Incandescent Lamps - construction, working and types

Incandescent Lamps

There are two types of incandescent lamp which is mentioned below:

• 1. Vacuum lamps / Filament lamps

• 2. Gas filled incandescent lamp

These two types are described in Detail below.

1. Vacuum lamps /Filament lamps:

These lamps can operate upto the temperature of 2000°C. The construction is shown in below figure.

Construction Of incandescent lamp

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 of filament lamps / vacuum lamps:

✓ Operating power factor 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.

Disadvantages of filament lamps / vacuum lamps :

✓ 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.

2. Gas filled incandescent lamp

Construction:

Construction is same as vacuum lamps.

Working:

A metal filament can work in an evacuated bulb upto 2000°C without oxidation and it it is worked beyond this temperature it vaporises quickly and blackens the lamp.

For higher efficiency it is necessary to use working temperature more than 2000°C keeping down the evaporation, which is possible by filling the bulb with an inert gas-argon with a small percentage of nitrogen.

Nitrogen is added to reduce the possibility of arcing. Krypton is the best gas for this purpose but it is so expensive that it is used only in special purpose lamps, such as miner lamps.

The tungsten filament can safely be burn at temperature of 2400°C to 2750°C according to the size of the lamp.

However, due to presence of gas there is heat loss due to convection currents. This loss depends upon the surface area of filament.

As such, coiled coil filaments, which take much less space compared with coiled filaments are used with such lamps.

A coiled coil filament is made by winding tungsten wire on a fine iron wire to form a spiral which is again wound on to a thick iron wire to form a coiled coil. (Iron is later on dissolved out by acid).

Helical filaments also have a slower rate of tungsten evaporation. This evaporation

eventually causes bulb blackening because the tungsten vapour condenses as a black film on the inner surface of the bulb.

In a gas-filled lamp, the hot gas carries the tungsten vapour upward, Therefore, a black spot forms at the top of the bulb instead of spreading over the entire inner surface, as in a high-vacuum bulb. 

Chemicals called 'getters' are often placed

inside the bulb to capture tungsten vapour and thereby reduce the rate of blackening.

A piece of wire mesh called a collector grid

may also be attached to each lead in wire to attract the particles of tungsten vapour. Efficiency of coiled coil lamp is about 12 lumen/watt.

For low wattage lamps, however, the heat loss due to introduction of gas is more than in medium wattage lamps, so for low wattage (up to 40 watts) vacuum type lamps are used.

Operating Characteristics of Gas-discharge Lamps:

1. Starting:

In comparison with ordinary instant start filament lamps, the starting time and to achieve full brightness by the lamp is much more in two phases

(1) actual ignition

(2) run-up time phase

2. Burning position:

It depends on Lamp construction burying time depends on mechanical and thermal balance inside the lamp.

3. Run-up time:

As noted in 1st item, the first phase is ignition from this up to the stable operation is the run-up time.

4. Re-ignition time:

It is the time from extinguishing of the lamp and the moment when vapour pressure dropped for enough to allow the lamp to re-strike.

5. The emitted light:

If we compare incandescent lamp and gas discharge lamp we have to note that in case of incandescent lamps all the wavelengths in the visible region are present hence said to have a continuous spectrum.

Where as, in gas discharge lames limited no of wavelengths in the visible region are present called as line-spectrum. Again it is dependent on composition of gas, vapour pressure etc.

6. Operating temperature:

High temperature and use of reflectors affect the life of the lamps. Even there is a possibility of unsatisfactory performance and even failure in operation.

7. Ambient temperature:

Ambient temperature affects the striking voltage of gas discharge lamps. Suitably designed ballast therefore be used. Use of double envelope has no effect of ambient temperature.

8. Power factor improvement:

Ballasts are generally inductive type. When used in circuit the p.f. reduces hence to improve the p.f. a suitable value capacitor is to be used in parallel to ballast coil.

9. Voltage fluctuations of supply:

The lamp voltage is determined by vapour pressure and distance between the electrodes. 

Ballast should be capable to absorb these

fluctuations of supply voltage and to limit the current to give as far as possible constant

luminous flux.