How fluorescent lamps work

When the switch is turned on, the power supply voltage is immediately applied to the two poles of the starter through the ballast and the lamp filament. The voltage of 220 volts immediately ionizes the inert gas of the starter to produce a glow discharge. The heat of the glow discharge causes the bimetal to thermally expand and contact the two poles. The current forms a path through the ballast, the starter dipole and the filaments at both ends. The filament is quickly heated by the current and emits a large amount of electrons.

At this time, since the two poles of the starter are closed, the voltage between the two poles is zero, the glow discharge disappears, and the temperature inside the tube is lowered; the bimetal is automatically reset and the two poles are disconnected. At the moment when the two poles are disconnected, the circuit current is suddenly cut off, and the ballast generates a large self-induced electromotive force, which acts on both ends of the tube after being superimposed with the power supply voltage. A large amount of electrons emitted when the filament is heated is moved from a low potential end to a high potential end at a great speed under the action of a high voltage across the tube. During the acceleration process, the argon molecules in the tube are collided to rapidly ionize. Argon gas ionizes heat, which causes the mercury to generate vapor, which in turn is ionized and emits intense ultraviolet light. Under the excitation of ultraviolet light, the phosphor inside the tube wall emits almost white visible light.

After the fluorescent lamp is normally illuminated. Since the alternating current continuously passes through the coil of the ballast, a self-induced electromotive force is generated in the coil, and the self-induced electromotive force hinders the current change in the coil. At this time, the ballast acts as a step-down current limiting, so that the current is stabilized in the rated current range of the lamp. Inside, the voltage across the lamp is also stable within the rated operating voltage range. Since this voltage is lower than the ionization voltage of the starter, the starter connected in parallel at both ends will no longer function.

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