is 1.5-2.5mm.
Features of Reed Pipe
advantage
1. The advantages of reed switches are their small size and light weight, which makes them easy to install and inconspicuous. Due to the small size of the operating switch, there is no need for complex cams or cranks, so there will be no metal fatigue, and an almost unlimited service life is guaranteed. And can be installed in a limited space, very suitable for micro equipment. Reed switches and suitable magnets are cheap and easy to obtain. The most important thing about the reed switch is a non-contact sealed switch.
2. The switching components of the reed switch are sealed in an inert gas and not in contact with the external environment, which greatly reduces the oxidation and carbonization of the contacts caused by the sparks during the opening and closing of the contacts. And to prevent the corrosion of the contact point of impurities such as external steam and dust. Long working life.
3. The reed is thin and short, and has a higher natural frequency, which improves the on-off speed of the contact. Its switching speed is 5-10 times faster than that of a general electromagnetic relay.
shortcoming
1. First of all, the contacts and reeds are quite small and delicate, so they are difficult 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 switch capacity is small, the contact is prone to jitter and the contact resistance is large.
2. Reed switches have voltage and current ratings. Although power W = voltage I * current U, the same power may be obtained from different voltage and current combinations. Remember not to exceed the rated current. For example, 10V*1A = 10W, and 1V*10A=10W at the same time, in the second case, the current will be too large. If you want to use a large current, a relay circuit composed of a relay coil and a reed switch is a more suitable choice.
3. There are many troubleshooting procedures. The faulty reed switch needs to be detected with special instruments (such as AT value tester, insulation withstand voltage tester, internal resistance tester, etc.).
4. Not suitable for product design with small error range: The AT value range is large, and from the cost point of view, it cannot be guaranteed that the AT values ​​of batch products are the same, and the matching magnets are not the same.
5. Since the reed switch is quite fragile, if the lead wire is welded to a thicker device, it is easy to damage the glass and sealing parts. If you need to bend the lead wire, you need to select the bending point of the lead wire appropriately.
Comparison of Reed Switch and Hall Effect Sensor
Reed switches and Hall-effect sensors are both magnetic sensitive components, and with the development of technology, the application of Hall sensors has become more and more extensive. So in the application, whether to choose a reed switch or a Hall-effect sensor, we will compare them here.
1. Hall-effect sensors are generally low in price. With the continuous improvement of sensor technology, current Hall-effect sensors no longer need expensive power supply circuits, such as Honeywell’s SL353, which integrates power circuits and operating voltages. The range can be from 2.2VDC to 5.5VDC without a significant increase in cost. However, the reed switch does not require external power supply, and can realize the state conversion of attracting and releasing under the action of a magnetic field.
The Hall sensor needs external power supply. With the rapid development of electronic technology, the power consumption of the Hall sensor is getting smaller and smaller. For example, Honeywell's SL353LT has an average operating current of only 1.8uA at room temperature, which is a battery-powered application. Ideal choice.
2. The reed switch sensor has no wear components, but it is still electromechanical. Its working life depends on the elasticity of the shrapnel. It can work up to a million times under low load (10mA and lower than 5V or lower). The Hall-effect sensor is an all-electronic solid-state structure, and there is no limit to the number of times it works.
3. The contact of the reed switch sensor has a very low on-resistance when it is turned on, the typical value is as low as 50mΩ, the signal range can be directly switched from several nanovolts to kilovolts, and the current is from microamp to ampere. However, it is worth noting that the higher the working current of the reed switch, the more heat it generates, and the higher the temperature, the elasticity of its shrapnel will decrease, and the number of operations and releases will drop rapidly.
Therefore, although the reed switch can work millions of times, its premise is low load, and its work under high load conditions such as high current or high voltage is at the expense of the number of operations, that is, its life. The Hall-effect sensor uses semiconductor materials. If you need to drive a high-current system, you can also use an external power amplifier. However, for most current battery-powered and low-energy applications, the Hall-effect sensor is also an ideal choice.
4. The reed switch sensor is sealed, so it can work in almost any environment. It has no effect on the temperature environment, and the typical working temperature ranges from -50°C to +150°C, without special additional conditions, restrictions or costs. The working temperature range of current Hall-effect sensors has also been greatly improved. For example, Honeywell’s SL353 has a normal working range of -40°C to +80°C, but the maximum rated range can be from -40°C to +150°C.
5. The electrical (metal) part of the reed switch sensor is sealed in a glass shell, which is fragile. Its ability to resist impact and vibration is poor, it is easily damaged during transportation and processing, and processing loss is large, which affects the product and life.
When installing on a printed circuit board (PCB), attention must be paid to the deformation of the PCB and the different thermal expansion characteristics of the metal and glass of the reed switch. The stress caused by these factors may damage the glass-metal seal of the reed switch.
Therefore, when installing the reed switch on the PCB, it is recommended to form a lead angle and leave an appropriate space between the PCB and the reed switch. At the same time, when welding, especially manual welding, it is necessary to pay attention to the control of welding temperature and welding time.
6. Hall sensors are getting smaller and smaller, such as Honeywell's SL353, which adopts S0T-23 package, connects the pins together, and the size is in the range of 2.8*2.9mm, and no other external components are needed. SMT technology can be used in the processing of the Hall sensor, the processing cost is low, and the processing process is not easy to be damaged.
7. The reed switch sensor reed is light and short. It adopts a mechanical structure and has a natural frequency. The reed switch cannot be used for occasions higher than its natural resonant frequency. When the reed switch is dropped from a height of more than 30cm to a hard surface (ground), its electrical characteristics (starting, releasing, etc.) may be changed. The Hall-effect sensor will not be affected and work more reliably.
8. The reed switch has excellent insulation from input to output and the insulation resistance of the switch is high. When it is disconnected, the leakage current is zero. However, due to its semiconductor structure, the leakage current of the Hall sensor is usually not zero, which is not suitable for application to Where zero leakage current is required.
Through the above comparison, we can see that Hall-effect sensors and reed switches have their own advantages and disadvantages, and they have no technical obstacles to interchange in many applications.
Application of Reed Pipe
Reed switches are used in mobile phones, program-controlled switches, photocopiers, washing machines, refrigerators, cameras, disinfection cupboards, door sensors, electromagnetic relays, electronic weighing apparatuses, level gauges, remote gas meters, water meters, IC card gas meters, water meters, electronic gas meters It is well used in water meters. As long as automatic switches are used in electronic circuits, reed switches can basically be used.
Automobile: pressure sensor, liquid level sensor, position sensor, current sensor, vibration detector, temperature detector.
Home appliances: counters for water and electricity meters, temperature sensors and position sensors for electric woks and rice cookers, air-conditioning humidifiers, etc.
Health equipment: bicycle speedometer, running machine, position switch of sports equipment.
Toys: The position switch of electronic toys controls music, flashing and other effects.
Reed switch relays are used in IC multimeters, control systems, and digital test equipment. Security system door and window magnetism, door lock.
Application of Reed Switch in the Automobile Industry
Application of Reed Switch in Reed Relay
Reed switches can be used in reed relays. One or more reed switches in a coil to obtain a reed relay with multiple pairs of poles.
The working current of the reed relay is relatively low, and provides high operating speed, good performance, which is unreliable to switch very small currents by traditional contacts, high reliability and long life. In the 1970s and 1980s, millions of reed relays were used for telephone communication.
In particular, they are used to switch the telephone communication of TXE households in the UK. The inert atmosphere around the reed contacts ensures that oxidation will not affect the contact resistance. Sometimes mercury wetted reed relays are used, especially in high-speed counting circuits.
Advantages of Reed Relay:
(1) Small size and light weight;
(2) The reed is light and short, and has a natural frequency, which can increase the on-off speed of the contact. The on-off time is only 1 to 3 ms, which is 5 to 10 times faster than a general electromagnetic relay;
(3) The contact is isolated from the atmosphere, and there are rare gases in the tube, which can reduce the oxidation and carbonization of the contact, and because of the sealing, it can prevent the corrosion of the contact by external organic vapor and dust impurities.
Application of Reed Switch in Oil Level Sensor
The reed switch is an excellent substitute for various vehicle fuel tank oil level sensors. The oil (liquid) level sensors are divided into two categories: one type uses a sliding potentiometer as the basic detection element, which is driven by a float. Use an ohmmeter to detect the resistance value, so as to achieve the purpose of displaying the oil level, but when the oil dirt covers the potentiometer, its resistance value will change, causing too much error, or even unusable, this type of fuel tank sensor becomes a short life and easy Damaged parts.
The other is to use inductance coils as the basic detection element. It uses a float to drive the inductance coil, change the oscillation frequency of the shock circuit, and then measure the oil (liquid) level by detecting its frequency with a frequency meter. But its structure is complicated, debugging is troublesome, cost is high, price is high, and it cannot be widely used. Therefore, the liquid level sensor produced with the characteristics of long life of the reed switch, safe and reliable action, and non-sparking, is an excellent substitute for the various fuel tank oil level sensors currently used in vehicles.
Application of Reed Switch in Magnetic Sensor
In addition to being used in reed relays and sensors, reed switches (reed switches) are widely used in circuit control, especially in the field of communication. A mechanical system commonly used in reed switch magnet drives, proximity sensors. Examples are door and window sensors in anti-theft alarm systems and anti-tampering methods.
The laptop uses a reed switch to put the laptop in sleep/hibernation mode when the lid is closed. The rotational speed sensors of bicycle wheels and automobile gears use reed switches, which simply drive the magnets on each wheel through the sensors.
Previously used for magnetic reed switches in computer terminals, where each key has a magnet, and keyboards with reed switches that are actuated by pressing the keys; cheaper switches are being used. For pipe organs used by electric and electronic pedal keyboards, electronic organ players often use reed switches to touch the glass shell to protect it from dirt, dust, and other particles.
They may also be used to control diving equipment such as flashlights or cameras, which must be sealed to maintain pressurized water.
Application of reed switch in high-risk environment
The reed switch can be a high vacuum type or an inert gas filled type. The reed switch seals the spark generated by the contact when the control circuit is turned on and off in the tube body, so that it is completely isolated from explosive gases and coal dust to prevent explosion and improve safety. The purpose of the coefficient.
The glass tube of the dry reed tube is equipped with two strong magnetic reeds, which are placed at one end of the tube so that they face each other with a certain gap. The glass tube is filled with inert gas, and the contact part is plated with rhodium or iridium to prevent the activation of the contact. Dry reed pipes use coils or permanent magnets to induce N pole and S pole for the reed, and then start to attract due to this magnetic attraction.
When the magnetic field is released, due to the elasticity of the reed, the contact immediately returns to its original shape and opens the circuit. Explosion-proof reed switch buttons and switches are suitable for mines containing explosive gas and coal dust, as well as dangerous places such as factories and ships containing explosive gas. They are used for the control of intrinsically safe control circuits and non-intrinsically safe control circuits.
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