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What is reed switch

What is reed switch?

Reed switch, also known as clarinet, is an electric switch operated by the applied magnetic field.

In 1936, Walter B. Ellwood of Bell Telephone laboratory invented the magnetic reed switch, and applied for a patent in the United States on June 27, 1940, with the patent number of 2264746.
The basic type is to seal two pieces of magnetic reed in the glass tube. Although the two pieces overlap, there is a small gap between them. When the external magnetic field is applied, the two magnetic reeds will contact and then conduct. Once the magnet is pulled away from the switch, the reed switch will return to its original position.
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Reed Switch

Structure of magnetic reed switch

The reed switch is sealed in a glass tube by two magnetic reeds (usually composed of iron and nickel). The two magnetic reeds are overlapped, but there is a small gap between them. The external appropriate magnetic field will make the two magnetic reeds contact.
The contacts on the two reeds are coated with hard metal, usually rhodium and ruthenium, which greatly improves the life of switching times. The glass tube is usually filled with nitrogen or some equivalent inert gas. In order to improve the switching voltage performance of some reed switches, the inside of the switch is made into a vacuum state.

Principle of magnetic reed switch

The working principle of magnetic reed switch is very simple. Two overlapped magnetizable reeds (usually composed of iron and nickel) at the ends are sealed in a glass tube. The two reeds are overlapped and separated by a small gap (only about a few microns). The contacts on the two reeds are coated with hard metal, usually rhodium and ruthenium, which greatly improves the switching efficiency Number and product life. The glass tube is filled with high-purity inert gas (such as nitrogen). In order to improve its high-pressure performance, some dry reed switches will make the interior into a vacuum state.
The reed acts as a magnetic flux conductor. Before operation, the two reeds are not in contact; when the magnetic field is generated by the permanent magnet or electromagnetic coil, the applied magnetic field makes the two reeds have different polarity near the end point. When the magnetic force exceeds the spring force of the reeds, the two reeds will pull in and conduct the circuit; when the magnetic field weakens or disappears, the two reeds will turn on the circuit, The reed is released due to its own elasticity, and the contact surface will separate, thus opening the circuit.
When there is no magnetic field, the switchable spring is in contact with the normally closed plate. When the magnetic field of sufficient strength is generated, the spring will move to the normally open plate, and both the normally open plate and the normally closed plate are fixed. The two fixed pieces and the swing switchable reed are both ferromagnetic pieces, but the surface part of the normally closed reed contact is welded on the reed by non-magnetic metal. When placed in a magnetic field, the fixed plates on both sides of the normally open and normally closed have the same polarity, and the polarity is opposite to that of the swing spring. The non-magnetic metal at the normally closed end will isolate the magnetic flux. Therefore, when the magnetic force between the normally open end and the swing spring is large enough, the swing spring will contact and close with the normally open spring.
Generally, there are two ways to make the reed of the reed switch close:

Using permanent magnets

As shown in the figure below, the basic operation of the reed switch under the magnetic field generated by the permanent magnet is that the two reeds have opposite polarity and generate enough suction between the two reeds to contact each other.

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Using external coils

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Principle of clarinet 2
As shown in the figure above: place the magnetic reed switch at the central axis of the coil. The magnetic field is the strongest in this part. The two reeds are of opposite polarity, generating enough suction between the two reeds to contact each other. When there is a permanent magnet close to 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 attraction to contact each other. The reed is tempered to eliminate remanence, so when the magnetic field recedes, the magnetic field on the reed disappears. If there is any residual magnetic force on the reed, the characteristics of the reed switch will be changed. In the manufacturing process, proper manufacturing process and annealing treatment are very important.
According to the characteristics of the reed switch, the reed switch can be made into a very small size and volume of switching components, and the switching speed is very fast, and has excellent reliability. The orientation and direction of the permanent magnet determines when and how many times the switch is turned on and off.

Characteristics of magnetic reed switch

Advantages of reed switch

The contact is sealed in the glass tube together with inert gas and is not affected by the external atmosphere.
The action answers quickly.
The operating system and circuit are located on the same axis, suitable for high frequency transmission applications.
Small size, light weight.
The contact has excellent corrosion resistance and wear resistance, long service life and stable switching action.
Combined with permanent magnet, the contactless switch can be formed economically and simply.

Disadvantages of reed switch

The contact and reed of magnetic reed switch are rather small and delicate, which is not suitable for dealing with the reed switching caused by large voltage or current. Reed switches provide typical voltage and current ratings for switches. It is only necessary to switch the high current between the operating relay coil and the relay circuit of the reed switch.
There are many troubleshooting procedures. The fault reed needs to be detected by special instruments (such as at value tester, insulation voltage tester, internal resistance tester, etc.).
It is not suitable for the product design with small error range: the at value range is large, from the perspective of cost, it can not guarantee that the at values of batch products are the same, and the matching magnets are not the same.
The magnetic spring switch has large machining loss. The magnetic spring switch is packaged in glass, which is easy to be damaged in the process of transportation and processing, affecting the product and life.

Construction of magnetic reed switch

Reed contact

Good electrical connection is achieved by plating a thick layer of non-magnetic precious metal on the contact part of the two reeds. Silver with low resistivity is more suitable for coating material than gold with corrosion resistance. There are also mercury wet reed tubes, and the contacts of the wet reed tubes must be installed in pairs.

Reed contact structure of magnetic reed switch

Two spring wires are made of nickel / iron (ferronickel) alloy (52% nickel).
The affected magnetic spring lead through the magnetic field must be ferromagnetic.
Three of the most popular material properties are easy to anneal ferromagnetism: iron, cobalt and nickel.
The tips of the two reed contacts are plated or sputtered with rhodium, ruthenium or iridium.

Contact material

Rhodium contact

Rhodium plated contacts are 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

The mercury contact magnetic reed switch has the characteristics of no jump operation, so it does not need additional circuit to suppress the jump. They have high switching power, low and stable contact impedance and long working life. They can also be used for high surge current switches.

Ruthenium contact

The hardness of ruthenium is higher than that of rhodium. Rhodium plated contact has good mechanical wear and heat loss characteristics, but it is only used for small load switch. Because of these characteristics of ruthenium, Huanen electronics has successfully developed ruthenium oxide coated double plated contacts on rhodium or platinum. These double plated contacts have excellent switching characteristics from low load to heavy load.

Glass sealing

The temperature expansion coefficient (TCE) of NiFe alloy used for outer package of glass tube is completely matched.
Both ends of the glass tube are heated and the glass melts to form an air tight 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 a vacuum. The vacuum normally supports high voltage switches (over 1000 Volts).

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Airtightness of reed glass

Working characteristics

Pull in (PI) is to open the contact point of the reed switch.
Disconnect (do) the contact point of the reed switch.
Most companies measure ampere turns (at) in reed switches, while others use millitesla (MT) as the unit of magnetic measurement.


Incentive form

The most common way to excite reed switches is to use a magnet. Typical incentive forms are shown in the following figure:

Level incentive

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Horizontal excitation of reed switch

Vertical excitation

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Vertical excitation of reed switch

Lateral incentive

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Transverse excitation of reed switch

Rotational excitation

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Rotating excitation of magnetic reed switch

Selection of magnets

Generally speaking, the selection of magnet for reed switch (Reed) needs to consider different application factors, such as working temperature, demagnetization effect, magnetic field intensity, environmental characteristics, movement and purpose, etc.

NdFeB magnet

  • The highest energy products;
  • Very high remanence coercivity;
  • The price is relatively low;
  • The mechanical strength is higher than samarium cobalt;
  • Some grades can be used up to 200 ° C;
  • Hydrogen is not recommended;
  • The bonding method can be machined, but not thread.

SmCo magnet

  • High magnetic energy product;
  • Suitable for high performance applications;
  • High resistance to demagnetization;
  • Excellent thermal stability;
  • High corrosion resistance;
  • The most expensive magnet;
  • It can be used up to 300 ° C;
  • Free cutting – should not be used as a structure.

Alnico magnet

  • It is cheaper than rare earth magnet;
  • The maximum working temperature is 550 ℃;
  • Minimum temperature coefficient;
  • Low coercivity compared with other types;
  • High induction level.

Ferrite magnet

  • Crispy;
  • The thermal stability is poor;
  • The cheapest of all types;
  • It can be used up to 300 ° C;
  • Grinding is required to meet strict tolerances;
  • High corrosion resistance.

What kind of material and shape is the best magnet for the clarinet?
In order to activate the reed (reed switch), magnet (magnetic field) is necessary. Different magnet materials have the characteristics of positive or negative bias, which mainly depends on the size, shape and environmental factors of the magnet.
What are the commonly used magnet shapes for Clarinet?
Rectangular, circular and cylindrical magnets are mostly used for reed pipes, and square magnets are mostly used for small braids.
How far does the reed fit with the magnet?
Some require that the linear distance between the magnet block and the clarinet is less than or equal to 20 mm. Some distances are about 5mm, and the specific induction distance can only be known by matching with the magnet.
What kind of magnet is used for reed?
The performance of different magnets is different. Ferrite has good temperature resistance and low cost. NdFeB has strong magnetic force and moderate price (higher than ferrite). Samarium cobalt is rarely used as reed. You can do it if you want.

Performance of magnetic reed switch

Reed switch is a unique technology, fully sealed features make it can be used in almost all environments. Although the structure of the magnetic spring switch is simple, its manufacturing process contains a number of processes. Over the years, the size of reed switch has developed from about 50 mm (2 inches) to as small as 6 mm (0.24 inches). The emergence of these small sizes enables it to be applied to more fields, especially to meet the needs of RF and fast time domain.
The performance of the reed switch is as follows:

  • Capable of switching up to 10000 volts;
  • Able to switch up to 5 amps;
  • It can switch or load as low as 10 nanovolts without signal loss;
  • It can switch or load as low as 1 femtoamps without signal loss;
  • It can switch or load up to 6 gigahertz with little signal loss;
  • The insulation between contacts is 10 Ω;
  • The contact resistance is usually 50 milliohms (m Ω);
  • In the case of normally open, it can provide bistable function without consuming any energy or power;
  • The operation time ranges from 100 μ s to 300 μ s;
  • It can be used in the extreme temperature range from – 55 ° C to 200 ° C;
  • Able to operate in a variety of environments. Including air, water, vacuum, oil, fuel and dust environment;
  • It can withstand up to 200gs impact force;
  • It can withstand the vibration from 50 Hz to 2000 Hz under the magnetic induction intensity up to 30 GS (Gauss);
  • It has long service life and can be operated for up to one billion times under the working condition of 5 V and 10 mA.

Application of magnetic reed switch

Magnetic reed switch has been widely used in home appliances, automobile, communication, industry, medical treatment, security and other fields. Briefly explain the application of magnetic spring switch in magnetic spring relay and dry spring sensor.

Reed relay

Place a reed switch around the coil so that the current through the coil is equivalent to the magnetic field generated by a permanent magnet.
A coaxial shield is placed around the switch to switch the high frequency signal to 20GHz.
Because the dry spring switch has no wearing parts, the contact can switch the low level signal.
Dry spring relay is widely used in the whole field of measurement.
Test system, matrix, RF, modem, alarm, very suitable for reed relay:

  • 1. High cycle count
  • 2. High voltage applications
  • 3. Low current, low voltage switch

Reed sensors

Using the magnetic spring switch, the dry spring sensor can sense all kinds of movements using permanent magnets.
Dry reed switches draw zero current when they are on, which makes them an ideal choice for energy-saving applications.
Even when air, plastic and metal are separated, magnets can be applied
Magnets and reed switches are generally separated by physical enclosures or other obstacles.
Dry spring sensor is used to detect movement, count, detect liquid level height, liquid level measurement, switch in harsh environment, implant equipment, etc.

Source: China Permanent Magnet Motor Manufacturer –



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