ELECTRIC LAMPS – PART – 02 - OPERATION OF FLUORESCENT TUBE LIGHT WITH GLOW STARTER

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 MATERIALS

1. Zinc Silicate                   Green  5200

2. Calcium Tungstate          Blue  4600

3. Magnesium Tungstate      Blue weight  4800

4. Zinc Beryllium Silicate    Yellow white 6000

5. Cadmium Silicate            Yellow pink  6100

6. Cadmium Borate             Pink 6200

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.

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 a glass of tube of 25 mm diameter.

2. Length varies from 0.6 m to 1.5m

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]

STROBOSCOPE

Scientific instrument that provides a flashing light synchronized with the periodic movement of an object; can make moving object appear stationary.

APPLICATIONS OF STROBOSCOPE

1. In medicine, stroboscopes are used to view the vocal cords for 

    diagnosis of conditions that have produced dysphonia.      

   [Dysphonia is the medical term for disorders of the voice]

2. Flashing lamp strobes are used in night clubs and for lighting 

    effect where they give the impression of dancing in slow motion.

3. It is also used for frequency measurement.

STROBOSCOPIC EFFECT IN A TUBE LIGHT

In any type of discharge lamp this discharge is actually extinguishes twice per cycle of the supply, so there is a slight stroboscopic effect. [After every half-cycle, the current in an AC circuit is zero. At that instant the tube remains off.]

Due to this the periodic fluctuations in the light output of a lamp caused by the cyclic variations of the current on AC circuits.

This effect causes multiple image appearance of moving objects and makes the movement appear jerky.

This effect is more at lower frequencies.

METHODS OF MINIMIZING THE EFFECT

1. By using 3 lamps on the separate phases of a 3-phase supply.

2. By using twin tube lights with one of the chokes having a

    capacitor in series with it and the lamp.

3. By operating the lamp from a high frequency supply.