What is the resistance of the reed switch. Reed switches: methods of control, examples of use. Characteristics of high voltage reed switches and high power reed switches

Their design has a number of serious shortcomings. Among them, first of all, it should be noted the unreliability of the contact system, as well as rubbing metal parts, the wear of which reduces the overall performance of the device. As a result, hermetic magnetically controlled contacts were created - a reed relay, the principle of operation of which made it possible to get rid of the disadvantages inherent in electromagnetic devices.

Characteristics of reed relays

A reed switch is a device consisting of two contacts made of a ferromagnetic alloy. They are placed in a special flask that allows you to control their work. In the case of approaching the contacts of a permanent magnet, a circuit occurs with the formation of a continuous circuit. For this reason, reed relays are known as limit switches.

All reed switches are marked according to their application. For example, the KEM designation refers to the switching of electrical mechanisms, the letter "A" means the ability to work in any climate, the letter "B" implies the operation of the device only indoors. MCA is a magnetic switch for any climatic conditions.

The resistance of a standard switching track reed switch a is approximately 0.2 ohm. For a reed switch operating in opening, this indicator is at least 1 kOhm. These indicators allow you to significantly speed up the switching of circuits. Magnetic switches of this type are used for power voltage circuits and have improved performance. Opening magnetic reed switches are used in many circuits, mainly for computer or security systems, control sensors and many other devices.

The principle of operation of the reed relay

In the operation of a normally closed reed switch, the principle of the interaction of forces arising between magnetic bodies is used. In the electromagnetic field, impulses appear and are transmitted, electrons begin to move, causing movement and deformation of the conductive contacts.

A change in the position and state of the magnetic limit switch in a particular device or in a circuit leads to opening of the contacts. A further change in their position occurs under the action of other moving elements - buttons, end springs, disks, etc. Thus, the contacts are switched on and off alternately.

This principle of operation has become the basis for the operation of an intermediate reed switch acting on a short circuit. Its design consists of two cores and a sealed, durable glass container filled with gas or a gas mixture. The balloon itself is under the constant action of an electric current. Gases prevent oxidation of metal cores.

When connected to such a reed switch with direct current, a powerful one forms around the cores. The presence of special gaps greatly facilitates the passage of this field between the parts of the relay. Then comes the emergence of an autonomous magnetic flux moving in a given direction. The bonding of the cores is greatly accelerated by coating them with precious metals with lower resistance than conventional material.

A constant magnetic flux is provided by the design features of the reed relay. The homogeneity and integrity of the parts is created by casting and stamping, and welding processes are used to connect them together. Therefore, the relay coil is magnetized to a minimum degree. According to this scheme, a reed relay operates, the principle of operation of which is quite simple. If the DC supply is interrupted, the contacts open and the magnetic flux disappears.

Switching devices, or contacts, are used in radio engineering and electronic devices. In an electromagnetic relay, contacts are an unreliable design; there are rubbing metal parts. They wear out, the performance of the relay decreases. Reed switches are magnetically controlled hermetic contacts. Switches on reed switches were invented for high-quality operation, increasing service life. The first devices based on reed switches appeared in the last century in the 30s, and the reed switch was invented in 1922.

In modern times, hermetic contacts are not widely used, they are gradually being replaced by Hall sensors. But there are places where the reed switch has no competitors, it is easy to use, has a dry contact, galvanic isolation. Until now, magnetic contact is used in electronics. Reed switches are installed where the durability of switching, reliability of operation is needed. They are included in various sensors, relays, position switches.

Kinds

Like all contact groups, hermetic contacts are divided into types according to their functions:

• Closing.
• Switching.
• Opening.

According to the manufacturing technology and design, reed switches are divided into groups:

• Dry.
• Mercury.

Dry magnetic contacts work like normal contacts. In mercury samples, contacts with a drop of mercury are located inside the glass case. A drop of mercury is needed to wet contacts in operation, improve contact, reduce transition resistance, and eliminate contact bounce.

Chatter is the vibration of the contact group when triggered by a circuit or opening. With one operation, false switching of the transmission signal occurs, and the response time increases. If the bounce is in the audio amplifier when the signal is turned on, then the sound will be distorted, the operation of the amplifier will be disrupted. When using a reed switch in digital circuits, it is necessary to suppress bounce with RS flip-flop filters or RC chains. Reed contacts are used in microcontroller circuits, in which the bounce of reed switches is eliminated with the help of programs, which reduces the speed of the system.

Device

The design of the magnetically controlled contact is made of a glass container. The cylinder contains contacts made of magnetic cores, which are welded from the ends of the flask. The outer elements of the magnetic cores are connected to the power supply. This is visible on the diagram.

1. The flask is glass.
2. Switch contact.
3. Stationary contact.

The most common closing hermetic contacts. They have contacts made of rectangular wire, with ferromagnetic properties. Also, the cores can be made of permalloy wire. It depends on the size and power of the reed sensor. Contact plating is also made of rhodium, gold, etc.

An inert gas is pumped into the flask, or a vacuum is created. This prevents corrosion and rust from developing in the reed switch sensor. In the production of reed switches, it must be taken into account that there is a gap between the cores.

Reed switch operation

A simple relay with closing contacts has two cores with contacts having increased magnetic permeability. They are in a sealed glass container, with an inert gas, or a mixture of gases. The pressure in the cylinder is 50 kPa. The inert environment does not allow the contacts to oxidize.

The reed switch bulb is placed inside the control winding connected to direct current. When the power is turned on, a magnetic field is formed on the relay, passing through the contact cores, along the gap and closes along the control coil. The magnetic flux creates a pulling force that connects the contacts to each other.

To make the resistance of the contacts as small as possible, the touching surfaces are coated with silver, radium, palladium, etc. When the power is turned off, the force disappears in the coil of the reed switch electromagnet, the springs open the contacts. In reed relays there are no friction surfaces of parts, the contacts have many functions, they perform the work of a magnetic circuit, a conductor and a spring.

To reduce the dimensions of the magnet coil, increase the current density. Apply wire in enamel to wind the coil. Reed switch parts are stamped, connections are made by soldering or welding. Reed switches use magnetic shields to reduce the area of ​​the turn-on state.

The springs in the reed relays are installed without additional tightness, they turn on immediately, without wasting time to start. Instead of an electromagnet, permanent magnets can also be used. Such reed switches are called polarized. The pressing force of the reed relay contacts is determined by the magnetic force of the coil, unlike conventional electromagnetic relays, in which the force depends on the springs.

On opening, the reed switch works differently. The system of relay magnets, under the action of electromagnetic force, magnetizes the cores of the same name, which repel each other and open the circuit.

In a reed switch with switching, one of the 3 contacts is closed, made of non-magnetic metal. The remaining two contacts are made of a ferromagnetic composition. Under the influence of a magnetic field, open contacts close, and a closed non-magnetic one opens. Although there is always a magnetic field, like the Earth's field, but such a field is not enough to trigger the reed switch, so it is neglected.

The use of reed switches

Reed sensors and switches use:

• Medical devices and communication devices.
• Apparatus for submariners.
• Synthesizers and keyboards.
• Testing devices, meters.
• Devices of automatic equipment and safety.

In security systems, sensors on reed switches are used as relays. The security sensor includes a magnet and a reed switch. The simplest reed relay consists of a winding and a reed switch.

The advantages of relays on reed switches can be called:

• Small dimensions, simple device.
• Protection against moisture, burning of the contact group.
• There are no rubbing parts.

Such reed switches are widely used, but they also have disadvantages, such as susceptibility to mechanical damage. This is a big minus for application in many systems.

Reed switches are indispensable in alarm systems. Installing the sensor is not difficult. When the door is closed, the reed contact is closed. When the door is opened, the magnet attached to the jamb moves away from the reed switch, the magnetic force decreases, the power circuit opens. This serves as a signal to trigger the alert circuit.

The situation is similar with the use of reed switches in elevators. To determine the location of the elevator car, reed switches are used. With the help of magnets and a reed switch, it is easy to control lighting equipment. Reed switches are also present in electricity meters.

When using reed relays or sensors, you can give some tips that take into account the nuances of using such devices:

• When installing reed switches, if possible, avoid sources of ultrasound, it can adversely affect the electrical parameters of the sensor, change them.
• A nearby source of magnetic field can also change the characteristics and properties of the magnetic switch.
• Reed relays and sensors are afraid of shock and mechanical damage. The inert gas inside the sensor may be released upon impact due to a leak in the gas reservoir. This will disable the reed switch.
• When soldering, the reed sensor manufacturer's instructions must be followed.

Gersikony

The relay on reed switches has a wide variation in the return coefficient due to manufacturing technology errors. To increase the rated power and switching current, auxiliary contacts are built into the reed relays to extinguish the arc.

Such relays are called hersicons, or power sealed contacts. Industrial production produces gersikons for current strength up to 180 amperes. They have a switching frequency of up to 1200 starts per hour. Gersikonami run asynchronous with a rated power of up to 3000 watts.

Ferrite Reed Relays

This is a special class of relays on reed switches with ferrite cores. They have a memory function. To make switching in this type of reed switches, it is necessary to apply a current pulse of reverse polarity in order to demagnetize the ferrite core. They are called memory hermetic contacts, or gezakonami.

Advantages of reed relays

• The absolute tightness of the contacts makes it possible to use them in aggressive environments, under conditions of dust, humidity, etc.
• Small size, light weight, simple sensor design.
• The increased operating speed makes it possible to use reed switches at high switching frequencies.
• Reliability of operation in a wide range of temperatures (from -60 to +120 degrees).
• Wide range of applications combined with relay functionality.
• Availability of galvanic isolation of switching circuits and controllability of relays on reed switches.
• Increased strength of electrical contacts.
• Long sensor life.

Disadvantages of reed switches

• Low sensitivity of reed switch magnets.
• Excessive susceptibility of the sensor device to magnetic fields. This requires protective measures against the effects of magnetic forces.
• Reed switch cylinder made of fragile material, sensitive to damage and shock.
• The switching power is small, both for reed switches and reed switches.
• At high currents, the contacts of the reed switches spontaneously open.
• When operating at low frequency voltage, the contacts open and close without supervision.

Reed switches are one of the switching elements in electrical circuits that are successfully used under certain conditions. In some cases, reed relays are a more efficient alternative to electromagnetic relays.

Scope of reed switches

Contact groups on reed switches are actively used in electrical circuits of a security alarm. A group of contacts on reed switches in one housing can simultaneously switch in several electrical circuits that are not connected to each other. In the alarm system, this is used to turn on the sound, light indication of the operation, to transmit signals to the on-duty control panel.

At enterprises with explosive impurities, reed switches are effectively used for switching electrical equipment for various purposes, since when closing and opening contacts, there are no sparks that go beyond the sealed glass bulb of the case. To start powerful electric motors, reed switches are used that can connect circuits with a load of up to 45 kW.

In addition to low-voltage equipment, there are models of reed switches that are used to close circuits with voltages from 1000 V to 100 kV, in relay protection of high-voltage overhead lines for power transmission. On such elements, arc-extinguishing structures and damping devices are installed to damp vibration vibrations of contacts. Reed switches for switching provide an opportunity to develop new trends in instrument making, automatic control and protection devices in relay systems.

The principle of operation of reed switches

The work is based on the use of magnetic field forces arising between the ferromagnetic elements in the reed switch. These forces can deform and move ferite contact plates, while they close or open. The magnetic field for the magnetization of ferromagnetic contacts in the area of ​​the device is created in two ways:

• Coil wound on the body, which is supplied with direct current;

Tip # 1 The magnitude of the magnetic flux can be adjusted independently by winding the wire around the coil body until the contacts actuate

• external permanent magnet.

The simplest reed switch design

Types of reed relays

The great demand for the use of reed switches in various industries, taking into account the production conditions, generates a large number of product models. All reed relays can be divided according to the type of contacts:

• With open contacts in the initial state;
• With closed contacts in the initial state;
• With combined groups of contacts, when normally closed and open reed switches are located in one housing.

By type of design, reed relays are divided into two types:

• Dry - with filling the flask with an inert gas or with a vacuum inside, this is done to increase the resistance of contacts to high current loads;
• Wet - reed switches at the contact points have liquid metal, mercury during vibration plays the role of a shock absorber, preventing opening.

Main technical characteristics of reed switches

Due to the wide variety of designs of reed relays with different functional purposes, there are characteristics that are relevant only for a particular type. Consider the main ones that are inherent in all types of reed relays:

• Vibration level- when the specified level is exceeded, the glass flasks of the reed switches can crack, the contacts close or open. That value is measured by the number of oscillations per second;
• Maximum contact voltage in a switched electrical network, it is measured in volts and kV, depends on the cross section and material of the contacts, is written as Umax;
• Permissible power, at which the contacts do not lose their ferromagnetic properties and the ability to perform their functions. The power of the reed switch is determined by the material and cross section of the contacts, the larger the cross section, the greater the electrical power of the network is allowed, indicated in the technical documentation as Pmax, measured in W; kW;
• Number of switching cycles- the number of openings and closings before the wear of the contacts, at which they can no longer fulfill their functional purpose. In some technical sources, this is called a work resource, denoted as N max, where N is the number of operations, usually calculated from 4-5 billion;
• Release time- the time interval from the moment the coil is de-energized to the transition of the contacts to the initial state 0.2 - 1 μs;
• Reaction time - time from the moment the current is applied to the coil to the closing or opening of the contacts 0.5 - 2 μs;
• Contact capacity - Ck, can only be in the open state of contacts, depends on the gap between them and the geometric dimensions of the contact plates.

The last two parameters in the technical documentation can be formulated as the speed of closing and opening contacts in milliseconds, written as Tcp and Temp. These values ​​show the speed of the reed switch, small-sized models have a higher speed. The frequency of switching cycles can reach 1000 Hz.

• Breakdown voltage - the voltage value (tens of kV) at which an electric arc or spark breaks through the ferrite contacts in the open state. This voltage characterizes the electrical strength of the reed switch, which largely depends on the materials from which the contacts are made, coatings and the gap between them;
• Field strength - the value at which the switching of contacts occurs, sometimes this parameter is called the magnetic force Vav - operation. Operation is understood as the closure of contacts and Votp. Releases mean the opening of contacts.
• Contact junction resistance - has two meanings, very small values ​​​​are measured in the closed state Rk (contact). In the open state, Riz (insulation) is the insulation resistance within tens of MΩ.

Table:CHARACTERISTICS OF REED CONTACTS FOR CLOSURE OF CONTACTS

Parameters of switching and measuring reed switches

reed switches with high power

Features of reed contact control

There are two control methods, each of which has its own design features:

Control by means of a magnetic field from a permanent magnet.

The reed switch is installed motionless, the magnet moves in space relative to the reed switch, when approaching a distance when the magnetic field strength is sufficient to switch the contacts, it triggers. Similarly, when the magnet is removed from the reed switch, the field weakens, and the reed switch contacts return to their original state.

A classic example of such an option is the use of a reed switch in burglar alarm systems, when the reed switch is installed on the door frame, and the magnet on the door, or vice versa.

Example of mounting reed sensors on a door
A - contacts are in the open state;
B - contacts are closed, the alarm is triggered:

Tip #2 In this case, it is recommended to use cylindrical sensors in a plastic case. They discreetly fit into the drilled holes in the frame and door. For masking, you can glue elastic plugs of the corresponding color on top.

Depending on the operating conditions and functional purpose, design solutions may be different:

• The magnet can rotate around the axis, changing the polarity, thereby switching the contacts of the reed switch.
• A shielding magnetic shutter can move between the reed switch and the magnet to shunt the field;
• Any element can be movable, several, elements or all, a shutter, a magnet and a reed switch, all determine the conditions of a particular object.

Reed switch control by means of a coil through which direct current is passed

This method has been widely used in the design of reed relays with a small number of contact groups. One or more reed switches are placed in the hollow core of the body, on which the winding is wound.

An example of such use is current protection sensors in power networks supplying equipment. The coils are wound with wire thick enough to withstand the current loads used in the manufacturing process. If the current is exceeded, the magnetic field disconnects the contacts of the reed switch, the equipment is de-energized. The adjustment is carried out by moving along the threaded connection of the reed switch inside the coil along the axis.

Advantages of reed switches

• Unlike conventional relays with electromagnetic coils and a core, reed relays do not have mechanical elements, a lever drive for moving contacts and a steel core in the coil. This results in a smaller design.
• Many indicators of reed relays are hundreds of times higher than conventional relays, insulation resistance, breakdown voltage, and, accordingly, electrical strength.
• It is obvious that ordinary relays cannot be compared with reed switches in terms of speed. Switching frequency of contacts on reed switches 1000 Hz;
• The service life of reed switches is calculated in billions of switching cycles;

disadvantages

Despite all the perfections, there are also disadvantages:

• Not much power
• Not a large number of contacts in one flask;
• In dry versions, there may be mechanical rattling of the contacts;
• Fragile glass bottle body;
• In an unshielded case, there may be the influence of third-party magnetic fields.

A Brief History of Reed Switches

Switching devices or simply contacts are very widely used in various electrical and radio equipment. In order to improve the performance properties, especially the service life and reliability of the connection, and have been developed magnetically operated sealed contacts named reed switches.

The first samples of such contacts appeared in the 30s of the last century, and the first magnetically controlled contact was invented in 1922 in St. Petersburg by Professor V. Kovalenkov, for which he was issued a USSR author's certificate No. 466. The design of such a contact is shown in Figure 1.

Such contact is arranged as follows. Contacts 1 and 2, also made of soft magnetic material, are attached to the core 3 of a soft magnetic material through insulating spacers 5. When current is passed through coil 4, a magnetic field arises in core 3 and magnetizes contacts 1 and 2, which close. The opening of the contacts occurs when the current through the coil stops.

Figure 1. Magnetically controlled contact by Professor V. Kovalenkov

In fact, it was the very first magnetically controlled contact, only without a sealing shell. Such a contact was first placed in a sealing shell by the American engineer W.B. Ellwood only in 1936. In the seventies of the last century, reed switches reached their maximum development, and were widely used in various electronic devices.

Currently, reed switches are used less intensively, since they have been "forced out". But in some cases, reed switches remained out of competition, due to ease of use, galvanic isolation from the power source, and “dry contact” properties, so reed switches are still used in various circuits and devices.

In cases where high reliability and durability of the switching element is required reed switches are simply irreplaceable. As an integral part, reed switches are included in the design of various sensors, electromagnetic relays, especially low-current ones, as well as position switches and some other devices.

Varieties of reed switches

Just like ordinary contacts, reed switches can be normally closed (1 normally open contact), switching (1 changeover contact) and open (1 normally closed contact). This division is based on functional features.

According to their structural and technological features, reed switches are divided into two large groups: with dry contacts and with mercury contacts. The first variety is called dry reed switches, and the second is mercury reed switches. Actually, there is nothing special in the operation of dry reed switches, compared to conventional contacts.

In mercury reed switches inside a sealed glass case, in addition to contacts, there is also a drop of mercury. The purpose of this mercury drop is to wet the contacts during operation to improve the quality of the contact by reducing the contact resistance, and also to get rid of contact bounce.

Chatter is the vibration of contacts during closing and opening, which, with a single operation, leads to multiple switching of the transmitted signal, and in addition to a significant increase in the response time.

Imagine that such a bounce would be present during the switching of the input signal! In the case when such a rattling contact works in conjunction with digital microcircuits, it is necessary to take measures to suppress the chatter in the form of RC chains or.

Various contacts, including reed switches, are also used in, but in them, contact bounce is suppressed by software. It also slows down the performance of the system as a whole.

Reed switch design

The design of various types of reed switches is shown in Figure 2.

Figure 2. Reed switch design

All reed switches are sealed glass container, inside which is contact Group. The contacts are magnetic cores welded into the ends of the cylinder. The outer ends of the cores are intended for connection to an external electrical circuit.

The most widespread reed contact with closing contact group or, as shown in the figure "open". Each contact - the core is made of a ferromagnetic elastic wire, which is flattened to a rectangular shape. For the manufacture of cores, permalloy wire with a diameter of 0.5 - 1.3 mm is used, depending on the power of the reed switch and, accordingly, its dimensions.

Directly contacting surfaces are plated with noble metal, gold, palladium, rhodium, silver and alloys based on them. Such a coating not only reduces, but also improves the corrosion resistance of the contact surface.

The internal space of the cylinder is filled with an inert gas (hydrogen, argon, nitrogen or their mixture) or simply evacuated, which also helps to reduce contact corrosion and increase their reliability. During manufacture, the cores are arranged in such a way that there is a gap between them, by the way, of a certain size.

Rice. 3. Reed switch

The principle of operation of the reed switch

In order to trigger the contact group, it is necessary to create a magnetic field of sufficient strength around the reed switch. In this case, it does not matter at all how this field will be created, either simply by a permanent magnet, or by an electromagnet. The lines of force of the external magnetic field magnetize the internal contacts - the cores of the reed switch, as a result of which they overcome the elastic forces, attract and close the electrical circuit.

The contacts will be in this state as long as there is a magnetic field of sufficient strength around them: it is enough to turn off the electromagnet or remove the usual permanent magnet away, as the contacts immediately open. The next operation of the contacts will occur when the magnetic field reappears. From all of the above, we can conclude that the contacts perform three functions at once: elastic elements (springs), a magnetic circuit, and the actual conductive contacts.

A reed switch operates somewhat differently, working to open. Its magnetic system is designed in such a way that when exposed to a magnetic field, the contacts - the cores are magnetized in the same way, therefore they repel each other, breaking the electrical circuit.

In a switching reed switch, one of the three contacts, as a rule, normally closed is made of non-magnetic metal, and both normally open contacts are made of ferromagnetic, as mentioned above. Therefore, when a magnetic field is applied to the reed switch, the normally open contacts simply close, and the non-magnetic normally closed one, remaining in its original place, opens.

Note. Normally open contact, which is open in the absence of a control action, in this case a magnetic field. Respectively normally closed contact closed in the absence of a magnetic field.

Of course, there is always a magnetic field, such as the Earth's magnetic field. And it seems impossible to say about the absence of a magnetic field at all. But the Earth's magnetic field is not enough to trigger the reed switch, so it can be neglected and we can say about the absence of a magnetic field, in this case, an external one.

Continue reading the next article.

Article continued:

Boris Aladyshkin

Reference information on domestic and foreign reed switches - designation and marking, technical characteristics, types and types. Magnetically controlled hermetic contacts (reed switches) are widely used in electronic equipment. They are used in the design of relays, logic elements, various sensors, toggle switches, limit switches and switches.

Due to the sealing of the contacts, the reliability of switching and the stability of the contact resistance have increased. The small size of the moving parts made it possible to increase the maximum switching frequency tenfold compared to electromagnetic relays. The response time of the reed switches does not exceed 2 ms, and the maximum number of responses reaches a million.

Symbol for reed switches

• the first element - defines the conventional name of the reed switch. MK - magnetically controlled sealed contact, KEM - electromagnetic contact, KMG - magnetically controlled contact with increased contact pressure (for switching high currents - more than 5 A);
• the second element indicates the reed switch switching system: A - closing, B - opening, C - changeover, D - transitional;
• the third element - the letter "P" is present only in mercury reed switches;
• the fourth element - a two-digit number shows the length of the balloon in millimeters;
• the fifth element - indicates the functional purpose of the reed switch: 1 - low and medium power, 2 - high power, 3 - powerful, 4 - high-voltage, 5 - high-frequency, 6 - "with memory", 7 - special (with increased resistance to external factors and the nature of the load), 8 - measuring.
• the sixth element indicates the serial number of the development.

According to the type of contacts, reed switches are distinguished as closing and switching, according to the state of the contact surface - dry and liquid. Inside the cylinder of dry reed switches are inert gases. The contacts are ferromagnetic springs coated with . Reed switches are also subdivided into low-power (switched power up to 60 W) and high power (up to 1000 W), low-frequency and high-frequency, low-voltage (switched voltage up to 250 V) and high-voltage (over 250 V), there are reed switches with "memory" and special ones. Next, we give reference parameters of domestic reed switches, and at the end of the article - imported reed switches.

Characteristics of switching and measuring reed switches

Parameters of closing reed switches of miniature type

Parameters of closing reed switches of standard and intermediate types

Characteristics of high voltage reed switches and high power reed switches

Technical characteristics of high-frequency reed switches

Main parameters of memory reed switches