How Electronic Ballasts Work

Electronic ballasts keep fluorescent lights bright with less energy.

The ballast in a fluorescent lamp is a device that starts the lamp and regulates the voltage, frequency and amount of current that goes into the lamp. Fluorescent lamps can’t be connected to the household electric supply except through a ballast. Starting in the mid-1980s, fluorescent light makers began switching from electromagnetic ballasts to more efficient solid-state electronic ballasts.

How Lights Work

To understand what electronic ballasts do, you need to know how fluorescent lights produce illumination. They work by passing a stream of high-energy electrons between positive and negative electrodes located at each end of a glass tube filled with mercury vapor and an inert gas like argon or krypton. A starting current heats a filament to provide the electron stream. As the electrons collide with mercury atoms floating amidst the gas, they ionize the mercury atoms into a plasma, exciting the atoms to emit electrons vibrating in the ultraviolet light range. The invisible ultraviolet light strikes a phosphorescent compound coating the inside of the glass tube. The compound absorbs energy from the ultraviolet light, causing the compound to glow with visible light that illuminates the room.

Role of Ballast

Ballasts provide the power needed to heat the electrodes and establish the initial stream of high-energy electrons that excite the mercury plasma. They also regulate the amount of electric current flowing through the lamp to ensure stable light output. Old-style electromagnetic ballasts used transformer coils to step up the voltage from the 120 volts of line current to the roughly 400 volts needed to excite the mercury plasma. The new electronic ballasts leave voltage unchanged and instead use solid-state electronic devices to step up the alternating current’s frequency from the 60 cycles per second of line current to 20,000 cycles per second. The high frequency AC imparts the energy needed to excite the mercury plasma and start the fluorescence.

Electronic Mandate

Since 2006, federal law has mandated electronic ballasts in all new fluorescent lights installed in office, commercial and industrial buildings, and as of 2010 mandated all replacement ballasts in those fixtures must be electronic. Electronic ballasts are favored because they use roughly 25 percent less electricity to produce 10 percent more light from standard fluorescent tubes. That translates to estimated annual energy savings of between $6 and $7 per two-tube fluorescent fixture.

Quiet Electronics

Electronic ballasts also don’t emit the humming noise common with electromagnetic ballasts because they don’t have vibrating laminated transformer cores. They also are lighter, weighing about a third less then electromagnetic ballasts, and they produce flicker-free light. Electronic ballast makers have models with the same physical footprint as electromagnetic ballasts so they can exactly replace worn out electromagnetic ballasts.

References

• Phillips Advanced Lighting: ABCs of Electronic Fluorescent Ballasts
• National Lighting Products Information Program: Electronic Ballasts