Light emitting diodes, commonly called LEDs, are real unsung heroes in the electronics world. They do dozens of different jobs and are found in all kinds of devices. Among other things, they form numbers on digital clocks, transmit information from remote controls, light up watches and tell you when your appliances are turned on. Collected together, they can form images on a jumbo television screen or illuminate a traffic light.
Basically, LEDs are just tiny light bulbs that fit easily into an electrical circuit. But unlike ordinary incandescent bulbs, they don't have a filament that will burn out, and they don't get especially hot. They are illuminated solely by the movement of electrons in asemiconductor material, and they last just as long as a standard transistor. The lifespan of an LED surpasses the short life of an incandescent bulb by thousands of hours. Tiny LEDs are already replacing the tubes that light up LCD HDTVs to make dramatically thinner televisions.
In this article, we'll examine the technology behind these ubiquitous blinkers, illuminating some cool principles of electricity and light in the process.
What is a Diode?
A diode is the simplest sort of semiconductordevice. Broadly speaking, a semiconductor is a material with a varying ability to conduct electrical current. Most semiconductors are made of a poor conductor that has had impurities (atoms of another material) added to it. The process of adding impurities is called doping.
In the case of LEDs, the conductor material is typically aluminum-gallium-arsenide (AlGaAs). In pure aluminum-gallium-arsenide, all of the atoms bond perfectly to their neighbors, leaving no free electrons (negatively charged particles) to conduct electric current. In doped material, additional atoms change the balance, either adding free electrons or creating holes where electrons can go. Either of these alterations make the material more conductive.
A semiconductor with extra electrons is called N-type material, since it has extra negatively charged particles. In N-type material, free electrons move from a negatively charged area to a positively charged area.
A semiconductor with extra holes is called P-type material, since it effectively has extra positively charged particles. Electrons can jump from hole to hole, moving from a negatively charged area to a positively charged area. As a result, the holes themselves appear to move from a positively charged area to a negatively charged area.
A diode consists of a section of N-type material bonded to a section of P-type material, with electrodes on each end. This arrangement conducts electricity in only one direction. When no voltage is applied to the diode, electrons from the N-type material fill holes from the P-type material along the junction between the layers, forming a depletion zone. In a depletion zone, the semiconductor material is returned to its original insulating state -- all of the holes are filled, so there are no free electrons or empty spaces for electrons, and charge can't flow.
To get rid of the depletion zone, you have to get electrons moving from the N-type area to the P-type area and holes moving in the reverse direction. To do this, you connect the N-type side of the diode to the negative end of a circuit and the P-type side to the positive end. The free electrons in the N-type material are repelled by the negative electrode and drawn to the positive electrode. The holes in the P-type material move the other way. When the voltage difference between the electrodes is high enough, the electrons in the depletion zone are boosted out of their holes and begin moving freely again. The depletion zone disappears, and charge moves across the diode.
If you try to run current the other way, with the P-type side connected to the negative end of the circuit and the N-type side connected to the positive end, current will not flow. The negative electrons in the N-type material are attracted to the positive electrode. The positive holes in the P-type material are attracted to the negative electrode. No current flows across the junction because the holes and the electrons are each moving in the wrong direction. The depletion zone increases.
How Can a Diode Produce Light?
Lightis a form of energy that can be released by anatom. It is made up of many small particle-like packets that have energy and momentum but no mass. These particles, called photons, are the most basic units of light.
Photons are released as a result of moving electrons. In an atom, electrons move in orbitals around the nucleus. Electrons in different orbitals have different amounts of energy. Generally speaking, electrons with greater energy move in orbitals farther away from the nucleus.
For an electron to jump from a lower orbital to a higher orbital, something has to boost its energy level. Conversely, an electron releases energy when it drops from a higher orbital to a lower one. This energy is released in the form of a photon. A greater energy drop releases a higher-energy photon, which is characterized by a higher frequency. (Check out How Light Works for a full explanation.)
As we saw in the last section, free electrons moving across a diode can fall into empty holes from the P-type layer. This involves a drop from the conduction band to a lower orbital, so the electrons release energy in the form of photons. This happens in any diode, but you can only see the photons when the diode is composed of certain material. The atoms in a standard silicon diode, for example, are arranged in such a way that the electron drops a relatively short distance. As a result, the photon's frequency is so low that it is invisible to the human eye -- it is in the infrared portion of the light spectrum. This isn't necessarily a bad thing, of course: Infrared LEDs are ideal for remote controls, among other things.
Visible light-emitting diodes (VLEDs), such as the ones that light up numbers in a digital clock, are made of materials characterized by a wider gap between the conduction band and the lower orbitals. The size of the gap determines the frequency of the photon -- in other words, it determines the color of the light. While LEDs are used in everything from remote controls to the digital displays on electronics, visible LEDs are growing in popularity and use thanks to their long lifetimes and miniature size. Depending on the materials used in LEDs, they can be built to shine in infrared, ultraviolet, and all the colors of the visible spectrum in between.
A tube light works by using the principle of fluorescence. The fluorescent tube contains two filaments, one at each end of the tube, which glow to heat up the gas contained inside the tube.
The inner surface of the tube is coated with compound of elements such as phosphorus having fluorescence. While manufacturing, the tube is filled with a small amount of a gas such as mercury vapor (for a white color), carbon dioxide (for green), neon (for red color), etc.
The power supply is delivered to the tube through starter and choke. When the supply is turned on, the two filaments glow and the contacts of the starter becomes open. This action provides a high voltage across the tube and this ionizes the warmed-up gas inside the tube. This ionized gas excites the fluorescent coating inside of the tube so that it gives out visible light.
Choke:
The choke is a current limiter. A high initial voltage is required to fire the tube, and then the current must be limited to keep the bulb from burning up.
Most ballasts nowadays are actually solid state electronic. They can run the tube at a higher frequency than the AC line making it more efficient, last longer, and correct the power factor making the load on the electrical system less.
All circuit of neon tube light is like this: L(live), inductor, first heater, starter, second heater, N(neutral) connected in series. Starter is gas glow bulb where one of electrodes is made of bi-metal, and parallel capacitor for RFI. When you switch on starter bulb is short circuited and heating current flows through series circuit. After some time (few second) gas bulb is heated enough and bi-metal electrode break the series circuit. Sudden reduce in circuit current causes induction of high voltage on inductor and so on the opposite electrodes of neon tube. Neon tube starts to glow giving the light. At the same time the starter bulb continue to glow and keep hot preventing bi-metal electrode to close again. Start of neon tube may be clean or through few attempts (flickering). Inductor limits the neon tube voltage when it glows to abt. 50V.
Starter:
Fluorescent startersare used inseveral typesof fluorescent lights. The starter is there to help the lamp light. Whenvoltageis applied to the fluorescent lamp, here's what happens:
The starter (which is simply a timed switch) allows current to flow through the filaments at the ends of the tube.
The current causes the starter's contacts to heat up and open, thus interrupting the flow of current. The tube lights.
Since the lighted fluorescent tube has a low resistance, the ballast now serves as a current limiter.
When you turn on a fluorescent tube, the starter is a closed switch. The filaments at the ends of the tube are heated by electricity, and they create a cloud of electrons inside the tube. The fluorescent starter is atime-delay switch that opens after a second or two. When it opens, the voltage across the tube allows a stream of electrons to flow across the tube and ionize the mercury vapor.
Without the starter, a steady stream of electrons is never created between the two filaments, and the lamp flickers. Without the ballast, the arc is a short circuit between the filaments, and this short circuit contains a lot of current. The current either vaporizes the filaments or causes the bulb to explode.
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!! I HOPE THIS HELPED YOU !!
Thursday, 5 April 2012
HOW RADIO WAVES ARE PRODUCED
Introduction: Light Waves and the Electromagnetic Spectrum
Light consists of electromagnetic (EM) waves. An EM wave is composed of an electric field and a magnetic field that are oscillating together. The fields are oriented perpendicular to each other, and the wave travels in a direction perpendicular to both of the fields (see image at right). These waves can also be thought of as particles called photons: massless packets of energy that travel at the speed of light. In fact, EM radiation behaves as both a particle and a wave at the same time. EM waves can be characterized by any of three properties: wavelength (λ) - the distance between two adjacent crests of the wave, frequency (f) - the number of wave oscillations per second, or the energy (E) of the individual photons in the wave. For all types of EM radiation, the simple relationships between wavelength, frequency, and energy are:
where wavelength is measured in units of length such as meters (where 1 cm = 10-2 meters, 1 micrometer = 10-6 meters, etc.), frequency is measured in units of Hertz (Hz), where 1 Hz = 1 wave crest per second (e.g. 1 MHz = 106 Hz, 1 GHz = 109 Hz); c is the speed of light, which is about 3 x 108 meters per second (or 186,000 miles per second); and h is Planck's constant, which is equal to 6.63 x 10-27 erg/s, where an erg is a unit of energy. Remarkably, all forms of EM radiation (visible light, x-rays, radio waves, etc.) travel at the speed of light, regardless of their energy. Since the energy of an EM wave is directly proportional to its frequency and inversely proportional to its wavelength, the higher the energy of the wave, the higher the frequency, and the shorter the wavelength.
The different wavelengths of EM radiation cause the radiation to react differently with different materials, such as our eyes or detectors in telescopes. The way visible light of different wavelengths interacts with our eyes gives rise to "colors", with the shorter wavelengths (about 0.0004 mm) appearing as blue light and the longer wavelengths (about 0.0007 mm) appearing as red light. Even shorter wavelengths of EM radiation (such as x-rays) can pass right through tissues in our bodies. Radiation at longer wavelengths (e.g. infrared) cannot be seen by our eyes, but can be felt as heat. Radio waves are EM waves with the longest wavelengths, from 1 mm - 100 km. The image below shows the entire electromagnetic spectrum, from shorter wavelengths to longer wavelengths.
Short wavelength High frequency High energy
Long wavelength Low frequency Low energy
Just as the EM spectrum is divided up into different regions depending on wavelength, the radio region of the EM spectrum can also be divided up into different regions or "bands". These are the bands in which astronomers use radio telescopes to observe the radio waves emitted by astronomical objects. The most common radio band names and their corresponding wavelengths/frequencies are:
Band
Wavelength
Frequency
P-band
90 cm
327 MHz
L-band
20 cm
1.4 GHz
C-band
6.0 cm
5.0 GHz
X-band
3.6 cm
8.5 GHz
U-band
2.0 cm
15 GHz
K-band
1.3 cm
23 GHz
Q-band
7 mm
45 GHz
Astronomers must build special telescopes and detectors in order to detect EM radiation of different wavelengths. For example, optical telescopes are designed similar to the human eye, with a lens to focus incoming light onto a detector. Since radio waves have a much longer wavelength than optical light, radio telescopes are designed much differently, although the basic principles are the same. Learn more about how radio telescopes work.
Electromagnetic radiation is emitted by charged particles such as electrons when they change speed or direction (or accelerate). In the following sections we will see how this basic concept applies to the primary astrophysical processes that emit radiation at radio wavelengths.
In general, electromagnetic radiation is emitted by one of two means, eitherthermal or non-thermal mechanisms. Thermal emission, which depends only on the temperature of the emitting object, includes blackbody radiation,free-free emission in an ionized gas, and spectral line emission. Non-thermal emission, which does not depend on the temperature of the emitting object, includes synchrotron radiation, gyrosynchrotron emission, and amplified emission from masers in space.
Thermal Emission
Blackbody Radiation
Thermal emission is perhaps the most basic form of emission for EM radiation. Any object or particle that has a temperature above absolute zero emits thermal radiation. The temperature of the object causes the atoms and molecules within the object to move around. For example, the molecules of a gas, as in a planet's atmosphere, spin around and bump into one another. When the molecules bump into each other, they change direction. A change in direction is equivalent to acceleration. As stated above, when charged particles accelerate, they emit electromagnetic radiation. So each time a molecule changes direction, it emits radiation across the spectrum, just not equally. As a result, the amount of motion within an object is directly related to its temperature.
You can explore this for yourself by placing a cast-iron pan on a stove, heating it for a few minutes, and then placing it to the side. It is hot enough to be emitting a noticeable amount of infrared radiation (or heat), which you can detect by placing your hands near it. If you were to put more heat into the iron, it would eventually emit higher and higher energy wavelengths, until it would glow on its own, emitting visible light as well as infrared radiation.
Scientists call this blackbody radiation. A blackbody is a hypothetical object that completely absorbs all of the radiation that hits it, and reflects nothing. The object reaches an equilibrium temperature and re-radiates energy in a characteristic pattern (or spectrum). The spectrum peaks at a wavelength that depends only on the object's temperature. All objects in the universe behave this way. The image at left shows blackbody spectra for objects at three different temperatures: 5000 K, 4000 K, and 3000 K. It is apparent from the image that objects at lower temperatures emit more radiation at longer wavelengths. The objects in the image emit at or near the visible range of the electromagnetic spectrum; in order for an object to emit thermal radiation at radio wavelengths, it must be much colder than these objects.
The unit of temperature that astronomers typically use is called the Kelvin, and its symbol is K (no degree symbol is used). To convert from degrees Celsius to Kelvin, add 273 to the temperature in Celsius. So, if an object has a temperature of 100° C, its temperature in Kelvin is 100+273 = 373 K. Objects that are cooler than about 1000 K emit more infrared than visible light, such as the Earth orbrown dwarfs (dim, cool objects too massive to be planets but not massive enough to be stars). Hotter objects, like stars, emit mostly optical light. Very hot objects emit mostly ultraviolet radiation, such as white dwarfs (dying stars that have burned up all of the hydrogen in their cores). The major difference in the type of energy emitted by these objects is their temperature.
The Sun and other stars are, for all intents and purposes, considered blackbody radiators. By looking at the frequency or "color" of the radiation they emit, scientists can learn about the temperature of these bodies. For example, cooler stars appear red and hotter stars appear bluish-white.
One of the most famous examples of a "perfect" blackbody is known as the Cosmic Microwave Background (CMB) radiation. This is the radiation permeating the entire Universe that was released during the Big Bang explosion and has been cooling for the last 15 billion years. Today the CMB radiation is so cold (only 2.725 K, or -270° C), that most of the radiation is emitted at radio wavelengths of a few centimeters (also called "microwave" radiation; see image at right). Astronomers were able to obtain the spectrum of the microwave background using a specially designed satellite called the Cosmic Background Explorer (COBE). The blackbody nature of the microwave background spectrum matches the predictions of the Big Bang theory extremely accurately, thus confirming the theory that the microwave background radiation was created in the Big Bang explosion. The Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001, has observed the microwave background to an even higher level of sensitivity, giving astronomers greater insights into the origin and evolution of the Universe.
Another form of thermal emission comes from gas which has been ionized. Atoms in the gas become ionized when their electrons become stripped or dislodged. This results in charged particles moving around in an ionized gas or "plasma", which is a fourth state of matter, after solid, liquid, and gas. As this happens, the electrons are accelerated by the charged particles, and the gas cloud emits radiation continuously. This type of radiation is called "free-free" emission or "bremsstrahlung". The image at left shows the emission of a photon when a negatively charged electron (green particle) changes direction or accelerates due to the presence of a nearby positively charged ion (red particle). Some sources of free-free emission in the radio region of the EM spectrum include ionized gas near star-forming regions or Active Galactic Nuclei (AGN).
Spectral line emission involves the transition of electrons in atoms from a higher energy level to lower energy level. When this happens, a photon is emitted with the same energy as the energy difference between the two levels. The emission of this photon at a certain discrete energy shows up as a discrete "line" or wavelength in the electromagnetic spectrum.
An important spectral line that radio astronomers study is the 21-cm line of neutral hydrogen. This line is emitted by the following transition: the hydrogen atom consists of one electron orbiting one proton in the nucleus. Both the electron and the proton have a "spin". In the lowest energy state, or "ground" state, the spins of both particles are in opposite directions. When the atom becomes excited, either by absorbing a photon of energy, or by bumping into other atoms, the electron absorbs a small amount of energy, and the spin of the electron "flips," so that the spins of both particles are in the same direction. When the atom reverts back to its natural state, it loses this energy by emitting a photon with a wavelength of 21 cm, in the radio region of the electromagnetic spectrum.
Non-thermal Emission
Synchrotron Emission
Non-thermal emission does not have the characteristic signature curve of blackbody radiation. In fact, it is quite the opposite, with emission increasing at longer wavelengths.The most common form of non-thermal emission found in astrophysics is called synchrotron emission. Basically, synchrotron emission arises by the acceleration of charged particles within a magnetic field. Most commonly, the charged particles are electrons. Compared to protons, electrons have relatively little mass and are easier to accelerate and can therefore more easily respond to magnetic fields.
As the energetic electrons encounter a magnetic field, they spiral around it rather than move across it. Since the spiral is continuously changing the direction of the electron, it is in effect accelerating, and emitting radiation. The frequency of the emission is directly related to how fast the electron is traveling. This can be related to the initial velocity of the electron, or it can be due to the strength of the magnetic field. A stronger field creates a tighter spiral and therefore greater acceleration.
For this emission to be strong enough to have any astronomical value, the electrons must be traveling at nearly the speed of light when they encounter a magnetic field; these are known as "relativistic" electrons. (Lower-speed interactions do happen, and are called cyclotron emission, but they are of considerably lower power, and are virtually non-detectable astronomically).
As the electron travels around the magnetic field, it gives up energy as it emits photons. The longer it is in the magnetic field, the more energy it loses. As a result, the electron makes a wider spiral around the magnetic field, and emits EM radiation at a longer wavelength. To maintain synchrotron radiation, a continual supply of relativistic electrons is necessary. Typically, these are supplied by very powerful energy sources such as supernova remnants, quasars, or other forms of active galactic nuclei (AGN).
It is important to note that, unlike thermal emission, synchrotron emission is polarized. As the emitting electron is viewed side-on in its spiral motion, it appears to move back-and-forth in straight lines. Its synchrotron emission has its waves aligned in more or less the same plane. At visible wavelengths this phenomenon can be viewed with polarized lenses (as in certain sunglasses, and in modern 3-D movie systems).
Another form of non-thermal emission comes from masers. A maser, which stands for "microwave amplification by stimulated emission of radiation", is similar to a laser (which amplifies radiation at or near visible wavelengths). Masers are usually associated with molecules, and in space masers occur naturally in molecular clouds and in the envelopes of old stars. Maser action amplifies otherwise faint emission lines at a specific frequency. In some cases the luminosity from a given source in a single maser line can equal the entire energy output of the Sun from its whole spectrum.
Masers require that a group of molecules be pumped to an energized state (labeled E2 in the diagram at right), like compressed springs ready to uncoil. When the energized molecules are exposed to a small amount of radiation at just the right frequency, they uncoil, dropping to a lower energy level (labeled E1 in the diagram), and emit a radio photon. The process entices other nearby molecules to do the same, and an avalanche of emission ensues, resulting in the bright, monochromatic maser line. Masers rely on an external energy source, such as a nearby, hot star, to pump the molecules back into their excited state (E2), and then the whole process starts again.
The first masers to be discovered came from the hydroxl radical (OH), silicon oxide (SiO), and water (H2O). Other masers have been discovered from molecules such as methanol (CH3OH), ammonia (NH3), and formaldehyde (H2CO).
To get rid of the depletion zone, you have to get electrons moving from the N-type area to the P-type area and holes moving in the reverse direction. To do this, you connect the N-type side of the diode to the negative end of a circuit and the P-type side to the positive end. The free electrons in the N-type material are repelled by the negative electrode and drawn to the positive electrode. The holes in the P-type material move the other way. When the voltage difference between the electrodes is high enough, the electrons in the depletion zone are boosted out of their holes and begin moving freely again. The depletion zone disappears, and charge moves across the diode.
