A lightbulb socket, lightbulb holder, light socket, lamp socket or lamp holder is a device which mechanically supports and provides electrical connections for a compatible electric lamp base. Sockets allow lamps to be safely and conveniently replaced (re-lamping). There are many different standards for lampholders, including early de facto standards and later standards created by various standards bodies. Many of the later standards conform to a general coding system in which a socket type is designated by a letter or abbreviation followed by a number. The most common type of sockets for mains electricity are Edison screws, used in continental Europe and North America, while bayonet mounts dominate in the Commonwealth countries, except Canada, and in the automotive industry. Fluorescent lamps typically require a two-pin, unthreaded socket. Not all lamps require a socket; for example, some miniature lamps have wire leads suitable for direct connection to screw terminals or other wires, and some reflector lamps provide screw terminals for electrical connections.
History Early experimental incandescent lamps employed wire leads which had to be connected to screw terminals, but this was inconvenient for commercial use. The Edison organization used simple wooden receptacles with internal copper strips for lamps on the commercial steamship SS Columbia, the first ship to use electric light bulbs. These sockets included switches, but required bulbs to be mounted upright. The Edison organization developed a screw-base in 1880 which was initially made of wood but later made of plaster of Paris. Many competitive designs of lamps and sockets appeared in the early era of incandescent lighting, which often were incompatible with other designs.
Construction and materials The construction of a lampholder socket defines and limits its intended primary use. Ceramic insulation can withstand considerably higher operating temperatures than bakelite or other plastics. The electrical components and wires must be designed to carry the intended current plus a safety factor. The contact surface area, thickness and conductivity of the metal, connection methods and maximum operating temperature must all be considered in the design of a new socket. In addition, mechanical factors such as shape of the socket, fixture mounting and attachment, lamp support, ease of re-lamping and total cost of manufacture must be considered. Sockets designed for ordinary household and industrial use have much more design leeway than those used in precision applications. The lampholder must be located far enough from the filament that the metals with the lowest melting point will remain solid. Historically this metal was a tin/lead solder whose melting point might be as low as 180 °C (356 °F) Due to the thermal changes from ambient temperature to full operating temperature, the design of a socket must allow for a considerable amount of expansion and contraction. Spring elements are required to accommodate these dimensional changes. However, the temperature at which a metal loses its spring is far below the melting point. This is why some older sockets that no longer work can be restored by prying up the base spring slightly. Lampholder failures are usually caused by mechanical abuse or by overheating. A socket with a built-in switch is far more likely to fail in normal use as the switch parts wear out. Insulation failures are usually caused by impacts or by difficulty inserting or removing a lamp. Sockets used outdoors or in damp areas often suffer from corrosion which can cause the lamp to "stick" in the socket and attempts to change a lamp can result in breakage of either the lamp or the lampholder. The corrosion is not only environmentally produced but may be a result of the current flowing through the parts when there is appreciable resistance between the parts. Fixtures in such environments may require gaskets or other waterproofing methods to prevent buildup of moisture in the socket area.
Edison screw bases
E10s Miniature (Flashlight lamp) E11s Mini-Candelabra E12s Candelabra E14s European E17s Intermediate E26s Medium E26d Three-way Medium (modified socket with additional ring contact for 3-way lamps) E27s European E39s Mogul E39d Three-way Mogul (modified socket with additional ring contact for 3-way lamps) E40s European Skirted (PAR-38) The light bulb commonly used since the early 20th century for general-purpose lighting applications, with a pear-like shape and an Edison screw base, is referred to as an "A-series light bulb." This most common general purpose bulb type would be classed as "A19/E26" or the metric version "A60/E27".
Bi-post
With bi-post bases, the lamp orientation is fixed so that the filament will always be in the focal plane. Filament configurations such as the C13D (coiled, zigzagged) emit far more light perpendicular to the zigzag than parallel to it.
Mogul bi-post (G38) can handle up to 100 amps and is used with searchlights and film & stage lighting fixtures of 1000 watts or larger. Incandescent, halogen and HMI light sources use this design. Medium bi-post (G22) is used with film & stage lighting fixtures between 250 and 1000 watts. Mini bi-post (G4-G6) Common types:
G4 – 4 mm (0.15748 in) pin spacing GU4 & GZ4 – same as G4 and only denote what lamp mount clip is needed to hold the actual light bulb in place G5.3 – 5.3 mm (0.20866 in) pin spacing GU5.3, GX5.3, GY5.3, GZ5.3 – same as G5.3 and only denote what lamp mount clip is needed to hold the actual light bulb in place G6.35 – 6.35 mm (0.25 in)spacing GY6.35 & GZ6.35 – same as G6.35 and only denote what lamp mount clip is needed to hold the actual light bulb in place G8 – 8 mm (0.31496 in) pin spacing GU8 – same as G8 and only denotes what lamp mount clip is needed to hold the actual light bulb in place GY8.6 – 8.6 mm (0.33858 in) pin spacing G9 – 9 mm (0.35433 in) pin spacing G12 – 12 mm (0.47244 in) pin spacing
Bi-pin connector
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