A ballast can be as simple as a series resistor; this approach is commonly used with small neon lamps. For high-power lamps, though, too much energy would be wasted in a resistive ballast, so more complex circuits are used. These frequently depend upon the reactance of inductors, capacitors, or both. In a theoretical (perfect) reactance, no power would be lost while limiting the current flow.

Electromagnetic ballasts

An electromagnetic lamp ballast uses electromagnetic induction to provide the proper starting and operating electrical condition to power a fluorescent lamp, neon lamp or high intensity discharge (HID) lamp. The electromagnetic ballast used in an electric circuit limits the current flow to the lamp. It does not change the frequency of the power; the connected lamp illuminates on each half-cycle of the mains power so it flickers at 100 Hz (for most of the world) and 120 Hz (for those portions of the world that have 60 Hz mains frequency; see alternating current). Some ballasts that connect to two or more lamps vary the phase relationship between the multiple lamps to mitigate the flicker of the lamps. These ballasts are often called lead-lag ballasts because the current in one lamp leads the mains phase and the current in the other lamp lags the mains phase.

Prior to 1980 in the United States, PCB-based oils or asphalt were used for ballast insulation (see transformer oil). These ballasts tend to be large and heavy.

Electronic ballasts

An electronic lamp ballast uses solid state electronic circuitry to provide the proper starting and operating electrical condition to power one or more fluorescent lamps and more recently HID lamps. Electronic ballasts are generally smaller, lighter, and more efficient (and thus run cooler) than magnetic ballasts. Electronic ballasts usually change the frequency of the power from the standard mains frequency to 20,000 Hz or higher, substantially eliminating the stroboscopic effect line frequency (50 or 60Hz) flicker associated with fluorescent lighting (see photosensitive epilepsy) and lamps actually operate at about 9% higher efficiency above approximately 10KHz. Lamp efficacy increases sharply to about 10KHz and continues to improve until approximately 20KHz*(IES Lighting Handbook 1984)

There are three starting methods available for electronic ballasts.

Instant Start

Starts lamps without heating the lamp end heaters at all using a high voltage (around 600V). It is the most energy efficient type, but gives the least number of starts from a lamp. This is the best type for installations where lamps are not turned on and off very often.

Rapid Start

Applies voltage and heats the filament simultaneously. Provides superior lamp life and more cycle life, but uses slightly more energy for lamp end heaters.

Programmed Start

More advanced version of rapid start. Applies filament power first, then applies voltage to the lamps. Gives the best life and most starts from lamps. This is the preferred type of ballast for applications with very frequent power cycling such as vision examination rooms and restrooms with a motion detector switch.

They are often based on inverter/converter style power supplies, rectifying the input power and then chopping it at a high frequency, much like the first part of a switch-mode power supply.

Cite: Wikipedia