Incandescent Lamp
(a) Vacuum Lamps / Filament Lamps :
(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:
- High melting point.
- Low vapour pressure.
- High resistivity.
- Low temperature coefficient.
- Ductile.
- 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%.
(A) Vacuum/Gas filled incandescent Lamp
- 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.
- Operating p.f. is unity.
- Operates directly on normal voltages.
- Good radiation characteristic in luminous range.
- Available in various shapes and sizes.
- Rating 15 watts, 25 watts, 40 watts, 60 watts, 100 watts, 200 watts and 500 watts.
- Very cheap in cost.
- Handy and easily fixed up.
- Produces yellow light.
- Life is less compared to other lamps.
- Power consumption is more.
- Filament may break due to vibrations, shocks.
- Frequent switching produces thermal stress on filament.