Ballasts are used to regulate the amount of electric current a light bulb draws from a power source. They are also used to provide starting voltages for certain types of light bulbs.
According to the Lighting Research Center, without the use of a ballast, light bulbs connected to high-voltage power sources would draw in uncontrollable amounts of electrical current. The bulb would immediately overheat and burn out.
Two basic types of ballasts are used for lighting. Magnetic ballasts used to be the most widely used type of lighting ballast, but they are steadily being phased out by electronic ballasts because electronic ballasts do not "flicker" as often as magnetic ballasts.
Some applications call for the use of adaptable ballasts, a type of electronic ballast. This type of ballast is very useful because it can be employed to operate several light bulbs at a time.
Several types of light bulbs utilize ballasts. All fluorescent light bulbs employ ballasts, as do high-intensity discharge (HID) bulbs like metal halide, high-pressure sodium and mercury vapor bulbs.