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. 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.
Description The reed switch contains a pair 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 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. Uses
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 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. Magnetic sensors
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.
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.
They may also be used to control diving equipment such as flashlights or camera, which must
be sealed to keep pressurized water out. At one time brushless DC electric motors used
reed switches to sense the rotor's position relative to the field poles. This allows switching
transistors to act as a commutator, but without the contact problems, wear and electrical
noise of a traditional DC commutator. The motor design could also be 'inverted', placing
permanent magnets onto the rotor and switching the field through the external, fixed coils.
This avoided the need for any rubbing contact to provide power to the rotor. Such motors
were used in low-power long-service-life items such as computer cooling fans and disk drives.
As cheap Hall effect sensors became available, they replaced the reed switches and gave longer
service lifetimes. Reed switches may be selected for a particular
application when a solid-state Hall device is not suitable. Reed switches can be used
to greatly reduce leakage current when compared with solid state devices; this may be useful,
for example, in medical devices requiring protection of a patient from tiny leakage
currents. The reed is hermetically sealed and can therefore operate in almost any environment,
such as where flammable gas is present or where corrosion would affect open switch contacts.
A reed switch has very low resistance when closed, typically as low as 50 milliohms,
whereas the Hall Effect can be in the hundreds of ohms.
The reed switch principle can be applied to directly switch a variety of loads ranging
from nanovolts to kilovolts, femtoamperes to amperes, and DC to radio frequency. The
Hall effect devices have very limited ranges of outputs and generally do not control a
final device such as a lamp, solenoid, or motor. Reed sensors can withstand higher voltage
than typical Hall devices. Further reading
Books "Electric Relays: Principles and Applications",
Vladimir Gurevich, CRC Press, London - New York, 2005, 671 p.
"Electronic Devices on Discrete Components for Industrial and Power Applications", Vladimir
Gurevich, 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 External articles and references
Citations
General Reed Switch FAQ
Glossary of Reed Switch and Magnet Related Terms
Advanced information about Reed Switches Reed Switch Information
Glossary of Commonly Used Terms Relating to Reed Switch Products