Lightbulb socket

Light bulbs are usually inserted in light bulb sockets which provide electrical connections to the bulb and support it in the lighting fixture. The use of sockets allows lamps to be safely and conveniently changed out at the end of life. There are many different standards for these sockets, created by de facto and by various standards bodies. A very few miniature lamps have wire leads suitable for direct connection to wires; some reflector lamps have screw terminals for wire connections.

Construction and materials

The construction of a 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, bulb 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 socket 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.

Socket 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 bulb. Sockets used outdoors or in damp areas often suffer from corrosion which can cause the bulb to "stick" in the socket and attempts to change a bulb can result in breakage of either the bulb or the socket. 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

Main article: Edison screw

• Miniature E-10 (Flashlight bulb)
• Miniature E-11
• Candelabra E-12
• European E-14
• Intermediate E-17
• Medium E-26/ E-27 (sizes are interchangeable)
• 3-Lite (modified medium or mogul socket with additional ring contact for 3 way bulbs)
• Mogul E-40
• Skirted (PAR-38)

Bayonet styles

A bayonet style light bulb

Main article: Bayonet mount

• Miniature bayonet
• Bayonet candelabra
• Bayonet Candelabra with prefocusing collar
• Medium prefocus
• Mogul prefocus

Bi-post

The Bi-post base has several advantages

• Bulb orientation is fixed so filament will always be in the focal plane. Filament configurations such as the C13D (coiled, zig-zagged) emit far more light perpendicular to the zig zag than parallel to it.

• Mogul bi-post can handle up to 100 Amps and is used with searchlights, film & stage lighting fixtures 1000 watts or larger. Incandescent, halogen and HMI light sources all use this design.
• Medium bi-post is used with film & stage lighting fixtures between 250 and 1000 watts (the development of the T14 base up to horizon design for ellipsoidal spotlights was one of the most important innovations of the mid 20th century).
• Mini bi-post

Bi-pin connector

Main article: Bi-pin connector

• Medium bi-pin is used on each end of a T12 fluorescent bulb
• mini bi-pin is used with MR16 halogen bulbs

The 2 pin socket is an update of the bi-post design with smaller pins designed to reduce the cost of manufacture. The 1000 watt FEL medium 2 pin base halogen lamp allows designers to insert the bulb into the end of the ellipsoidal reflector through a smaller hole than previously possible with conventional incandescent bulbs. This improves efficiency compared to the older side inserted bulb or a double ended bulb which requires two holes. One variation is the Polarized 2 pin socket - used primarily in projectors, which defines the exact positioning of the filament on one side. This improves the "point source" characteristic necessary for building complex optical systems.

Another facet of the two pin design is that many new designs of bulbs use baseless glass envelopes. The wire leads are thickened and crimped in the glass envelope of the bulb base. The MR16 is an example of this design. You may argue that the reflector itself is the base, but the industry has decided that the reflector is an assembly where the actual bulb is inserted into the reflector with the leads sticking out and a ceramic paste used to glue it in.

Wedge base

Miniature lamps may have a wedge base made of glass or plastic.

Fluorescent tube standards

Main article: Fluorescent-lamp formats

• T-5 mini
• T-8 medium
• T-12 large

Phillips Compact Fluorescent types

• G-23
• GX23
• GX32d-2
• GX32d-3

Lamp base styles

Abbreviation	Term
Cand	Candelabra
DC Bay	Double contact bayonet candelabra
DC Pf	Double contact prefocus candelabra
EMEP	Extended mogul end prong ferrule contact
F	Ferrule contact
Mc	minican
Med	Medium
Med Bp	Medium bipost
Med Pf	Medium Prefocus
Med Skt	Medium Skirted
Med 2P	Medium two pin
MEP	Mogul End Prong
Mog	Mogul
Mog Bp	Mogul bipost
Mog Pf	Mogul prefocus
MS	Miniature screw (with reference shoulder)
MSP	Medium side prong
G38 Bp	G38 mogul bipost
Rect RSC	rectangular recessed single contact
RM2P	Rim mount two pin
RSC	Recessed single contact
S	Metal sleeve
SC Bay	Single contact bayonet
SC Pf	Single contact prefocus
SFc 10-4	Sleeve with threaded pin
SFc 15, 5-6	Sleeve with threaded pin
ST	Screw terminal
TB2P	TruBeam two pin
Tf	Trufocus (also four pin)
TLMS	Tru-Loc miniature screw
2B	Two button
2PAG	Two pin all glass
2PAGC	Two pin all glass with ceramic cover
2PM	Two pin miniature
2PP	Two pin prefocus
3P	Three prong

Some of these base styles are obsolete and are included for reference. The trend in recent years has been to design newer bases to reduce waste of raw materials and make it easier to replace light bulbs.

References

• http://www.goodmart.com/info/base_general.aspx
• http://www.lightbulbsdirect.com/page/001/CTGY/Base
• http://www.gelighting.com/na/business_lighting/education_resources/literature_library/catalogs/downloads/Lighting_and_Ballasts_Section_D_Appendix_and_Glossary.pdf