SODIUM VAPOR LAMP

 A sodium vapor lamp is a cold cathode and low-pressure lamp. A sodium vapor discharge lamp consists of a U-shaped tube enclosed in a double-walled vacuum flask, to keep the temperature of the tube within the working region. The inner U-tube consists of two oxide-coated electrodes, which are sealed with the ends. These electrodes are connected to a pin-type base construction of a sodium vapor lamp. This sodium vapor lamp is a low luminosity lamp so that the length of the lamp should be more. In order to get the desired length, it is made in the form of a U-shaped tube. This longU[1]tube consists of a small amount of neon gas and metallic sodium. At the time of start, the neon gas vaporizes and develops sufficient heat to vaporize metallic sodium in the U-shaped tube.

Working

Initially, the sodium is in the form of a solid, deposited on the walls of the inner tube. When sufficient voltage is impressed across the electrodes, the discharge starts in the inert gas, i.e., neon; it operates as a low-pressure neon lamp with pink color. The temperature of the lamp increases gradually and the metallic sodium vaporizes and then ionizes thereby producing the monochromatic yellow light. This lamp takes 10–15 min to give its full light output. The yellowish output of the lamp makes the object appears gray.

 In order to start the lamp, 380 – 450 V of striking voltage is required for 40- and 100-W lamps. These voltages can be obtained from a high reactance transformer or an autotransformer. The operating power factor of the lamp is very poor so that a capacitor is placed to improve the power factor to above 0.8. More care should be taken while replacing the inner tube, if it is broken, then sodium comes in contact with the moisture; therefore, fire will result. The lamp must be operated horizontally or nearly so, to spread out the sodium well along the tube.

 The efficiency of the sodium vapor lamp is lies between 40 and 50 lumens/W. Normally, these lamps are manufactured in 45-, 60-, 85- and 140-W ratings. The normal operating temperatures of these lamps are 300°C. In general, the average life of the sodium vapor lamp is 3,000 hr and such bulbs are not affected by voltage variations.

Following are the causes of failure to operate the lamp, when:

 1. The cathode fails to emit the electrons.

2. The filament breaks or burns out.

3. All the particles of sodium are concentrated on one side of the inner tube.

4. The life of the lamp increases due to aging.

 The average light output of the lamp is reduced by 15% due to aging. These lamps are mainly used for highway and street lighting, parks, railway yards, general outdoor lighting, etc.