1. INTRODUCTION
Fluorescent lamps have applications in most areas
of lighting, where they give longer lifetime and lower
power consumption for equivalent light output
compared to filament bulbs, with the disadvantage
of higher initial cost. In operation, a voltage is applied
across the ends of a sealed glass tube containing
mercury vapour, causing the vapour to be ionised.
This ionised vapour radiates light in the ultra violet
part of the spectrum, which is converted to visible
light by a fluorescent coating on the inside of the
tube.
1.1 Driving a fluorescent tube
When the lamp is first turned on, the mains voltage
across it is not sufficient to cause the initial ionisation
of the vapour. A starting element is thus needed to
provide a high voltage pulse across the tube to start
the process.
Once the gas in the tube is ionised however, its
impedance becomes negative; that is to say the
more current that flows through the lamp, the more
conductive it becomes (this is because the increased
current increases the degree of ionisation of the
gas). Therefore, some form of limiter must be added
to prevent the current increasing to a level where the
lamp is destroyed. The current limiter is usually
known as a ballast