30 شهریور 1402

زمان مورد نیاز برای مطالعه : 10 دقیقه

' 10

Nearly 70 years have passed since the production of fluorescent lamps. These lamps are one of the most economical lamps for providing lighting due to their high light output and suitable light color. The light output of these lamps is 3 to 5 times higher than incandescent lamps and their lifespan is 10 to 20 times longer.

The bulb of fluorescent lamps is filled with low pressure mercury vapor gas, and as a result of electric discharge in mercury vapor gas, ultraviolet light in the range of 185 to 253.7 nm and a small amount of green-blue light are emitted depending on the chemical and gas pressure. This light turns into visible light when it hits the fluorescent inner coating layer. The inner surface of the bulb is covered with a thin layer of phosphor to absorb ultraviolet radiation and convert it into visible waves.

The efficiency of this operation is about 50%. Because the cathodes get hot when the lamp is turned on, these lamps are also called hot-cathode lamps. The cathode is covered with tungsten filaments with a thin layer of barium carbonate and when heated, provides additional electrons to help start the electrical discharge. Of course, excessive heating of this filament reduces the life of the lamp. To reduce the radiation of ultraviolet rays to the environment, they use glass impregnated with soda. Quartz is much more transparent than glass and only absorbs wavelengths shorter than 0.18 µm.

As a result, UV light bulbs are made of quartz. The amount of mercury used is low (mainly 12mg). Recently, in fluorescent lamps, an alloy of mercury with several other metals is used, which has reduced the amount of mercury to 5 grams.

All types of fluorescent lamps are widely used for providing lighting, especially indoor lighting, due to their high light output and suitable light color. The bulb of these lamps is filled with mercury vapor gas at low pressure and due to electric discharge in mercury vapor gas, ultraviolet light is emitted, this light is converted into visible light due to contact with the fluorescent inner coating layer.

The start of electrical discharge can be done in two ways:

1- Use of preheating electrode and current

2- Use of magnetic field (lamps without electrodes)

One type of fluorescent lamp is a fluorescent lamp with an electrode. To check the electrical discharge mechanism in this group of lamps, we first consider the electrical circuit of the fluorescent lamp:

At the first moment when the key is connected, the electric current passes through the starter. The starter consists of two metals that are placed at a small distance from each other inside a chamber filled with neon gas.

At the first moment, when the switch is connected, the line voltage is placed on these two metals, and as a result, electric discharge begins in the neon gas.

As a result of electric discharge, the space inside the chamber and then one of the metals, which is made of two different materials, gets heated and due to expansion, it tilts to the other side, until the two metals stick to each other.

When the two metals are connected to each other, the electrical discharge in the neon gas stops and the inside of the chamber starts to cool. During this time, the electric current passes through the choke, starter and electrodes located at both ends of the lamp and causes the gas inside the lamp to heat up

As the space inside the starter cools down, the metals contract and return to their original place. As a result, the electric current is suddenly interrupted. Since a choke is placed in the path of electric current, a sudden interruption of the current causes a high electric voltage to be induced in both ends of the choke, this voltage plus the network voltage of the two ends of the lamp falls and the electric discharge in the mercury vapor gas that previously heated by electrodes, it becomes possible.

If for any reason (such as low ambient temperature or wear of electrodes) the temperature of the lamp is not high enough, this voltage will not be enough for electric discharge and the lamp will not turn on.As a result, the electric discharge takes place again in the neon gas inside the starter and the cycle is repeated again and continues until the temperature of the gas inside the bubble increases enough to allow the electric discharge in it.

This is why the fluorescent lamp blinks before turning on.

To set up and work a fluorescent lamp, two basic operations must be done:

1- A relatively large voltage should be placed on both ends of the lamp so that the electric discharge starts and the lamp turns on.

2- After lighting the lamp, limit the circuit current.

The above mechanism can be implemented with the help of a choke (self) and starter in the way described, or with the help of an electronic circuit of the electronic ballast of the lamp. Fluorescent lamp starter electronic circuits have many advantages.

 We know that the electrons of each atom are circulating around the nucleus in certain orbits that have certain energy levels. Applying a sudden high voltage (in the way explained) causes these electrons to receive energy and leave their original energy levels and go to higher energy levels. After the current normalizes and the voltage reaches the grid voltage, the electrons They return to their previous levels and release the energy received in the previous stage in the form of ultraviolet light.

Ultraviolet light is converted into visible light by contact with fluorescent materials. In this category of lamps, the electrode plays an important role in starting the electrical discharge. Most of the fluorescent lamps used in the world are of this type.

The first generation of fluorescent lamps were lamps with a diameter of m38. These lamps were produced in the range of 40, 20 and 65 watts. Currently, the production and use of this type of lamps has been stopped in many countries of the world.