Lighting- control systems - Intelligent Buildings and Building Automation - page 209

The fluorescent lamp is one of the large family of discharge lamps. In these
lamps, light is generated by an electrical discharge within a gas or vapour.
Straight tubular lamps have been the main type of fluorescent lamps and are
still among the most commonly used fluorescent lamps today. Long life and
high efficiency are the main features of fluorescent lamps. Recent develop-
ments in the electronics industry allow electronic components (typically
ballasts) to be made very small in size to serve as the internal components
of a fluorescent lamp. This has allowed the lamps and associated electronic
components to be integrated as compact fluorescent lamps. Such lamps are
nowadays very widely used in the traditional applications of incandescent
lamps and special lamps.
10.2.2 Ballasts and dimming ballasts
Ballasts are the devices required to control the starting and operating voltages
of electrical gas discharge lamps, such as fluorescent lamps, neon lamps and
HID lamps. The lighting ballast is used to limit the flow of current through
a lamp, which can be a very simple resistor or rather more complex devices
(such as electronic ballasts).
Ballasts are necessary to operate discharge lamps because they have
negative resistance, meaning they are unable to regulate the amount of cur-
rent that passes through them. Therefore a ballast must be used to control
current flow, otherwise the lamp could fail. Electromagnetic ballasts use
electromagnetic induction to provide the starting and operating voltages of
gas discharge lamps.
Electromagnetic ballasts limit the flow of current to the light but do not
change the frequency of the input power. The lamp then illuminates on each
half-cycle of the power supply. This is why many fluorescent and neon lights
flicker visibly. Since the light illuminates on half-cycles, the rate of flicker is
twice the frequency of the power source, with a result that the light flickers
at 100 Hz or 120 Hz. A more modern type of ballast is electronic instead of
electromagnetic. Electronic ballasts use solid state circuitry to transform volt-
age. But unlike electromagnetic ballasts, they can also alter the frequency of
power. This means that electronic ballasts can greatly reduce or eliminate any
flicker in the lamps. Because it uses solid-state circuitry instead of magnetic
coils, it is also more efficient and therefore runs at a lower temperature.
The ballast can also be configured to change the current flowing through
the lamp while receiving a signal from a control device and afterwards,
and consequently achieving a gradual controlled reduction in lamp output.
Dimming ballasts are available for both linear and compact fluorescent
lamps. Dimming fluorescent lamps can provide significant benefits to owners
of commercial lighting systems including the following:
￿ Flexibility: allowing the lighting system to adapt to multiple activities and
the change of space use.