About reed contacts
Reed switches consist of two overlapping ferromagnetic contact tabs , usually nickel/iron, which are melted in a glass tube in a protective atmosphere. eingeschmolzen sind. The reed contacts are open when the device is not in use. You can find out more about reed contacts in our guide.
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What are the advantages of reed contacts?
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How are reed contacts characterized?
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What types of reed contacts are available?
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What types of reed contacts are available?
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What are reed contacts used for?
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What must be observed when installing and operating reed contacts?
Reed switches consist of two overlapping ferromagnetic contact tabs , usually nickel/iron, which are melted in a glass tube in a protective atmosphere. The reed contacts are open when the device is not in use. If the tongues are affected by a magnetic field of sufficient strength and the field lines pass through them, they become magnetic with opposite polarity, so they connect and close the contact. The magnetic field can be generated by a permanent magnet or a concentric solenoid . . The combination of reed contact and solenoid coil is a reed relay. The contact tabs are covered with a special contact material such as rhodium, ruthenium or similar on the surfaces on which they touch, in order to maintain a low contact resistance over a long period of time and many switching cycles.
1 glass tube with protective atmosphere
2 ferromagnetic tabs
3 field lines
4 permanent magnet
Principle of the reed switch or magnetic contact: The field lines of the magnet run through the ferromagnetic contact tabs. They will pull on and close the contact.
1 glass tube with protective atmosphere
2 Reed contact
3 field lines
4 Solenoid
What are the advantages of reed contacts?
Reed contacts are non-contact switching elements. Because the tabs are hermetically enclosed in a glass tube, environmental influences such as dirt, moisture, aggressive media, etc. have no influence on the electrical properties of the contacts . For the same reason reed contacts can also be used in potentially explosive environments or production areas, because a spark occurring at the contact point during the switching process cannot penetrate outwards. If reed contacts are operated with a permanent magnet, the switching process is carried out without additional power supply . Only the reed contacts actuated by electromagnets require an operating voltage like conventional electromagnetic relays. Reed contacts are very reliable switches with low contact resistance and long service life.
How are reed contacts characterized?
The magnetic field strength required for safe operation of reed contacts is specified with the AW value. "AW" stands for "Ampere-windings", "AT" for "Ampere Turns" internationally and results from the multiplication of the number of windings of a solenoid around the reed contact with the current flowing through this coil. There are two AW values for a given number of windings: The "tightening value" AW-an (PI) for the current at which the reed contacts are securely closed and the "waste value" AW-ab (DO) for the current at which the contacts open again. The two values differ, the ratio between AW-an and AW-ab is called "hysteresis" and is expressed as a percentage.
In practice this means: High AW value – a strong magnetic field or a magnet is required to actuate the contacts at a small distance; the reed switch is not sensitive. Low AW value – a weaker magnetic field is sufficient to actuate the contacts or a magnet at a greater distance; the reed switch is sensitive.
Other important technical data, as with other contact components and switches, are the maximum permissible switching current, the maximum permissible continuous current, the maximum permissible switching voltage and the maximum permissible switching capacity.
Closer in glass tube
What types of reed contacts are available?
- Most types are normally open contacts or normally open contacts . These are referred to as "Form A" or internationally as "SPST-NO" (Single Pole Single Throw – Normally Open).
- There are also NC contacts . These are referred to as "Form B" or internationally as "SPST-NC" (Single Pole Single Throw – Normally Closed).
- In addition, changeover contacts or changeover contacts are offered. These are referred to as "Form C" or internationally as "SPDT" (Single Pole Double Throw).
NC contacts and changeover switches are designed in such a way that a ferromagnetic contact tongue with mechanical spring force is resting on a non-ferromagnetic contact tongue. When a magnetic field is activated, the ferromagnetic tongue moves and opens the contact or switches to the other contact.
1 glass tube with protective atmosphere
2 movable ferromagnetic contact tongues
3 ferromagnetic contact tongues
4 non-magnetic contact tongues
What types of reed contacts are available?
The simplest version of a reed contact is the version in the glass tube . The connection is made via solder or cable lug connections. The respective dimensions depend on the electrical data, i.e. maximum current and voltage values. These components can be installed in any position in technical devices. It is important to have the correct position with regard to the control solenoid . Especially small versions can be mounted on the surface.
Because the glass tubes can break easily, there are reed contacts for some applications, which are installed in a plastic housing. These types have fixing devices and are easy to install.
Reed sensor in plastic housing with suitable actuating magnet
Reed switch in plastic housing for mounting on printed circuit boards by means of solder pads
What are reed contacts used for?
Reed contacts can be used for many purposes, e.g. as reed sensors in industrial applications , in machine controls and in safety engineering . Together with suitable actuation magnets, they are installed as magnetic contact on windows and doors for house alarm systems. Reed contacts can also be used as reed sensors for monitoring safety grillees and doors on machines. Similarly, many tasks of switching contacts in electromechanical systems can be solved, e.g. Limit stop, travel limit and positioning .
Actuated by a solenoid coil, i.e. as a reed relay, reed contacts represent sensitive switching elements with low contact losses and high reliability. They are therefore particularly suitable for switching very low voltages and currents.
What must be observed when installing and operating reed contacts?
The maximum values for currents, voltages and switching capacity specified in the technical specifications should not be exceeded during operation, otherwise the electrical properties of the contacts will deteriorate or they may even be destroyed. The contacts in the circuit must be effectively protected against overvoltages, which occur, for example, when switching inductive loads (snubber diodes, varistors, RC elements). Like other mechanical switches, reed contacts can bounce. If these are used as sensors in digital systems, circuit-related measures must be taken to check the signal.
The glass tubes must be mounted free of mechanical stress so that they do not crack or leak in the short and long term.