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ultraminiature

Reed switch

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. Elwood. 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; normally closed and opening when a magnetic field is applied; or one normally open and one normally closed.

The switch may be actuated by a coil, making a reed relay, or by bringing a magnet near to the switch.

Description

The reed switch contains two magnetizable and electrically conductive metal reeds which have end portions 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 contacts to pull 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 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 energy 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.

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.

Uses

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 switches as well as in door and window sensors in burglar alarm systems and tamperproofing methods; however they can be disabled by a strong, external magnetic field. 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. Speed sensors on bicycle wheels use a reed switch to actuate briefly each time a magnet on the wheel passes the sensor.

Reed relays

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 for temporarily storing information in mid-20th Century 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.

Further reading

Books:

  • 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.Journals:
  • 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

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