reed switch - wikipedia, the free encyclopedia

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Reed switchFrom Wikipedia, the free encyclopedia

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, ornormally closed and opening when a magnetic field is applied. The

switch may be actuated by a coil, making a reed relay,[1] 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.

An example of a reed switch's application is to detect the opening

of a door, when used as a proximity switch for a burglar alarm.

Contents

1 Description

2 Uses

2.1 Reed relays

2.2 Magnetic sensors

3 Further reading

4 External articles and references

Description

The reed switch contains a pair (or more) of magnetizable, flexible, metal reeds whose end portions are separ

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, whenmagnetic field ceases. Another configuration contains a non-ferrous normally-closed contact that opens when

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" contactsuch

switches must be mounted in a particular orientation otherwise drops of mercury may bridge the contacts eve

when not activated.

Since the contacts of the reed switch are sealed away from the atmosphere, they are protected against atmosp

corrosion. The hermetic sealing of a reed switch make them suitable for use in explosive atmospheres where t

sparks from conventional switches would constitute a hazard.

Reed relay and reed switches

Showing the contacts clearly


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24/12 Reed switch - Wikipedia, the free encyclopediaOne important quality of the switch is its sensitivity

he

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

number of turns. Typical pull-in sensitivities for

commercial devices are in the 10 to 60 AT range. T

lower the AT, the more sensitive the reed switch.

Also, smaller reed switches, which have smaller pa

are more sensitive to magnetic fields, so the smalle

reed switch's glass envelope is, the more sensitive

In production, a metal reed is inserted in each end

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

concentrate the heat in the small sealing zone of th

glass tube. The thermal coefficient of expansion of

glass material and metal parts must be similar toprevent breaking the glass-to-metal seal. The glass

used must have a high electrical resistance and mu

not contain volatile components such as lead oxide

and fluorides. The leads of the switch must be han

carefully to prevent breaking the glass envelope.

Uses

Reed relays

Main article: Reed relay

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 b

contacts sticking closed either from residual magnetism or welding.

Magnetic sensors

In addition to their use in reed relays, reed switches are widely used for electrical circuit control, particularlyhe

communications field.

Reed switches actuated by magnets are commonly used in mechanical systems as proximity sensors. Example

A TXE-3 reed relay

Reed switch diagrams from Ellwood's patent, U.S. Patent

2,264,746 (http://www.google.com/patents?vid=2264746) , Electromagnetic switch


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door and window sensors in burglar alarm systems and tamperproofing methods (however they can be disably.wikipedia.org/wiki/Reed_switch


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24/12 Reed switch - Wikipedia, the free encyclopediaa strong, external magnetic field). Reed switches are used in modern laptops to put the laptop on sleep/hibern

mode when the lid is closed. Speed sensors on bicycle wheels and car gears use a reed switch to actuate brief

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 play

often use reed switches, where the glass enclosure of the contacts protects them from dirt, dust, and otherarticles.

They may also be used to control diving equipment such as flashlights or camera, which must be sealed to ke

pressurized water out.

In recent times, solid state Hall effect sensors have begun to replace reed switches in most all the uses describe

here.

Further reading

Books

[1] (http://www.crcpress.com/shopping_cart/products/product_detail.asp?

sku=DK884X&isbn=9780849341885&parent_id=&pc=) Vladimir Gurevich "Electric Relays: Principl

and Applications", CRC Press, London - New York, 2005, 671 p.

[2] (http://www.crcpress.com/shopping_cart/products/product_detail.asp?

sku=69829&isbn=9781420069822&parent_id=&pc=) Vladimir Gurevich "Electronic Devices on Disc

Components for Industrial and Power Applications", CRC Press, London - New York, 2008, 418 p.

Journals

Miedzinski, B., and M. Kristiansen,Investigations of Reed Switch Dynamics and Discharge Phenome

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 ree

switches. IEEE Transactions on Components, Hybrids, and Manufacturing Technology, Apr 1992, Vol

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

Citations

1. ^ Rudolf F. Graf, "reed relay" Dictionary of Electronics; Radio Shack, 1974-75. Fort Worth, Texas.

General

Advanced information about Reed Switches (http://reed-switch-info.com/)

Reed Switch Information (http://www.eleinmec.com/article.asp?23)

Glossary of Commonly Used Terms Relating to Reed Switch Products


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