Reed relay

A reed relay (historically also Herkon relay ) is a relay that works with a reed switch (also called reed contact).

Principle structure of the reed relay: a reed contact is surrounded by a coil

Reed relay in the housing

Reed switch contacts are contact tongues that are melted into a glass bulb under vacuum or inert gas , which at the same time form the contact spring and the magnet armature. The name comes from the reed of woodwind instruments , English reed that is similar to the vibrating reeds. The contact tongues are made of precious metal coated ferromagnetic material (e.g. soft iron ). The contact is actuated by an externally acting magnetic field which , in the case of sensors or switching contacts , is generated electrically by a permanent magnet brought into the vicinity or, in the case of reed relays, in an associated magnetic coil . Due to the magnetic field, the two contact tongues are attracted and thus close. As soon as the magnetic field drops or the field strength falls below a certain level in the relay, the contact opens again due to the spring action.

construction

A reed relay consists of a magnetic coil, which is usually wound from enamelled copper wire . The reed switch sits in the middle of the coil.

Current sensors can be implemented with coils made of thick wire, for example to monitor the function of a warning lamp.

Reed relays are often magnetically shielded from the outside so that they can be packed tightly without influencing each other. There can be an electrical shield towards the switching contact, which at the same time forms the outer conductor of a coaxial cable for high-frequency applications.

properties

Reed relays are very reliable due to the hermetically sealed contacts and extremely durable with low switching capacities. Reed relays usually require relatively little control power and, in addition to transistors, can often be controlled directly with TTL or CMOS outputs. Due to the hermetically sealed switching contacts and the corrosion- protected contact layer , reed relays are particularly suitable for switching small signals up to femto amps and nano volts .

Low thermal voltages in the materials are important for switching small voltages: Reed relays can be specified with thermal voltages of <1 µV / K.

Reed relays can be manufactured for switching frequencies above 6 GHz by being manufactured as part of an adapted coaxial line with a defined impedance (e.g. 50 ohms). The insertion loss can be <0.2 dB. The standing wave ratio (VSWR) is <1.2.

Bounce-free switching and longer contact lifetimes with higher loads can be achieved with mercury-wetted contacts (today no longer permitted in the EU without a special permit ).

Properties (see also under reed switch ):

• especially for switching small signals up to femtoampere and nanovolt through contact materials such as gold, iridium or palladium
• Hermetically sealed switching contacts, therefore very robust against water, air, vacuum, oil, petrol, dust etc .; Ex-protected and certified designs
• no mechanical wear at the movement points, mechanical service life up to several billion (10 ⁹ ) switching cycles
• low contact resistance (typ. 50 mOhm), low capacitance and high insulation
• low control power with small size
• short switching and bounce times
• also for voltages up to 10,000 volts
• comparatively low switching capacities
• sensitive to capacitive loads and the self-induction of inductive loads

Reed relays are available as normally open, normally closed, changeover and bistable.

history

The reed contact was patented in 1936 by WB Elwood, who was working at Bell Laboratories at the time. However, at that time it was not yet possible to manufacture reed contacts on a larger scale, since materials technology was not yet able to provide suitable materials for the contact blades. The first reed contacts available on a larger scale were not manufactured until the late 1950s. The main area of ​​application was in the 1960s to 1980s telecommunications technology in the exchanges.

Types

Various reed relays, some with several reed contacts

The contacts of reed relays are protected from corrosion and oxygen by their design . They are therefore extremely reliable and are particularly suitable for small switching capacities with frequent and very seldom (e.g. once in 10 years) operation. In comparison to similarly dimensioned normal electromagnetic relays, reed relays can also be designed for very high switching voltages (up to about 10 kV) by increasing the internal pressure of the filling gas.

Reed relays are also available in DIL and SMD designs, i.e. H. in the form of housing designs for circuit board assembly that are common in microelectronics.

With reed contacts for position detection, the reed contact is operated by a permanent magnet ; as soon as the magnet approaches, the switching contacts are drawn towards each other and close a circuit. Such contacts are often housed robustly encapsulated in cuboid or cylindrical housings and have molded connection lines. The actuating magnets are often designed to match or they are embedded in machine parts.

NC contacts and other functions

With the basic function of a reed contact, its contact is closed by an axial magnetic field. However, there are also so-called switching (changeover switches) or opening reed contacts, which have a reversed switching behavior. With these, the opening contact tongue is made of non-ferromagnetic material. Switching reed relays can also be built with such contacts. The manufacturing costs of these reed changeover contacts are relatively high. In addition, these contacts are not suitable for switching larger currents, since the contact force is not supported by the magnetic field. The closing and bounce time is higher.

Alternatively, the functionality of a reed relay can also be inverted with an additional transistor circuit. The circuit complexity and the current consumption are increased, however, since a permanent quiescent current flows through the transistor circuit .

Another possibility to invert the switching function is to attach a smaller permanent magnet to the glass body of the reed switch. The polarity of the external field must be reversed to this. The magnetic fields of both magnets compensate each other and the contact opens. Without an external field, the reed contact is closed by the integrated magnet. Other functions can also be implemented in this way:

• Reed relay with self-holding function: the field of the integrated permanent magnet is not enough to close the reed contact, but it can keep it closed. In this way, a field-reinforcing current pulse through the relay coil leads to a (permanent) pick-up and a field-compensating current pulse to a drop-out.
• Reed relays with increased pick-up sensitivity: the field of the integrated permanent magnet strengthens the external field, but only to such an extent that it can drop without a field. These relays require a prescribed polarity of the control voltage.

Applications

Reed relays are widely used there for switching low voltages and currents, where reliability and lowest contact resistance arrives (z. B. for signal switching in meters). Another application is high-frequency relays - here the contacts can be operated as the inner conductor of a coaxial line arrangement.

Reed contacts are often used in conjunction with permanent magnets for door contacts or for end position detection (e.g. on pneumatic cylinders and generally in automation technology). Reed contacts can also be switched through walls, such as in diving lamps or in level monitors.

Due to the encapsulated design, reed contacts are also approved in potentially explosive areas, such as those found in mines or in the chemical industry. For the same reason, they are suitable for use at great heights, where otherwise the contact distance would not be sufficient due to the greater throw distance in the thinner atmosphere.

A special area of ​​application is the use in medical implants. A reed contact is an easy-to-implement method for transmitting information. With cardiac pacemakers, it can be used, for example, to switch to a programming mode, while the implant works at a fixed rate. At this point it must be mentioned that a medical examination using magnetic resonance tomography can lead to this contact being closed, which is undesirable in this case.

They can be used instead of commutators in direct current motors (reed contact motors) . Some bike computers determine the speed and the cadence by means of reed contacts on the front wheel or the crank - in this area of ​​application for speed measurement, however, contactless Hall sensors are also very often used.

Reed relays with a thick winding attached to the outside are used as current sensors and can e.g. B. can be used for failure monitoring of incandescent lamps or for brake light control on motor vehicles.

Today, reed relays and reed contacts can often be replaced by Hall sensors that also work magnetically - in combination with electronic switches such as field effect transistors , functionally similar switch properties can be achieved, which, however, do without mechanical contacts and are therefore less mechanically sensitive (no breakage - and impact sensitivity, no special gas filling). Furthermore, Hall sensors can be designed for significantly higher switching frequencies than the mechanically moved reed contact tongues in order to be able to record higher speeds directly, for example. The disadvantage of Hall sensors is that they require auxiliary energy (operating voltage). This is why reed contacts are still often used today.

Reed relays have short pick-up times. The time from the tightening command to the closing of the contact can be less than 0.5 milliseconds for some designs.

Web links

Commons : Reed Relay  - Collection of Images, Videos, and Audio Files

• Interesting facts about reed technology

Individual evidence

1. ↑ Rudolf Scheidig: Herkon relay 80, a relay series with hermetically sealed contacts for printed circuits . In: SEL messages . 7, No. 1, 1959, pp. 6-8.
2. ^ Karl W. Steinbuch: Taschenbuch der Nachrichtenverarbeitung , 1st edition, Springer-Verlag OHG, Karlsruhe, Germany 1962, pp. 307, 431, 435-436.
3. ↑ Hilmar Beautiful Meyer: Quasi-Electronic Telephone Switching System HE-60 . In: International Telephone and Telegraph Corporation (ITT) (Ed.): Electrical Communication . 39, No. 2, Standard Elektrik Lorenz AG, Stuttgart, Germany, 1964, pp. 171, 244-259 [245-246, 251, 254-257].
4. ↑ Hoeckley Oden: Actual problem Of Telephone Switching - Quasi-Electronic Solutions For Switching Systems . In: Telecommunication Society of Austria (Ed.): The Telecommunication Journal of Australia . 14, No. 5/6, October 1964, pp. 342-355 [350, 355]. "The dry reed switch manufactured by SEL is sold under the registered name" Herkon "(hermetically sealed contact)."
5. ↑ Company information Meder Electronic
6. ↑ Elektronik No. 3, 2008, p. 16. ISSN  0013-5658
7. ^ Bruno Gruber: Reed switch with normally closed contact ( Memento from September 29, 2008 in the Internet Archive ).
8. ↑ Meder Electronic: Implantable cardiac pacemakers use reed sensors for transmission purposes ( Memento from December 6, 2011 in the Internet Archive ).
9. ↑ ( Page no longer available , search in web archives: G. Neubauer: Electromagnetic Fields and Man ) (PDF; 552 kB) Vienna University of Technology.