Not a single modern system of security, control, fire extinguishing, emergency warning can function without the use of sensors that connect it with the outside world. Sensors detect the presence of smoke, dust in the air, the movement of objects and many other changes.
The reed sensor is still used in many such systems due to its reliability.
Content
What is a reed switch
A reed switch is an electromechanical device that closes or opens electrical contacts under the influence of a magnetic field generated by an electromagnet or a permanent magnet.
The term "reed switch" means hermetic contact. This is due to its design. It consists of two ferromagnetic plates sealed in a glass capsule with two output contacts and filled with an inert gas. Such a shell minimizes environmental impact and ensures reliable operation of the device.
The flask may contain nitrogen, dried air, or other inert gas. Also, all the gas can be pumped out of the flask to a vacuum state. This achieves an increase in the level of switched voltage.
Purpose and scope
Reed sensors, despite being replaced by Hall sensors, still find application in many devices and systems:
- Keyboards for synthesizers and industrial equipment. The design of the sensors eliminates the possibility of a spark. Therefore, they are primarily used in explosive production, where combustible fumes or dust are present.
- Household counters.
- Automatic security and position control systems.
- Equipment operating under water or in high humidity conditions.
- Telecommunication systems.
- Medical equipment.
In security systems, devices consisting of a reed switch and a magnet are used. They report the opening or closing of doors.
Reed relays are also used, consisting of a contact sensor and a wire winding. Such a system has some advantages: simplicity, compactness, moisture resistance, and the absence of moving parts.
Reed switches are also used in special areas - these are protection mechanisms against overloads and short circuits of high-voltage and radio electrical installations. Also, these are high-power radars, lasers, radio transmitters and other equipment operating under voltage up to 100 kV.
Varieties
Depending on the normal state of the contacts, the devices are divided into:
- closed - the circuit opens under the influence of a magnetic field;
- switchable - under the influence of the field, one contact is closed, and in the absence of the field, the other;
- open - the operation of the reed switch occurs when a magnetic field appears.
Depending on the design, the sensors are:
- gas - the glass sleeve is filled with dry air or inert gas;
- mercury - on contacts, mercury is additionally applied, which helps to improve switching, minimizes resistance and removes vibration of the closed plates.
According to technical characteristics, reed switches are divided into:
- Reed switches.
- Gazakon is a device that has a memory function. That is, the position of the contacts is saved after the magnetic field is turned off.
- Hercotrons are relays with high-voltage insulation. Designed for operation in devices with voltage from 10 to 100 kV.
- Gersikon is a relay designed to control equipment and automation with a power of up to 3 kW. The design is characterized by an increased switching current and the presence of arcing contacts.
Due to the variety of designs, reed switches continue to be used in many areas.
Operating principle
The reed switch is similar in principle to a switch. The relay consists of a pair of conductive cores with a gap between them. They are hermetically sealed in a glass flask with an inert medium that excludes the oxidation process.
A control winding is placed around the bulb, fed with direct current. When power is applied, the winding generates a magnetic field that acts on the cores, and leads to the closure of the contacts between themselves.
When the coil is disconnected from the power supply, the magnetic flux disappears and the contacts are opened by springs.Reliability is ensured by the absence of friction between the contacts, which, in turn, act as a conductor, spring and magnetic circuit.
A feature of the reed switch is that no forces act on the relay springs at rest. This allows them to close the contact in a fraction of a second.
Permanent magnets can also be used. Such devices are called polarized.
Normally closed devices have a different principle of operation. Under the influence of electromagnetic force, a system of magnets charges the cores with one potential, causing them to repel each other, opening the circuit.
Switchable reed switches consist of three contacts. One of them is installed permanently and is not magnetized, the other 2 are made of a ferromagnetic alloy. When a magnetic field is applied, a pair of open contacts closes, opening a pair with a non-magnetic contact.
Reed sensor connection
The documentation supplied with the sensors provides comprehensive information on how to connect the reed switch.
For the functioning and safety of the sensor, the part of the relay that generates the magnetic field is mounted on the moving part of the structure. The reed switch itself is attached to a permanently installed element of a structure or building.
The movable part tightly adjoins, acting by the magnetic field of the coil on the contact network of the reed switch and thereby closing the electrical circuit. The system sensor informs about the correct functioning of the system. As soon as the coil located on the moving part ceases to act on the sensor, the network opens and the automation reports a violation of the integrity of the system.
According to the installation method, sensors are:
- hidden fastening;
- external fastening.
Depending on the physical properties of the surface on which the reed switch is connected, there are:
- sensors for mounting on steel structures;
- sensors mounted on magneto-passive structures.
When installing a reed switch, it is necessary to remember some features of the installation:
- It is recommended to avoid location near ultrasonic sources. It is able to have a negative impact on the parameters of the sensor.
- Avoid location near the source of an extraneous magnetic field.
- Protect the sensor flask from shock and damage. Otherwise, the gas will evaporate, contact will be broken, and the cores will quickly become unusable.
Reed switches cannot switch high currents due to the low power of the cores. Therefore, they cannot be used to turn on and off powerful electrical devices.
They are included in a low power switching circuit to control the relay that controls the equipment.
Advantages
Reed sensors have the following advantages:
- Full tightness allows their use in fire hazardous rooms and aggressive environments.
- Instantaneous operation allows their use in devices with a high switching frequency.
- Elimination of contact bounce in mercury sensors. They are used in equipment with increased requirements for signal purity.
- Small dimensions from 4 mm, simple design, low manufacturing cost.
- High functionality and versatility of the relay.
- Ability to switch low-power signals.
- Large operating temperature range - from -55 to + 110 ºC.
- High core strength.
- No friction surfaces.
High versatility, reliability and price still allow reed switches to compete with direct competitors.
Flaws
Like all devices, reed switches also have disadvantages:
- Low sensitivity magnets.
- High susceptibility to external magnetic fluxes. As a result, additional screens may be required.
- Sometimes the contacts, after removing the magnetic field, may remain in a closed position from which they cannot be removed.
- The capsule is made of thin glass and is easily destroyed by falls and bumps.
- When a voltage with a low frequency is applied, the contacts spontaneously open and close the circuit.
- When high currents are applied, the core contacts can spontaneously open.
For these reasons, when using the relay, a number of restrictive measures specified in the accompanying documentation must be observed.
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