Fluorescent lamp - a source of low pressure light, where ultraviolet radiation, as a rule, mercury discharge is converted by a layer of phosphor deposited on the walls of the flask of the device into visible. Consider the difference between devices and halogen and other similar devices.
The history of the development of fluorescent lamps
Fluorescence phenomena began to be studied in the 19th century. Among the scholars we single out Michael Faraday, James Maxwell and George Stokes. The most remarkable invention is called the Gissler flask. This scientist tried to pump out air using a mercury pump. Discharge in the flask reached a high level - before it was not possible to create such conditions. At the same time, the released volume was filled with mercury vapor. Gissler discovered that by placing the electrodes at the two ends of a long bulb and applying voltage to them, he sees a green glow.
This is a glow discharge, the basis of the devices today. At low pressure, an electron beam forms between the cathode and the anode. In some places, elementary particles collide with few ions of gas, giving up energy. Due to electron transitions to new levels, luminescence is formed, the color depends on the chemical element used and other conditions. Gissler tubes from the 80s of the 19th century put into mass production. Mainly for entertainment and other related purposes. For example, the famous neon signs.
The causes of fluorescence varied. Often the effect was provoked by electromagnetic radiation. Famous entrepreneur Thomas Edison experimented with strands of calcium, exciting them with X-rays. Similar works were done by Nikola Tesla.
Varieties of luminescence
According to the reasons causing the phenomenon, luminescence is divided into classes:
- Cathodoluminescence occurs in the Gissler tubes.
- Photoluminescence: the glow of substances under the action of waves close to the visible range.
- Radioluminescence is identical to the previous one, exciting waves of greatly reduced frequency.
- Thermoluminescence: luminescence is produced by heating the body.
- Electroluminescence is noticeable on the example of LEDs.
- Bioluminescence. A prime example of a class is the ocean floor population.
Fluorescent lamps belong to the discharge, the discussion will begin with the process of ionization. Otherwise it will be uninteresting because of ignorance of the basis. Before the advent of LEDs, discharge lamps showed high luminous efficiency. They are up to 80% more economical than devices with filaments. A glow discharge is formed in a gas, vapor or mixture medium. When the medium is already ionized, there are no difficulties, but at the start it is necessary to use extremely high voltages reaching units of kV.
The discharge lamp with a small exception - in screwdrivers-indicators - works in tandem with the starter. Sometimes this part is incorrectly called ballast. These are different things:
- A starter( control gear) is the part of the circuit where a high voltage is generated to start the arc. As a result of an abrupt jump in the thickness of the gas or vapor, it breaks through, ionizes, and conducts current. Then the need to maintain high voltage on the electrodes disappears. The control gear works exclusively at the start.
- Ballast refers to a set of devices designed to compensate for the negative resistance of a fluorescent lamp. As the current rises, the conductivity between the electrodes increases. This process does not take an avalanche-like character; it excludes equipment failure due to ballast connected in series in a chain. It limits current growth to a specific level.
Ballast and ballast are difficult to separate. For example, a choke creates a sharp voltage surge at the right moment, its impedance simultaneously limits the amount of current.
. The principle of ignition of the arc and the design of the discharge lamp.
Fluorescent lamp consists of a long glass bulb, at the ends of which there are contact pads with electrodes. The design feature is such that in parallel with the lamp it is necessary to include part of the ballast. The electrode has two outlets, resembling a tungsten horseshoe. The difference between fluorescent lamps: a special substance that is illuminated by ultraviolet radiation is applied to the walls of a glass bulb. Recall that inside there are mercury vapor or a substance capable of maintaining a glittering discharge in the volume with the desired frequency of the wave at a relatively low starting voltage.
Let's figure out how the ignition. Parallel to the fluorescent lamp, a bimetallic relay is switched on. Through it, a small discharger feeds on the network voltage. It represents a greatly reduced copy of the main lamp and 220 V is enough for ionization. The glow discharger gradually warms up the bimetallic relay producing power. As the temperature rises, the contacts open. As a result, the arrester goes out, and the bimetallic relay, after a certain period, closes again. The cyclic process takes 1-2 seconds.
Let's see how to use the described fixture to kindle a fluorescent lamp. The effective voltage value of 220 V is not enough to ionize the gas in the flask. The designers went to the original course - they used a choke. This is an inductance coil with two windings on a common core. Wound so that with the sudden disappearance of a voltage surge of large amplitude. Description of work in the complex:
- Fluorescent lamp is powered through the choke, they are connected in series. The starter is connected in parallel to the flask through the horseshoe electrodes.
- As a result, if there is voltage at the initial moment of time, the arrester lights up and heats the relay. Contact resistance is low, 220 V is applied to the choke. There begins the process of storing reactive power.
- When the discharger heats the contacts of the bimetallic relay strongly, it breaks the circuit. As a result, the power to the choke disappears, resulting in an abrupt voltage surge. This causes a response, the amplitude of the pulse increases many times( to units of kV).
- The potential difference across the electrodes of a fluorescent lamp becomes so large that it ionizes the gas in the flask. The glow discharge process starts.
- As a result, the voltage on the starter drops, the discharger no longer ignites.
This is how the fluorescent lamp arc is ignited in standard mode.
fluorescent lamp circuit The system is called electrode preheating. As the bimetal relay is heated, the current passes through tungsten horseshoes, raising the temperature and facilitating the ignition process. If the room is too cold, the first time the process fails. Then the cycle repeats, the temperature of the tungsten electrodes becomes slightly higher. It looks like a quick blink of light when the switch is closed.
How to light a burnt fluorescent lamp
More often, a fluorescent lamp burns tungsten electrodes in the shape of a horseshoe. Then, through it, it is no longer possible to apply power to the starter connected in parallel with the flask. The scheme shown in the figure below is used. At the electrodes of the lamp is constantly maintained a high voltage( above 600 V).This ensures a glow discharge. The operating mode of the fluorescent lamp becomes intense, and the device will not be able to function for a long time.
Note that from the outside both outputs of each electrode are short-circuited. This ensures the work of remaining inside the tungsten electrode bits. Diodes are used for the correct switching of each half-wave of the supply voltage, the capacitors bring the level of potential difference to the specified one.
Difference between a fluorescent lamp and an
discharge lamp. The main feature of these devices is the presence of phosphor on the walls of the flask. The phenomenon of luminescence has been observed since ancient times. The most well-known property of phosphorus.
Many crystals under the action of ultraviolet radiation begin to glow, but the temperature does not change. Recall the law of wine for a completely black body. He states that the maximum radiation depends on the temperature and increases with its increase. To make the body red, its surface becomes hot, 500 degrees and higher. Other colors go higher in the spectrum, which means that the temperature rises more.
But the phenomenon of luminescence appears under normal conditions, even the frost is not a hindrance. It is known that at an absolute zero temperature the continuous emission spectrum of some bodies becomes simply discrete. Instead of a chaotic stream of quanta, orderliness is outlined. The phenomenon of luminescence does not disappear. This is easy to explain:
- At elevated temperatures, electrons pass between levels in a completely chaotic way. Each body glows when heated, depending on the specific temperature. For example, strong metals easily reach the desired condition, and the tree first turns black, actively being oxidized by atmospheric oxygen.
- The luminescence phenomenon is based on the principle of absorption of waves of a certain frequency by the body. Most often it is infrared or ultraviolet. The easiest way to give an example with a ball "pen for spies."Its ink characteristically glows when exposed to ultraviolet waves. Although first the paper looks white.
In a similar way, each body exhibits an absorption spectrum, and radiation occurs at a reduced wave. This is due to the fact that some of the energy incident on the material is dissipated as heat. It is said that the body radiates in the Stokes( on behalf of the scientist) region of the spectrum. There are substances in which the luminescence wave is higher than the exciting one. Then they say that the body glows in the anti-Stokes region of the spectrum. Finally, there are materials exhibiting both kinds of properties.
In the case of fluorescent lamps, the excitation wave is formed by a glow discharge of mercury vapor and lies in the ultraviolet range. The light emitted by the phosphor is visible. And here we come to an important characteristic - color temperature. If the phosphor gives a bright white light, they say, the shade is cold. This is good for creating a working rhythm of the brain. And the lamps are called daylight. More often found in practice.