What is a BALLAST?


A fluorescent lamp ballast is a device used to start and operate a fluorescent lamp and is a vital part of the lighting fixture.

It provides the three-step action needed by a fluorescent lamp: controlled energy to heat the electrodes (filaments); the right voltage to start the arc; and then impedance to limit the current to the proper value. To give proper lighting performance, the ballast must supply the specific electrical values established by the fluorescent lamp manufacturer.


The purpose of the ballast is to provide proper operation of the lamp.

A fluorescent lamp is an electrical discharge device. The lamp is filled with inert gas and a very small amount of vaporized mercury. The inner surface is coated with a fluorescent powder or phosphor. When proper voltage is supplied to the lamp, it ionizes the gas and sets the ions and the electrons in motion at high speeds between the electrodes (filaments) located at each end of the tube, forming an arc. The ballast supplies the energy to heat the filaments and the voltage to establish the arc.

Collisions between the electrons and the mercury atoms cause the emission of the ultraviolet energy. The fluorescent powder transforms this ultraviolet radiation into visible light. The ultraviolet wave lengths are contained within the lamp by the glass walls of the tube.

However, the ballast has another function: to prevent the destruction of the lamp. Unlike an incandescent bulb, if a fluorescent lamp were connected directly to an electrical outlet, it probably would not light. If it did light, the increase in current would soon destroy the lamp since, once the arc begins, the impedance (resistance) of the lamp drops to a low value.

So the ballast must provide additional impedance to limit the current to the proper value. Thus, the ballast controls the flow of the electricity, much as a valve controls the flow of water through a pipe. When improper electrical values are supplied by the ballast, the light output and life of the lamp may be greatly reduced.

Disclaimer: This information is intended to be general in nature and is not complete regarding ballasts or lighting fixtures. You should consult a qualified electrical engineer for specific applications. See our Terms and Conditions of sale and our warranty which are controlling and supersede this data.