Lightbulb socket

Lightbulb sockets, lamp sockets or lampholders provide electrical connections to the lamps and support it in the lighting fixture. The use of sockets allows lamps to be safely and conveniently replaced (re-lamping). There are many different standards for these lampholders, created by de facto and by various standards bodies. A general coding system is a letter or abbreviation followed by a number. Some miniature lamps have wire leads suitable for direct connection to wires; some reflector lamps have screw terminals for wire connections.

The most common type of sockets for mains electricity are Edison screws, used in continental Europe and the United States, while bayonet mounts dominate in the Commonwealth countries and in the automotive industry. Fluorescent lamps require a different, typically four-pin design. A broad variety of other socket designs exists, applied for specialized lamp constructions.

The construction of a lampholder socket defines and limits its intended 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 (360 °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.

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