ELECTRIC LAMPS - PART - 02 - OPERATION OF FLUORESCENT LAMP

ELECTRIC LAMPS - PART - `02
OPERATION OF FLUORESCENT TUBE LIGHT WITH GLOW STARTER [LOW PRESSURE MERCURY VAPOUR LAMP]
FLUORESCENCE
A few materials have the property of reflecting light incident on them; reflected light has a longer wave length than that of the incident light. This property is called fluorescence and the materials are known as fluorescent or phosphors.

FLUORESCENT LAMPS
Lamps which are coated with fluorescent, discharge ultra-violet radiations. They emit radiation of longer wavelength, in the visible spectrum.
Fluorescent lamps are more energy efficient than incandescent light bulbs of an equivalent brightness because more of the energy input is converted to usable light and less is converted to heat.
Fluorescent lamps require a ballast, which is a device used to provide and control the voltage in the lamp, and stabilize the current in the circuit.

FLUORESCENT TUBE LIGHT OPERATION
1. They produce light when an electric current passes between two electrodes (also called cathodes) in a tube filled with low-pressure mercury vapor and inert gases, such as argon and krypton.
2. Mercury is useful in lighting because it contributes to the bulbs' efficient operation and life expectancy.
3. The mercury within fluorescent lamps poses no health risk.
4. The electric current excites the mercury vapor in the tube, generating radiant energy, primarily in the ultraviolet (UV) range.
5. The energy causes a phosphor coating on the inside of the tube to "fluoresce," converting the UV light into visible light.

CONSTRUCTION DETAILS
1. It consists of a glass tube of 25 mm diameter.
2. Length varies from 0.6 m to 1.5 m
3. Inert gas used are Argon and krypton at low pressure (2.8
mm of Hg column) plus a few drops of mercury.
4. A tungsten filament which is coated with electron emitting
materials are connected across the ends of the tube.
Normally opened bimetallic strips start glow as a result
the strips are heated and are closed.
5. Voltage operated glow starter is used to start the operation
of the tube light.
6. A glow starter contains a glass bulb filled with a mixture of
hydrogen and helium used to produce a glow in the starter
and a bimetallic strip which is normally open.
7. A capacitor of 0.05 micro farad is connected across the
bimetallic strip
8. The capacitor gets charged due to the arc and radio
interference is controlled. The starter does not consume any
power.
9. A choke is provided as a ballast (an electrical device for
starting and regulating fluorescent and discharge lamps)
which gives high impulse about 1200 V for starting and later
on, maintain 230 V which is sufficient to maintain the
discharge.
10. When supply is switched on, full voltage appears across the
starter.
12. Due to small gap between the strips an arc is produced, the
strips are heated and are closed.
13. The circuit is closed and current flows through it.
14. After some time, the strips are cooled down and opened.
15. This interruption of current causes a very high voltage
induced in the choke which is sufficient to start the
discharge.
16. Once the discharge happens the starter is removed of
circuit as the discharge gets a low resistance path through
the tube.
17. Fluorescent tube lamp gives light like day light in which
colour distinction is possible.

ADVANTAGES
1. Its lamp efficacy is 40 lumens per watt.
2. The life of lamp is about 4000 hours.
3. Its light is uniform.
4. It gives pleasant light.
5. It consumes 50% less energy than incandescent lamp for the
same voltage.
6. It can be used in any direction.
7. It can be used in both ac and dc supply.

DISADVANTAGES
1. Stroboscopic effect.
2. Magnetic hum associated with choke causing disturbance.

APPLICATIONS
1. Linear fluorescent, U-tube, and Circline lamps are used for
general illumination purposes.
2. They are widely used in commercial buildings, schools,
industrial facilities, and hospitals.
3. Bug zappers contain a fluorescent lamp that emits ultraviolet
light, attracting unwanted insects.
[Bug zappers: An electrical device that can injure or kill by means of electric currents]