The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by W. B. Ellwood. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope. The contacts may be normally open, closing when a magnetic field is present, or normally closed and opening when a magnetic field is applied. The switch may be actuated by a coil, making a reed relay, or by bringing a magnet near to the switch. Once the magnet is pulled away from the switch, the reed switch will go back to its original position.
The reed switch contains a pair (or more) of magnetizable, flexible, metal reeds whose end portions are separated by a small gap when the switch is open. The reeds are hermetically sealed in opposite ends of a tubular glass envelope.
A magnetic field (from an electromagnet or a permanent magnet) will cause the reeds to come together, thus completing an electrical circuit. The stiffness of the reeds causes them to separate, and open the circuit, when the magnetic field ceases. Another configuration contains a non-ferrous normally-closed contact that opens when the ferrous normally-open contact closes. Good electrical contact is assured by plating a thin layer of non-ferrous precious metal over the flat contact portions of the reeds; low-resistivity silver is more suitable than corrosion-resistant gold in the sealed envelope. There are also versions of reed switches with mercury "wetted" contacts. Such switches must be mounted in a particular orientation otherwise drops of mercury may bridge the contacts even when not activated.
Since the contacts of the reed switch are sealed away from the atmosphere, they are protected against atmospheric corrosion. The hermetic sealing of a reed switch make them suitable for use in explosive atmospheres where tiny sparks from conventional switches would constitute a hazard.
One important quality of the switch is its sensitivity, the amount of magnetic field necessary to actuate it. Sensitivity is measured in units of Ampere-turns, corresponding to the current in a coil multiplied by the number of turns. Typical pull-in sensitivities for commercial devices are in the 10 to 60 AT range. The lower the AT, the more sensitive the reed switch. Also, smaller reed switches, which have smaller parts, are more sensitive to magnetic fields, so the smaller the reed switch's glass envelope is, the more sensitive it is.
In production, a metal reed is inserted in each end of a glass tube and the end of the tube heated so that it seals around a shank portion on the reed. Infrared-absorbing glass is used, so an infrared heat source can concentrate the heat in the small sealing zone of the glass tube. The thermal coefficient of expansion of the glass material and metal parts must be similar to prevent breaking the glass-to-metal seal. The glass used must have a high electrical resistance and must not contain volatile components such as lead oxide and fluorides. The leads of the switch must be handled carefully to prevent breaking the glass envelope.
One or more reed switches inside a coil is a reed relay. Reed relays are used when operating currents are relatively low, and offer high operating speed, good performance with very small currents which are not reliably switched by conventional contacts, high reliability and long life. Millions of reed relays were used in telephone exchanges in the 1970s and 80s. In particular they were used for switching in the British TXE family of telephone exchanges. The inert atmosphere around the reed contacts ensures that oxidation will not affect the contact resistance. Mercury-wetted reed relays are sometimes used, especially in high-speed counting circuits. Reliability is compromised by contacts sticking closed either from residual magnetism or welding.
In addition to their use in reed relays, reed switches are widely used for electrical circuit control, particularly in the communications field.
Reed switches actuated by magnets are commonly used in mechanical systems as proximity sensors. Examples are door and window sensors in burglar alarm systems and tamperproofing methods (however they can be disabled by a strong, external magnetic field). Reed switches are used in modern laptops to put the laptop on sleep/hibernation mode when the lid is closed. Speed sensors on bicycle wheels and car gears use a reed switch to actuate briefly each time a magnet on the wheel passes the sensor. Reed switches were formerly used in the keyboards for computer terminals, where each key had a magnet and a reed switch actuated by depressing the key; cheaper switches are now used. Electric and electronic pedal keyboards used by pipe organ and Hammond organ players often use reed switches, where the glass enclosure of the contacts protects them from dirt, dust, and other particles.
-  Vladimir Gurevich "Electric Relays: Principles and Applications", CRC Press, London - New York, 2005, 671 p.
-  Vladimir Gurevich "Electronic Devices on Discrete Components for Industrial and Power Applications", CRC Press, London - New York, 2008, 418 p.
- Miedzinski, B., and M. Kristiansen, Investigations of Reed Switch Dynamics and Discharge Phenomena When Switching Intermediate and Heavy Loads. IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Jun 1982, Volume 5, Issue 2 pg 231- 237. ISSN 0148-6411
- Hinohara, K., T. Kobayashi, and C. Kawakita, Magnetic and mechanical design of ultraminiature reed switches. IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Apr 1992, Volume 15, Issue 2, pg 172-176. ISSN 0148-6411 DOI 10.1109/33.142891
- Pinnel, M., Magnetic materials for dry reed contacts. IEEE Transactions on Magnetics, Nov 1976, Volume 12, Issue 6, pg 789- 794. ISSN 0018-9464
- Demirdjioghlou, S. and M. Copeland, Force measurements on magnetic reeds, IEEE Transactions on Magnetics, Jun 1968, Volume 4, Issue 2, pg 179- 183. ISSN 0018-9464
External articles and references
- ^ Rudolf F. Graf, "reed relay" Dictionary of Electronics; Radio Shack, 1974-75. Fort Worth, Texas.
Wikimedia Foundation. 2010.
Look at other dictionaries:
Reed switch — Saltar a navegación, búsqueda Reed switch … Wikipedia Español
Reed — can refer to: * REED, a theatre research centre at the University of Toronto, Canada* Edward C. Reed High School, Sparks, Nevada * Reed Airport * Reed College, Portland, Oregon * Reed (Company), group of companies offering employment related… … Wikipedia
Reed relay — A reed relay is one or more reed switches controlled by an electromagnet. The contacts are of magnetic material and the electromagnet acts directly on them without requiring an armature to move them. Sealed in a long, narrow glass tube, the… … Wikipedia
Switch — For other uses, see Switch (disambiguation). Electrical switches. Top, left to right: circuit breaker, mercury switch, wafer switch, DIP switch, surface mount switch, reed switch. Bottom, left to right: wall switch (U.S. style), miniature toggle… … Wikipedia
reed — noun 1》 a tall, slender leaved plant of the grass family, growing in water or on marshy ground. [Genera Phragmites and Arundo: several species.] ↘used in names of similar plants growing in wet habitats, e.g. bur reed. 2》 a tall, thin,… … English new terms dictionary
Nathaniel Reed — A photograph of Reed which appeared on the title page of his 1936 autobiography The Life of Texas Jack. Born March 23, 1862(1862 03 23) Madison County, Arkansas, United States … Wikipedia
Accordion reed ranks & switches — A reed rank inside accordions refers to a single full set of the reeds that are the means to achieve the instrument s sound range. These reed ranks are located in the reed chamber . Most accordions to this date typically have anywhere between 2 4 … Wikipedia
Dan Reed Network — Origin Portland, Oregon, United States Genres Hard rock Pop rock Funk rock Funk metal Years active 1984–1993 Labels … Wikipedia
Crossbar switch — In electronics, a crossbar switch (also known as cross point switch, crosspoint switch, or matrix switch) is a switch connecting multiple inputs to multiple outputs in a matrix manner. Originally the term was used literally, for a matrix switch… … Wikipedia
Steve Reed (footballer) — Infobox Football biography playername = Steve Reed fullname = Stephen Leslie Reed dateofbirth = birth date and age|1985|18|6 cityofbirth = Barnstaple, Devon countryofbirth = England height = height|ft=5|in=8 currentclub = Weymouth clubnumber = 14 … Wikipedia