What is a Discharge Lamp : Design & Its Working

The Discharge lamps are one of the reliable sources of artificial light energy, which fundamentally works by converting electrical energy to light energy by ionizing the gas particles. Discharge lamp gained popularity, in the mid-twentieth century. The best part of the discharge lamps, it can produce lights of different colors. Due to its working principle, it is cheap, long-lasting, and can be used for a number of applications. For a period, these discharge lamps are even used in aircraft and vehicles. But due to the advent of technology, they have been replaced with CFL bulbs and LEDs.

Design Features of Discharge Lamp

As shown in the diagram, a discharge lamp consists of one gas tube, wherein a gas at low pressure is maintained. The gas tubes are filled with noble gasses like argon, neon, krypton, etc. Additionally, they are also filled with sodium mercury amalgam.

The sodium and mercury amalgam is required to draw an arc. At the end of the gas tube, two electrodes are placed, which produce very high voltage. The electrodes are excited by an alternating voltage source. In order to produce a high voltage, a ballast is maintained in series with the alternating voltage source. The ballast works on the inductor principle. In some advanced lamps, electronic ballast is also used.

Symbolically a discharge lamp is shown as above. It has shown an ellipse, representing the glass tube, and two symbols at the end representing the electrodes.

Working of Lamp

The discharge lamps work on the fundamental principle that, light is emitted when the electrons have collided with each other. To obtain this, the first gas an ionized at very high voltage. At low pressure, gas maintained in a glass tube. At the end of the tube, two electrodes are placed which are excited with an alternating voltage source.

When the source produces high voltage, then the gas inside the glass tube gets ionized due to the ionization of the molecules. The ionized electrons tend to move from one end to the other end. In this process, the ionized electrons collide with the unionized electrons and emit light. Based on the properties of the gas different colors of light can be produced.

Discharge Lamp Types

There are three types of discharge lamps which are given as

Low-pressure Discharge

In low-pressure discharge lamps, the working pressure is much less than the atmospheric pressure. Like the fluorescent lamps, and the sodium lamps. They produce 200 lumens per watt. One advantage of low-pressure discharge is they have a long life span. The low-pressure discharge lamps require the ballast to create a high voltage. Ballasts fundamentally work on the principle of inductance. The voltage across the inductor is directly proportional to the product of inductance and the rate of change of current (di/dt). With increase.

High-Pressure Discharge

The working pressure of these lamps is much higher than the atmospheric pressure. Different categories are there in high-pressure discharge lamps such as metal halide lamps, high-pressure sodium lamps, and high-pressure sodium mercury lamps.

High-intensity Discharge

The high-intensity discharge lamps are very efficient as compared to pressure based discharge lamps. They have a long life span and can save a lot amount of energy. The high-intensity lamps use an arc array to produce intense light. For this they need high voltage, i.e. they need ballasts just like fluorescent lights. The ballast helps to build up the high voltage.

For this reason, when turned on, the high-intensity lamps take up to ten minutes to reach peak value. The high-pressure mercury vapor lamp, metal halide lamps, and high-pressure sodium lamps are some of the examples of high-intensity lamps.

Lamp Examples

We shall see the high-intensity discharge lamps for this case. The mercury lamps are mostly used for street lighting due to their long span of life. They provide about 50 lumens per watt. They also provide huge energy savings while usage. The metal halide lamps are mostly used for indoor purposes like stadiums, outdoor areas, sports facilities, etc.

They provide very bright and intense light. They are much simple in construction as compared to mercury lamps and provide more lumens per watts. The high-pressure sodium lamps provide a warm light with a comparative high life span and more lumens per watt.

Advantages

The advantages of the discharge lamp are

• Long life
• Less in cost
• Can be used for a number of applications
• Fewer heat dissipations as compared to fluorescent bulbs.
• Can be designed for different colors

Disadvantages

The disadvantages of the discharge lamp are

• More costly as compared to fluorescent bulbs. This factor is replaced by CFL bulbs

Applications

The applications of the discharge lamp are

• Street Lighting
• Sport Arenas
• Aircrafts
• Gymnasiums
• Industries

Hence we have seen the principle, and features of discharge lamps. It is interesting to think about the replacement of the ballistic coil in the lamp. Can we replace the coil in the lamp? What could be other elements which will give the same performance?