Incandescent Lamp Working and Bulb Construction

Incandescent Lamp
(a) Vacuum Lamps / Filament Lamps :

• These lamps can operate upto the temperature of 2000°C. The construction is shown in the Figure (A)
• The filaments used for this type of lamp are carbon, osmium, tantalum or tungsten. Tungsten is best of all.
• The filament materials have the properties:

1. High melting point.
2. Low vapour pressure.
3. High resistivity.
4. Low temperature coefficient.
5. Ductile.
6. Mechanical strength.

• The bulb is evacuated so that filament is not oxidized even at higher temperatures. Melting point of tungsten is 3400°C.
• The filament heats up immediately and its resistance increases immediately. Its luminous efficiency is 10 lumens/watts.
• As the time passes aging of tungsten starts and its efficiency decreases. The total depreciation of light output is roughly 15% over the useful life range.
• There is a effect on the performance of the lamp due to voltage variations. 5% increase in voltage increases lumens output by 20% but shortens the life by 50%.

• 5% decrease in the voltage reduces 20% lumens output but increases the life of lamp by two times. The normal life of filament lamp is 1000 working hours.

Advantages :

1. Operating p.f. is unity.
2. Operates directly on normal voltages.
3. Good radiation characteristic in luminous range.
4. Available in various shapes and sizes.
5. Rating 15 watts, 25 watts, 40 watts, 60 watts, 100 watts, 200 watts and 500 watts.
6. Very cheap in cost.
7. Handy and easily fixed up.
8. Produces yellow light.

Disadvantages :

1. Life is less compared to other lamps.
2. Power consumption is more.
3. Filament may break due to vibrations, shocks.
4. Frequent switching produces thermal stress on filament.

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