Reed switch: technical characteristics, principle of operation, examples of application. Magnetic reed switches Mercury reed switches

A reed switch (short for hermetic contact) is a mechanical sensor that closes its contacts when exposed to a magnetic field. The body of the reed switch is a sealed glass flask, inside which two metal plates are located parallel to each other. From an electrical point of view, a standard reed switch is equivalent to a single non-latching closing button.

Advantages of reed switches (English "Reed Switches"):

Low power required for control (50…200 mW);

Low resistance between closed contacts (0.05…0.2 Ohm);

High resistance between open contacts (10'"... 10'^ Ohm);

Full galvanic isolation due to the glass case;

Sufficient for practice speed (0.5 ... 1.5 ms);

Long service life (10^.. 10^ switching);

Mechanical stability, wide temperature range (-60…+150’С), possibility of operation in dusty and aggressive environments.

Depending on the principle of operation, reed switches are divided into closing and switching. Depending on the switched voltage, low-voltage (less than 1 kV) and high-voltage reed switches are distinguished. Depending on the length of the case - standard and miniature (less than 10 mm).

The main areas of application of reed switches: relays, keyboards, industrial and security sensors. The leading manufacturers of reed switches in the world market are the following companies: OKI (Japan), Hamlin Electronics (USA), Fujitsu (Japan), RZMKP (Russia), etc.

On Fig. 3.25, a ... and diagrams for connecting reed switches to the MC are shown.

Rice. 3.25. Diagrams for connecting reed switches to MK (beginning):

a) /1/ - this is a narrowly directed magnetic field sensor, consisting of a reed switch KEM-1A (SF1), placed in a metal case from a KBG-M capacitor with a slot;

b) a current sensor based on a KEM-2 reed switch with a wire wound around its balloon (8 turns of PEV-2.0). MK fixes the closing / opening of contacts SF1 at a trip current of 2 A and a release current of 1.5 A. The position of the reed switch inside the coil Y is selected experimentally;

c) chain C/, R1 eliminates false positives of reed switch SF1 and “bounce” of its contacts. To speed up the charge of the capacitor C /, you can temporarily switch the MK line to the LOW level output mode, while the resistor /? 2 limits the current through the key transistor MK;

Rice. 3.25. Diagrams for connecting reed switches to MK(the ending):

d) bias coil L2 increases the sensitivity of the current sensor. Coils L/, L2 are wound over the reed switch SFI and contain, respectively, 10 turns of PEV-0.8 wire and 200 turns of PEV-0.06 wire;

e) serial connection of reed switches ^SF / ... 5/77 with the logical function "AND";

f) parallel connection of reed switches 5F / ... 5/77 with the logical function "OR";

g) resistor R1 protects the MC input from powerful electromagnetic pickups on the contacts of the reed switch SF1. Capacitor C/ serves as the first stage of suppression of the "bounce" of mechanical contacts. The final filtering of the signal is carried out programmatically;

h) noise-resistant interrogation of the reed switch SF1, which can be closed at a frequency of up to 100 Hz. The function of the low-pass filter is performed by the chains RI, C / and R2, C2 \

i) capacitor C/ forms a short pulse of a HIGH level at the input of the MC when the reed switch SF1 is closed. The real state of its contacts can be determined by the voltage across the resistor R2 through the ADC MK (0.45 V - closed, 0 V - open). The large resistance of the resistor R1 reduces the current consumption in the reed switch circuit compared to the circuit on Rice. 3.25, w.

Modern technologies every day more and more simplify all spheres of human life. The subject of today's publication is one of these most useful devices. A modern reed switch, as one of the most practical and convenient switching means (connecting electromechanical contacts in an electrical circuit), is a pragmatic and effective solution for both special applications in the professional field and ordinary household use. For more information about what it is, a reed switch, how it works and what is the principle of its operation, whether it is possible to make and connect it with your own hands at home - find out further.

What is it: reed switch

The official scientific name of the reed switch is the formation of two words - hermetic and contact. In its essence, the name fully justifies its main functionality and purpose, since the reed switch is a switching (connecting) device in electrics, which operates on the basis of the physical laws of the electromagnetic field and current transmission through conductors.

A feature of this type of contact is its hermetic operating conditions.

It is surrounded by a completely sealed glass (most often) sheath, which provides complete insulation and reduces the level of external influences on the conductors.


Advantages of using reed switches, their properties and features:

  1. High efficiency and pragmatism. The technical characteristics of hermetically sealed contacts are high compared (proportional) to conventional relays (more details below).
  2. Smaller in size, in fact it is a small oblong ball with conductors.
  3. Durable, as the insulation allows the contacts to be subjected to external influences to a lesser extent, while avoiding most internal phenomena (bounce, friction, etc.).
  4. Minimum error and high response speed.
  5. Convenience and comfort of use due to the absence of possible sound or visual phenomena.
  6. Continuity of action.
  7. Simple connection.
  8. Possibility of use at minimal cost, as it works without the need for additional electricity to start the action.

Some contacts are specially treated with additional substances such as mercury to increase temporal resistance, safety and effectiveness. Such devices are called liquid because they are not in inert gas conditions, but in some conductive liquid consistency, such as mercury.

Technical characteristics of the reed switch

Despite the fact that the use of reed switches has been noted in history since the very beginning of the 70s, they are still relevant in many electrical appliances and circuits to this day. This is due to a number of their useful properties and advantages compared to other types of switching facilities. They are practical, durable, easy to put into action, while not spending a lot of resources.

The main disadvantages and nuances of operation:

  1. Despite the really fast response and operation, they are still unable to work with significant power parameters. Even the common 220 volts is a problem for the operation of a conventional contact.
  2. In reed switches using an inert gas (dry reed switch) there are several operational problems, some of which are electrical chatter (flashing, shorting) and friction.
  3. Against the background of high possibilities associated with a sealed bulb, in which the contacts are located, it is fragile and easily damaged.
  4. For operation, the full concentration of one single source of the magnetic field is necessary, otherwise the contact may be conditionally stretched.

The peculiarity of the reed switches requires special attention, when the contacts literally stick to each other. Sometimes, due to frequent actuation, the contacts can be in continuous contact with each other, which creates a closed flow.

Classification: reed relay

One result of the practical use of reed switches is the reed switch. The relay itself is an electromechanical mechanism that opens and closes the circuit at a certain interval. They were widely used and are still in use today. So, for example, you can often find a reed switch, a security sensor for a room or a car, and earlier even a computer keyboard was made on their basis.

Classification of reed relays according to the principle of magnetic field exposure (types):

  1. Normally open (NC) contact. Such a device, when it enters the area of ​​influence of the magnetic field, is triggered and the contact closing the circuit is connected.
  2. Normally closed contact. By analogy with an open one, it reacts to magnetic waves and opens the circuit when triggered.
  3. An alternative is a changeover contact. It is peculiar, but practical in application, since, reacting to a magnetic source, a circuit consisting of several relays closes in one gap and opens in another. When removed from the field - vice versa.

It is noteworthy that all three types are quite relevant today. In addition to the basic qualities inherent in reed switches, they can also operate at high temperatures.

The so-called hersikons (sealed power contacts) act as a modernization of such relays.

They are equipped with additional contacts designed for the formation of an electric arc on them. This increases the transferable power and reduces the errors in the operation of the reed switch. There are also so-called memory relays. They are so named because after the contacts are isolated from the magnetic field, they continue to remain in their original position, as they were in the field. This is because the contacts themselves are made of a special material that constantly has a certain level of charge, i.e. magnetized.

Working principle: reed switch

Devices operating on the basis of reed contacts are used in a variety of fields. So, a reed switch is a special triggering device that is used in security systems with the designation SMK, in the lighting system of premises, triggering alerts, and many others.

An example of the operation of a reed switch and its features:

  1. After opening the door, the device pre-installed in the door frame approaches the active magnetic field.
  2. Under the action of the field, the contacts are connected and create a directed current flow.
  3. The switch is activated and the light in the room is on until the reed switch leaves the field.
  4. A variant of spontaneous opening of the circuit is possible, as a result of which it will go out. This is due to the strong flow that the reed switch cannot withstand.

Despite all the features, such sensors and circuit breakers are widespread in protected areas. Deciphering the abbreviation of such QMS systems is a magnetic contact or magnetic contact signaling device. Please note that all reed switches are not designed for many standard volts. For example, 220V will most often require an additional reed switch upgrade.

Components: reed switch for opening

A sealed contact that opens the circuit operates according to the circuit of disconnecting the contact when it is placed in a magnetic field. This can be useful when it is necessary to stop the current supply in a constantly hot circuit. Such a scheme can be used both in domestic and in special conditions.


The main elements of reed switches:

  • Core built into the shell;
  • Contacts made of conductive material;
  • An inert gas (such as nitrogen) or a drop of conductive liquid (mercury);
  • Sealed bolon;
  • Insulation lining.

There is also a certain marking classification, for which a reference book is provided. The main designations of reed switches: A (closes), B (opens), R (mercury), M (liquid) and others.

Any mechanical contacts are subject to wear. To reduce the influence of this destructive factor, in the first half of the last century, magnetically controlled switching devices were developed, the contact group of which was placed in a vacuum flask. In the USSR, such elements were called "Reed switch", short for "sealed contact", in the English-language technical documentation the name "reed switch" is adopted.

Let's look at the principle of operation of these devices, design, main characteristics, advantages and disadvantages. At the end of the article, a couple of useful schemes will be given where reed switches are used.

Appearance and design features

These devices are a contact group made on the basis of a ferrimagnetic material, which is placed in a glass flask. Air is pumped out of it (conditions are created as close as possible to vacuum), as an option, filling with an inert gas is possible. The appearance of the device and its designation on the circuit diagrams are presented below.

The design can be seen in Figure 2.


Designation:

  • A - device outputs.
  • B is a glass flask.
  • C - contact group.
  • D - inert gas or vacuum.

Varieties

Switching devices of this class are usually divided, depending on the device of the contact group, into the following types:

  1. Elements with normally open contacts (the appearance of such a device is shown in Fig. 1).
  2. Elements with normally closed contact.
  3. With changeover contact.

In addition to the functional features listed above, there are also technological ones that divide hermetic switching devices into two groups: dry and mercury. A distinctive feature of the latter is that a drop of mercury is contained inside the flask. It serves to "wet" the contact group, this can significantly reduce the contact resistance and vibration (bounce) of the contacts during switching, which has a positive effect on the quality of the contact.

Operating principle

The operation of the device (closing, opening or switching contacts) is required to act on the element with a magnetic field, the intensity of which will be sufficient for switching. The source of such a field can be a conventional or electromagnet.

Under the influence of lines of force, the contacts are magnetized and, upon overcoming the elasticity threshold, they commute the circuit.


Accordingly, as soon as the magnetic field ceases to act on the contact group, it will return to its original state. That is, functionally, the contacts, in addition to their direct purpose, play the role of a magnetic circuit and an elastic element.

Devices with normally closed contacts act somewhat differently. Their ferrimagnetic elastic elements, falling under the influence of a magnetic field, acquire the same charge, which causes them to repel, breaking the contact.


Sometimes in such switches only one elastic element is made of a ferrimagnetic alloy, as a result of the approach of a magnet, it is attracted to it, turning off the circuit.

A similar principle is used in reed switches with a switching group of contacts, in which two of them are made of magnetic material. Under the influence of a magnet, they are attracted to each other, and the non-magnetic contact remains in its original position. As a result, the circuit is switched.

Main settings

The properties of sealed switches are determined by mechanical and electrical parameters. The first ones include:

  • N max - a number indicating the maximum allowable number of operations without changing the main characteristics.
  • V cp - value reflecting the intensity of the field required for the response of the device. In technical terminology, this characteristic is called the magnetomotive force.
  • V otp - the value corresponding to the opening force.
  • t cp - the time required for the operation of the contact group.
  • t otp - the time interval required for release.
  • The last two parameters are the most significant of the mechanical characteristics, since they describe the switching speed.
  • Now we list the main electrical characteristics:
  • R K - resistance between contacts in the closed state.
  • R FROM - resistance of open contacts.
  • U PR - breakdown voltage, this characteristic depends both on the previous parameter and the distance between the group of contacts. In addition, the dielectric strength is affected by the filling of the flask.
  • P max - switched power.
  • C K is the capacitance formed by open contacts.

How is management carried out?

The sealed switch can be controlled in two ways:

  • using a permanent magnet;
  • acting on a coil connected to a constant current source.

In the first variant, the control can be carried out by linear or angular movement of the permanent magnet. There is also a method in which the field is blocked using a special curtain.

As an example of the use of the magnetic control method, one can cite level sensors, as well as position sensors, burglar alarms, etc.

The second option allows you to create a relay based on a reed switch. Unlike the traditional design, such a device will be more reliable and durable, since it practically does not contain moving mechanical elements. As for the small number of contact groups, this disadvantage is easily eliminated by increasing the number of reed switches involved.


An example of the application of this control method is a current relay based on a reed switch. It is a coil wound with a thick wire, inside which a sealed switch is placed. This device can serve as a protection system against overload in DC circuits. The sensitivity of the device can be easily adjusted by linear movement of the switch inside the coil.

Advantages and disadvantages

Any design, in addition to advantages, is not without disadvantages. Knowing the strengths and weaknesses of the device, you can find the optimal scope for its application. Let's look at what the advantages of sealed switches are, these properties include:

  • High switching reliability. It is almost two orders of magnitude higher than that of open contact groups. This is achieved due to the high resistance between open contacts (R FROM), it can be calculated in tens of MΩ. An important role is played by the indicator of electrical strength (U PR), the breakdown voltage for some models exceeds 10 kV.
  • Speed ​​is also an undeniable advantage. The switching frequency of many models approaches 1 kHz. As for the parameters describing the switching speed, they are in the following ranges: t cp - from 0.4 to 1.8 ms, t otp - from 0.25 to 0.9 ms, which is much higher than similar characteristics of open contact groups.
  • Durability, the number of operations is in the billions, not a single open contact group can even come close to this milestone.
  • This type of switches is undemanding to match with the load.
  • The control can be carried out without the use of electricity.

Characteristic disadvantages:

  • Low switching power.
  • Small number of contacts.
  • Chatter when triggered (wet-type designs are free from this drawback).
  • Large dimensions for a modern radio engineering base.
  • Insufficient strength of the glass flask.
  • Sensitivity to external magnetic fields.

Despite the clear predominance of positive qualities, these devices are gradually being replaced by semiconductor counterparts, such as Hall sensors. The absence of chatter, small dimensions and higher strength played a decisive role.

Examples of practical application in everyday life

As promised at the beginning of the article, here are a couple of useful schemes that use reed switches. Let's start with universal lighting control in the hallway. The principle of operation is as follows: when you open the front door, the light automatically turns on, and after a few minutes it turns off. With a sufficient level of lighting, the light in the hallway does not turn on.


Designations:

  • Resistors: R1 - 68 kOhm, R2 - 33 kOhm, R3 - 470 kOhm, R4 - 10 kOhm, R5 - 27 kOhm.
  • Capacitors: C1 - 0.1 uF, C2 - 100 uF x 25 V, C3 - 470 uF x 25 V.
  • Zener diode and diodes: VD1 - KS212Zh, VD2 and VD3 - KD522 (1N4148), VD4 - KD209 (1N4004).
  • Transistors: VT1 and VT2 - ÌRF840.
  • SG1 - any conventional reed switch, for example, 59145-030.
  • FR1 - photoresistor, suitable for any type with a resistance in the light of at least 8 kOhm, in the dark - 120-180 kOhm.
  • Trigger D1 - K561TM2 (CD4013).

Setting up the circuit is reduced to the selection of resistance R1, to select the optimal delay time for turning off the lighting.

Now let's look at a simple home alarm circuit, where a typical reed sensor for a door is also used.


Designations:

  • Resistors: R1, R2 and R3 - 100 kOhm, R4 - 33 kOhm, R5 - 100 kOhm, R6 - 1 kOhm.
  • Capacitors: C1 - 100 uF x 16 V, C2 - 50 uF x 16 V, C3 0.068 uF.
  • Diodes and LED: VD1 and VD2 - KD522 (1T4148), HL1 - AL307B.
  • Transistors: VT1 - KT829, VT2 - K361.
  • Chip: K561LA7.
  • S1 - reed sensor 59145-030.

The AC-10 sound annunciator is used as a siren.

The circuit is powered by a 12 V battery with a capacity of 4 Ah.

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.

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