Screw bases and pin bases are the two main types of light bulb bases. The two types of bases differ in the way they connect the bulb to the source of electrical voltage in order to light the bulb.
Pin bases are used in MR16s, linear fluorescents, plug-in compact fluorescent bulbs, and some HID light bulbs. Whereas the screw base connects the light bulb to the voltage with two soldered contact wires on the metal base, pin base light bulbs have two pins sticking out of the base that connect the light bulb to the voltage. Electrical current then flows through the pins and into the light bulb to energize the filament or ballast to generate light.
The names of pin bases include a number that measures the distance in millimeters between the two pins from center point to center point. For example, bi-pin GU5.3 bases have two pins that are 5.3 millimeters apart between the center points of each pin. This base and bi-pin GU10 are used in MR16 light bulbs.
Medium bi-pin G13 bases are used in the vast majority of T8, T10, and T12 linear fluorescent light bulbs. Some linear fluorescents have a single pin Fa8 or recessed double contact R17d, but they are not very common. Miniature bi-pins are used in T5 linear fluorescents. T9 HID light bulbs use G12 and recessed single contact (R7S RSC) pin bases.
Plug-in compact fluorescent light bulbs are equipped with a two or four-pin base and use a ballast that has been built into the fixture. Plug-in CFLs cannot be interchanged with the ballast; depending on the ballast, only one or two light bulbs will be compatible with the fixture. The two-pin bulbs are for use with magnetic ballasts, and the four-pin bulbs are for use with electronic ballasts. The sockets used in plug-in CFL fixtures are designed to work with only one style of pin base. This ensures that the correct wattage light bulb is used to match the type of ballast inside the fixture. Some examples of plug-in CFL pin bases are G23, GX23-2, G24Q-2, and GX24Q-3.
Recent energy regulations have spurred the creation of the GU24 base, which has two pins that are 24 mm apart. Only energy-saving CFLs, CCFLs, and LEDs are manufactured with GU24 bases. Incandescent light bulbs are not manufactured with GU24 bases; therefore, a fixture with a GU24 socket will not accept an incandescent light bulb. This guarantees that the fixture is energy efficient.
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