|
Arabic
Bengali
Chinese
English
French
German
Hebrew
Hindi
Italian
Japanese
Korean
Malay
Polish
Portuguese
Spanish
Turkish
Ukrainian
Vietnamese|
ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Super-bright LED - the basis of energy-saving lighting. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Lighting More recently, the author of this article witnessed how a peddler in a subway car advertised an LED lamp. "The super-bright bulbs of this lantern," the seller famously shouted over the noise of a moving train, "use little energy, which means you don't have to change batteries often." Probably, there is some advertising truth in his words: everyone knows about incandescent lamps, but to mention a fundamentally new light source - perhaps they will think about whether these super-bright LEDs are so good, and whether a flashlight made on their basis will serve so reliably is unknown. Although very, very many are aware of such a trivial task as using an LED as a light signaling device. You can even say that in terms of their prevalence, conventional LEDs can easily compete with incandescent lamps, and in everyday life they are very common today - just remember household switches with light indication designed to search for them in the dark. Modern LED signaling LEDs (Light Emitting Diode) are produced in huge quantities, have a different glow color, which is very convenient for signaling devices, different designs. You can purchase two-color models that smoothly change their color depending on the ratio of input signals, you can - flashing when voltage is applied, you can - with a standard base for replacing incandescent lamps in signal fixtures. But which of the standard LED is the light source in the sense in which we understand the light source? After all, the maximum of what it is enough for is to highlight the liquid crystal indicator of a mobile phone. Isn't it hard to imagine that a person can live normally in the light of semiconductor light sources, that he does his daily work, reads a book, has pleasant conversations in a cozy atmosphere... Would you say fantasy? No, this is just the reality of the present time. The property of emitting light waves by pn junctions is a fundamental property of all semiconductors. But they are endowed with this ability to varying degrees. For example, the silicon pn junctions used to make transistors and ordinary diodes are completely unsuitable even for ordinary LEDs: they emit very few light waves. Semiconductors based on gallium compounds (gallium phosphide and gallium arsenide) radiate much better, therefore it is on their basis that the well-known red, yellow-green and green LEDs are produced. The luminous efficiency of these devices in the 60s of the last century was only 1,5 lm / W. Somewhat later, the results of research made it possible to increase the radiation efficiency of semiconductors up to 10 lm/W. The development of technologies for the production of gallium nitride has led to the emergence of blue LEDs. And here it is time to think about LEDs emitting white light. White LEDs first appeared on the world market in 1998. The efficiency indicators of solid-state light sources achieved to date are not impressive: the luminous efficiency of commercial samples of LEDs emitting in the red-yellow part of the spectrum is 65 ... 75 lm / W, in the green region - up to 85 lm / W, and in the white region luminescence up to 100 lm/W. On the way - commercial samples of white light with an efficiency of about 150 lm / W, and this is not the limit. That is, on average, over the 50 years of the existence of solid-state sources, their efficiency has grown by almost two orders of magnitude. In general, the light output of a "very average" LED with a "white" emission spectrum today is at the level of the light output of a good fluorescent lamp, and the increase in light output continues. And the high cost of producing solid-state sources pays off with a fantastic service life - more than 100000 hours of continuous trouble-free operation, as well as the highest mechanical and climatic reliability, uninterrupted operation at very low temperatures, the absence of harmful materials such as mercury, the possibility of elementary brightness adjustment, ensuring fire safety requirements in terms of low thermal radiation, low maintenance costs. True, there is a circumstance that introduces some dissonance into this "victory song" about the fantastic resources of super-bright LEDs. The fact is that light-emitting diodes tend to "age" in the process of operation, which is expressed in the loss of their emitting ability, and hence the efficiency of radiation. However, reputable world manufacturers of ultra-bright LEDs guarantee that their initial emissivity is maintained by 80% by half their service life. On Internet forums, the author of the article met peremptory statements about the real life of LED sources within 2 ... 3 thousand hours. This may turn out to be true only in two cases: when products of dubious production are used, they can really lose up to 40% of the radiation efficiency during those same 3000 hours of operation, or when the LEDs are operated in significantly higher than nominal operating modes. And now let's get acquainted with the technologies for obtaining white "solid-state" light from the "multi-color" radiation of standard LEDs. Currently, there are four methods for obtaining white light, all of which are actively used in the "solid state technology" industry. On fig. 1 shows the method of mixing different colors, namely the classic RGB triad, that is, red, green and blue. On one crystal of the LED source, multi-colored light-emitting crystals are closely arranged in a mosaic order, their light is focused by a lens so that the total emission spectrum is close to natural sunlight. By separately controlling the R, G and B channels, you can get any color (or shade of color) of the LED glow. The disadvantage of the method is also obvious: it is a significant laboriousness (and hence high cost) of manufacturing and the need for color balancing of the R, G, B channels, since LEDs of different colors have different radiation efficiency. However, this method is increasingly being used in the creation of color outdoor advertising displays.
The main provisions of the second method of obtaining white light are borrowed from the principles of operation of a fluorescent lamp. In this case (see Fig. 2), a special three-color phosphor is applied to the inner surface of the housing of the LED emitting waves in the UV range, which, under the action of radiation, begins to glow with white light. Among the shortcomings of the method, we should mention its not very high efficiency of light output. It is for this reason that the third and fourth methods turned out to be the most technologically advanced and most commercially profitable. But the most interesting thing is that these methods are a logical development of the second method, that is, they also use the luminescence effect.
The technology of the third method is based on the use of a blue LED, but the light-emitting crystal here is surrounded by a constructive reflector, on which a yellow glow phosphor is applied. Thus, when the colors are mixed, light is produced that has a spectral composition very close to white, as shown in Fig. 3.
The fourth method has little difference from the third and, in fact, is its logical development aimed at improving the spectral composition of the emitted light. This method is based on the same blue light emitting diode, the same constructive reflector is provided, but two types of phosphor are already deposited on it - with green and red glow colors (see Fig. 4).
The vast majority of commercial LEDs with an emission spectrum close to white light are made on the basis of single- and double-phosphor luminescence technology. For this reason, the light of such LEDs has a slight blue-violet "cold" tint. What can be said about the cost of "solid-state light" and the economic feasibility of its implementation? To date, "solid-state light" is the most expensive source of light energy, if, of course, only the cost of "producing" a unit of light energy is taken into account. The price of 1 lumen of "solid-state light" is still 30...50 times higher than the cost of 1 lumen produced by a classic incandescent lamp. For example, the author was able to purchase a 5 W LED lamp for $15, while a regular incandescent lamp with the same light output and 60 W power consumption costs a little less than $1. Another calculation shows that a matrix of 20 ultra-bright LEDs with a total cost of $20 in light output is close to a 20W halogen lamp with a cost of $1. But do not rush to draw conclusions. Comparing the service life of LED and classic incandescent lamps, as well as their luminous efficiency, we can say that the savings are obvious. It's just that the savings are not short-term, but long-term. According to experts, the dynamics of the decline in the cost of solid-state light sources will not be as fast as the increase in their light output: it is expected that the cost will fall by only 20% with a doubling of the efficiency of use. The promotion of LED sources to the markets follows the following scenario: at first they were used as a secondary (decorative) illumination, and today work is already underway to phase out incandescent and halogen lamps. Already, car manufacturers are actively developing high and low beam headlights based on white LEDs. The development achievements are impressive: a luminous flux of the order of 1000 lm has been obtained, which correlates with a standard xenon lamp. With direction indicators abroad, everything is much simpler - the technologies have been worked out and are being rapidly introduced. On fig. Figure 5 shows a 106 mm diameter industrial automotive LED low beam headlamp made from 4 ultra-bright LEDs.
And now we will talk about the optical characteristics of ultra-bright LEDs and, in particular, how these data are presented in technical documentation. Any LED emits a luminous flux in a direction, that is, unevenly, depending on the position relative to the observer. Some LEDs have a pronounced directionality: they shine like small spotlights. Others are like an incandescent lamp with a reflector - light waves here propagate in a fairly wide spatial sector. If there is a need to ensure uniformity of spatial radiation, a constructive assembly of LEDs directed in different directions helps out. The main spatial optical characteristic of an LED is its directivity. Manufacturers describe the type of directivity, firstly, by the radiation angle (radiation angle), and secondly, by the radiation pattern. If the first characteristic is just a bare "number", then the second is a much more informative graph. The type of radiation pattern is extremely important for a lighting design engineer to know. On fig. Figure 6 shows the most informative radiation pattern of the NSPW515BS white LED, which is manufactured by NICHIA, one of the world leaders in the LED industry. The right part of the diagram is made in polar coordinates, and the left part - in Cartesian. In such graphs, the argument is the angle of rotation relative to the main axis (the line of maximum radiation), and the function is a dimensionless quantity. The graph along the line of the function is normalized to the maximum radiation value, and the luminous intensity, given in mcd at a certain value of the forward current of the LED, acts as a normalizing value. In the radiation pattern, this parameter corresponds to a dimensionless "unit".
In some cases, when the radiation pattern is wide enough (such LEDs are usually intended only for non-directional lighting), the luminous flux value is given in lm, which is very convenient for calculating illumination using standard methods. Firms also provide in the technical documentation the type of spectral characteristic of the radiation of LEDs. For what? The fact is that the color temperature of light greatly affects the emotional state of a person. So far, LED lighting has had an image of "cold", "gloomy", "uncomfortable". However, warm white LEDs have recently appeared on the market, which mimic the light of an incandescent lamp. In particular, such LEDs are also in the nomenclature of NICHIA. The difference between the radiation of warm white LEDs and the radiation of white type is most clearly demonstrated in Fig. 7, which shows the spectra of the mentioned LEDs.
Let us analyze the presented spectra. The emission of a white type LED is rendered "pale" by a high-amplitude peak in the "blue" region of the spectrum, while in a warm white LED, the blue component is "crushed" by the more intense emission of a yellow phosphor, which colors the emission in a "warm" shade. On the other hand, it is necessary to evaluate the electrical parameters of the LEDs. This is most clearly described by the current-voltage characteristic (CVC), that is, the dependence of the current passing through the diode on the voltage applied to it (Fig. 8). When a reverse (blocking) voltage is applied, any diode, including an LED, does not conduct current. But, unlike rectifier diodes, LEDs do not allow large reverse voltages. The standard LED reverse voltage limit does not exceed 5 V, so it is recommended to be careful with "reverse polarity".
The direct branch of the CVC of LEDs differs from the CVC of conventional diodes only in the value of the opening voltage and the voltage drop in the open state. If germanium diodes open at a voltage of 0,1 ... 0,2 V, silicon - at 0,6 ... 0,7 V, then the opening voltage of the LEDs lies in the range of 1,2 ... 2,9 V. After opening, the voltage on the LEDs increases slightly with increasing current, stabilizing at a certain level already at a current of about 1 mA. From fig. 8 also clearly shows that the difference between the ignition voltage of the LED and the uncontrolled increase in current through it is only 0,3 V. An LED, like any semiconductor, cannot pass infinitely large currents - it will simply melt from heating. Therefore, it is necessary to use a ballast that will "repay" the excess voltage on itself and limit the flowing current. Since the LEDs are powered by a constant (or pulsed) voltage, the simplest ballast is predominantly the usual active resistance. There are also more complex and more economical types of ballasts based on electronic current sources. Author: B. Semenov
A New Way to Control and Manipulate Optical Signals
05.05.2024 Primium Seneca keyboard
05.05.2024 The world's tallest astronomical observatory opened
04.05.2024
▪ Samsung has released the first mobile phone with a hard drive ▪ Resistive memory prototype by Elpida ▪ The air in the subway destroys the human body ▪ PET plastic from biomass waste ▪ Wind turbines affect the climate
▪ section of the site RF power amplifiers. Article selection ▪ article Turbojet aircraft. History of invention and production ▪ article What is the eye made of? Detailed answer ▪ article Wet half bayonet. Travel Tips ▪ article Fixatories for hair. Simple recipes and tips ▪ article Rotating arm. Focus Secret
Home page | Library | Articles | Website map | Site Reviews
www.diagram.com.ua |
See other articles Section 
Latest news of science and technology, new electronics:
All languages of this page