Fluorescent Lamps

Author: E.E. Kimberly

Fig. 26-1. Fluorescent Lamp

The fluorescent lamp, shown in Fig. 26-1, consists of a tubular bulb with an electrode sealed in each end. A small amount of mercury sealed in the bulb produces a vapor, which is a conductor of electricity. When a sufficient voltage is applied to the electrodes, the column of vapor glows weakly. However, the inside of the tube is coated with a phosphor powder which, when excited by the mercury-vapor glow, responds by glowing brightly and is said to be "luminescent." The various dominant colors available from these lamps are obtained by proper choice of phosphor used in the coating, and the shades are affected by the vapor pressure.

Fig. 26-2. Choke Coil for Fluorescent Lamp

Fluorescent lamps, in common with all other electric-discharge sources of light, require auxiliary control devices. One of these devices, which is shown in Fig. 26-2, is usually an iron-core choke coil or impedance coil, called a ballast, in series with the lamp. When the lamp is turned on, the choke coil permits a voltage which will start the discharge; but, by its impedance, it reduces the voltage across the lamp after the discharge current begins to flow. Another of these control devices, shown in Fig. 26-3, is a starting switch which momentarily closes the circuit through a heater coil in one end of the tube and then opens it after the glow has started. The low power factor caused by the ballast reactor is improved by the addition of a shunt condenser or by using the lamps in pairs, one lamp of each pair having a condenser element in its ballast.

A tube with a heater in only one end will glow during only one half of every cycle, and would produce stroboscopic effects that are undesirable for most uses. For this reason, most fluorescent tubes are provided with a heater filament in each end, so that they glow during both halves of every cycle.

Fig. 26-3. Starter for Fluorescent Lamp

Fig. 26-4 shows such a tube with its ballast and starter. Fig. 26-5 shows a circuit diagram similar to that of Fig. 26-4 but including also a condenser which has been added to improve the power factor. Fig. 26-6 shows a circuit in which two lamps are used with a ballast having a high power factor.

Fig. 26-4. Fluorescent-Lamp Circuit With Ballast and Starter Switch

Fig. 26-5. Circuit for Single Fluorescent Lamp With Ballast Having High Power Factor

Fig. 26-6. Circuit for Two Fluorescent Lamps With Ballast Having High Power Factor

Fluorescent lamps have a spectrum which approaches daylight more closely than does that of incandescent lamps, and fluorescent lamps are sometimes preferred for that reason. In general, they lose their usefulness because of decrease in light output caused by darkening before they fail to operate.