Lightbulb socket

Light bulbs draw their power from wires via electrical/mechanical connections. It is impossible to discuss the sockets without referring to the specifics of the light bulb itself - as the two are intimately connected. The socket is the connector to the electrical power and consists of two parts - the male component attached to the light bulb and the female component attached to the power source via a system of plugs, receptacles, wires, circuit protectors and transformers (for AC sources).

There are many different standards for these sockets, created by de facto and by various standards bodies.

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. For many years, Underwriter's Laboratories in the USA has specified that copper switchboard bus bars carry no more than 1000 Amps per square inch and be spaced no less than 1/2 inch away from adjacent conductive surfaces to be UL rated when the power source is 120/208/240 volts.^[1] Similar restrictions exist in sockets - 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.

Incandescent filaments (the oldest electrical light source) produce light by passing current through a resistive element. The very first such elements successfully designed by Thomas Edison were of carbonized cardboard. This proved to be extremely fragile, and tungsten wire eventually became the standard as the melting point of tungsten was sufficiently high that a "white" light could be produced without the wire melting in normal use. The color temperature of a normal household light bulb is about 2750° Kelvin, which is a yellowish white. These bulbs last about 1000 hours in normal use. Light bulbs used in projection, photography, film and the television industry emit light at a slightly color temperature of 3200°K - but the expected life is much shorter (25 to 250 hours). Incandescent bulbs are extremely inefficient - only about 5% of the total energy applied produces visible light. The remaining 95% is emitted as infrared heat energy.

Light bulb designs are largely determined by the radiated power of the light source. The design must consider the total electrical energy converted to heat. The glass bulb must be far enough from the filament that the glass does not melt. Thus, different light bulb designs have evolved to meet specific needs. Just as the melting point of the glass must be considered so must the materials of the socket. 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). The majority of the metal used to make the socket is brass, which is easily formed or stamped into the appropriate shape. 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. (Caution: Never do this with an energized circuit!)

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.

The voltage rating of the circuit has much to do with the life expectancy of the bulb in extreme conditions. Automobiles use low voltage lamps with higher current because a thicker filament wire is more resistant to vibration.

Improvements in light bulb design such as the quartz bulb and the use of halide or halogen elements solve some problems but introduce others. Quartz can withstand considerably higher temperatures than glass, so bulbs can be made smaller. However the strength of a quartz bulb at operating temperature can be reduced by the presence of skin oils deposited when installing the bulb. This can cause the bulb to shatter. Some newer light bulb designs such as the MR16 include a glass UV filter and a glass reflector which combine to make it extremely difficult to actually touch the quartz. Halogen light bulbs add a trace amount of iodine inside the bulb. At operating temperature this vaporizes and ionizes the gases surrounding the filament. The result is that the tungsten boiled off the filament (which would normally coat the inside of the bulb) is returned to the filament. This prolongs the life of the filament.

Some types of socket are listed here.

"Decorative" and standard screw base (Edison screw) bulb bases

Miniature E-11
Candelabra E-12
European E-14
Intermediate E-17
Medium E-26 (This is what Americans call a "normal" lightbulb)
3-Lite (modified mogul socket with additional ring contact for 3 way bulbs)
Mogul E-40
Skirted (PAR-38)

Bayonet styles

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.
Sockets

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

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

Two Pin

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.

fluorescent tube standards

T-5 mini
T-8 medium
T-12 large

Specialty

Medium
Three Contact Medium 3C Med
...

Phillips Compact Fluorescent types

G-23
GX23
GX32d-2
GX32d-3
...

General Electric Compact Fluorescent types

...
...

SLI Compact Fluorescent types

...
...

Design and Use Terms

As light bulbs are designed to meet specific needs from general illumination to precision control of the light emitted, lightbulb sockets must be designed and chosen based on the needs of the application.^[2]

Large 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.

Bulb Dimensions

Abbreviation	Term
LL	Light length (filament length)
LCL	Light center length (optical center of the filament)
C-to-C	Contact to contact
MOL	Maximum overall length
Dia	Diameter
Max	Maximum

If the socket design dimensions are known it is easy to calculate how high the LCL of the bulb will be above the base of the socket. The MOL of the bulb plus the seated height of the bottom of the base will help determine the size of a container for the finished lighting fixture.

Common Burn Positions

Abbreviation	Term
BD	Base down
BD/45	Within 45 degrees of vertical base down
BD/Hor	Base down to horizontal
BU	Base up
Horiz	Horizontal
Univ	Universal

Examples using conventional incandescent bulbs (all require ceramic high temperature sockets and wiring):^[3]

A 1000 watt PS-52 (Pear Shaped - 52/8 = 6.5" maximum diameter) bulb is designed for universal base orientation. The long neck keeps the base cool enough in any position that solder on the base will not melt.
A 500 watt G-40 (Globe - 40/8 = 5" maximum diameter) bulb is specified as BD/Hor in order to keep the base below the highest part of the bulb so solder on the base will not melt.
A 750 watt T-12 (Tubular - 12/8 = 1.5" maximum diameter) bulb designed for BU use in ellipsoidal spotlights moves the filament to the lowest end of the glass bulb, creating the necessary distance to the base for cooling while having a small diameter so it can be inserted into an ellipsoidal reflector in such a way as to make the reflector more efficient.

Contemporary lightbulb construction for lower wattage (15 to 100watts) bulbs rarely uses solder to connect the bulb wire leads to the base. Instead the connection is spot welded. This allows the base to be made from aluminum rather than brass and keeps the cost down.

References

^ Electro Controls Wireman's handouts - circa 1974
^ 1990 Lampholders catalog from GTE Sylvania Emmissive Products
^ IES Lighting Handbook Second Edition (1952) (Illuminating Engineering Society)

[1] Electro Controls Wireman's handouts - circa 1974

[2] 1990 Lampholders catalog from GTE Sylvania Emmissive Products

[3] IES Lighting Handbook Second Edition (1952) (Illuminating Engineering Society)

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