Reed switch

Reed Switch Operation and Description
– Reed switch is an electromechanical switch operated by a magnetic field.
– Invented in 1922 by professor Valentin Kovalenkov at the Petrograd Electrotechnical University.
– Evolved into the reed relay in 1936 by Walter B. Ellwood at Bell Telephone Laboratories.
– Consists of a pair of ferromagnetic flexible metal contacts in a hermetically sealed glass envelope.
– Contacts are normally open, closing when a magnetic field is present.
– Commonly used circuit symbol.
– Contains magnetizable, flexible, metal reeds separated by a small gap when open.
– Reed switch can be actuated by an electromagnetic coil or a permanent magnet.
– Contacts return to their original position when the magnetic field is removed.
– Contacts are typically made of rhodium, ruthenium, iridium, or tungsten.

Reed Switch Sensitivity and Production
– Sensitivity is measured in ampere-turns (AT).
– Typical pull-in sensitivities range from 10 to 60 AT.
– Smaller reed switches are generally more sensitive to magnetic fields.
– Production involves sealing metal reeds in a glass tube using infrared heat.
– Reed switches are filled with nitrogen or vacuum-sealed for different applications.

Reed Switch Advantages
– Protected against atmospheric corrosion due to hermetic sealing.
– Suitable for use in explosive atmospheres where sparks from conventional switches would be hazardous.
– Can directly switch a variety of loads with different voltages and currents.
– Have small leakage currents compared to solid-state devices.
– Requires only two wires and zero power to operate.

Reed Switch Applications
– Commonly used to detect the opening of doors or windows for security alarms.
– Reed relays are used for low operating currents, high speed, and reliability.
– Used as proximity sensors in mechanical systems.
– Reed switches have been used in laptops and keyboards for sleep/hibernation mode.
– Speed sensors on bicycle wheels frequently use reed switches.
– Reed switches are commonly used in pipe organ and Hammond organ keyboards to protect them from dirt and dust.
– They are also used in diving equipment, such as flashlights and cameras, to maintain a seal under high pressure.
– Brushless DC electric motors used reed switches to sense rotor position, allowing for a commutator without contact problems.
– Reed switches were used in low-power, long-service-life items like computer cooling fans and disk drives.
– They are selected for sensor applications when solid-state devices do not meet specific requirements.

Reed Switch Life and Additional Information
– Reed switches have a mechanical motion below the fatigue limit, preventing them from breaking due to fatigue.
– Wear and life depend on the electrical load’s effect on the contacts and the specific reed switch used.
– Contact surface wear occurs when the switch contacts open or close.
– Manufacturers rate life in number of operations rather than hours or years.
– Higher voltages and currents cause faster wear and shorter life, with a range from thousands to billions of operations.
– Historically, ‘Herkon switch’ was occasionally used as a synonym for reed switch or reed contact.
– Herkon was a SEL/ITT trademark representing hermetically sealed contacts.Sources: