The ballast is the primary electronic device that serves the light-emitting tube in a fluorescent light fixture. Imminent ballast failure is indicated by longer start-up times, flickering at startup or intermittent cycling on and off of the lamp. A certain amount of noise is inherent in the operation of a ballast -- the unit is essentially a voltage transformer -- but increased noise is an indicator that the ballast is going bad. When a lamp simply doesn't light, try the tube in another fixture; if the tube illuminates, the issue is almost certainly with the original ballast.
How Fluorescents Function
Fluorescent lamps conduct electricity through a gas, rather than through a filament as is the case in a traditional incandescent bulb. The electrodes at either end of the tube emit electrons that pass through the gas, and in their passing they excite a coating of phosphor on the inside of the tube. The phosphor produces a light that is visible to the human eye. Fluorescent lamps are superior to incandescent bulbs in a number of ways -- they use less energy because they create less heat, they create more light with that reduced energy and they typically last longer.
Older-style fluorescent lamps had starters that functioned similar to a coil in an automobile; they "saved up" electricity and then sent a preheating bolt of power greater than mains current into the lamp to start it. Once lit, the lamp could then continue to operate on mains current alone. The starting pulse is called induced voltage. After the fluorescent lamp has lit, this voltage is no longer required. Newer fluorescents do not need a preheating device; they use a ballast. Properly functioning lamps that blink a few times before coming to full brightness have starters; those that do not blink have ballasts.
Internal Factors Influencing Ballast Life
Selecting the correct ballast for the lamp is imperative. Both the input voltage rating -- commonly 110 or 120 volts in the U.S. -- and the wattage rating must be specific. The necessary wattage is determined by the size of the tubes and is usually printed near one end electrode. Ballasts are heat sensitive. If they are incorrectly positioned in the fluorescent unit so that the heat created by their proper operation cannot dissipate, they will go bad early. Lamps that cycle on and off usually do so because a thermal cut-out in the ballast is sensing excessive temperatures, a sure sign the ballast is incorrectly located or failing.
External Factors Influencing Ballast Life
Ballasts are susceptible to the temperature at which they are asked to function. In abnormally cold conditions, a ballast-actuated fluorescent will blink at initiation just like an older style starter-equipped unit. In freezing conditions, a cold weather ballast should be used. Specialist ballasts must be used in conjunction with dimmer switches, properly called "rheostats." Using a dimmer with a regular ballast will cause it to go bad. Excessive humidity will reduce ballast life, often dramatically. Dampness in the atmosphere is turned to steam when the unit heats, and steam is corrosive. Fitting ballasts in waterproof housings only causes more problems with excessive heat. Aquarium lights typically have a much shorter life than office lights.