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Fluorescent Light - Starting And Running The Discharge

switch current electrodes voltage

Unlike the electrical circuit for an incandescent lamp, which contains only a switch, the control circuit for a fluorescent lamp must do two things. It must first provide a high voltage spike to strike the discharge, and it must thereafter control the current and voltage once the discharge is stable. The latter is important because the discharge itself is unstable and will terminate if the current is not controlled externally.

There are several types of starter circuits which all do two things. They supply a large current to the electrodes in order to produce electrons via thermioemission (the electrons "boil off" as the electrodes heat up) and they supply a high voltage to strike the discharge. Typical examples of these include the switch start, instant-start, and rapid start. The switch start has the advantage of being actively controlled and therefore avoids the misfirings which can have the deleterious effect of removing the coating on the electrodes and thus shorten the tube's life.

The switch is initially closed, thus shorting the electrodes and allowing a large current to flow which heats the electrodes to their operating temperature. After a short time (1-2 seconds), the switch is opened. The large voltage spike created by the sudden reduction of current through the ballast (an inductor) then strikes the discharge and the lamp lights up. The capacitor reduces the reactance of the inductive ballast.

The switch used to be an argon glow tube with a bimetallic electrode, but this function has been replaced in recent years with solid state circuitry which can be actively controlled.

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