What is led
LED is an abbreviation for LIGHT EMITTING DIODE, it is a semiconductor light-emitting device with two poles which emits visible light when a small amount of electric current through it. The first LED used in business was created in 1960. Its basic structure is like that: an electroluminescent semiconductor material is put on a shelf with down-lead, then the around is sealed with epoxy resin to protect inner core wire, so its anti-seismic performance is excellent.
LED’S composing
Different colors are due to different chemical ingredients
For example RED: aluminum-indium-gallium-phosphide
Green and blue: indium-gallium-nitride
White and other colors are made by mixture in proper proportion of RGB tricolor.
LED’s production process is similar to but the precision during process is worse then semiconductor, so its cost is comparative high at present.
LED working principle and structure picture
The hard core of LED is a wafer which is composed of P-type semiconductor and N-type semiconductor, in between of them there is a transition layer called P-N knot. In the P-N knot of some semiconductor material, redundant energy will be emitted in the form of light when minority carrier compound with majority carrier injected, so the electric energy converts directly into light energy. This diode made according to the principle of electroluminescence by injection is called Lighting-Emitting Device, which is generally called LED. When it works (two poles are connected with forward voltage), the electricity flows through anode to cathode, then the semiconductor crystal emits light of different colors from ultraviolet to infrared. The brightness of light is related to electric current.
LED’s advantages:
Super long life span of 100 thousand hours
Various colors can be produced (for example RGB)
Driving is simple (no need to touch off voltage)
Energy saving, higher efficiency then incandescence lamp and a potential to improve
Small size, flexible lamp design, each unit of LED flake is a 3-5mm square, so it can be made into apparatus of different shapes, thus suitable to be used in changeful environment.
Quick in response; the response time of incandescence lamp is millisecond, while LED’s is nanosecond.
Environmental friendly: no harmful Hg.
Low voltage, safety. According to different LED, low voltages of between 6-24V are used. So LED is a much safer electrical source then others using high voltage, it is especially suitable to be used in the public.
LED’s disadvantages
Lumen cost of per W is high
Luminous efficiency is lower than that of fluorescence lamp or HID
Need drivers to drive (not as simple as incandescence lamp)
LED’S application and market prospect
LED’S mainly application in lighting industry
Building’s outer lighting, casino or stage lighting, room lighting, road, sight lighting, sign and indication lighting, traffic signal lamp, video screen.
The familiar LED in the market at present:
LED ground lamp, LED underwater lamp, LED lawn lamp,
With the improvement of LED luminous efficiency and perfection and optimization of pivotal technologies, many enterprises promote solar energy semiconductor road lamp which is combined solar energy with high power LED.
With the enhancement of brightness during these years, LED is not only an indicator light, but very likely to become a lighting source.
LED technology is in beginning period right currently, it is hoped that there will be a great progress on enhancing luminous efficiency and reducing cost in the next 5 to 10 years
LED will be another strong competitor to traditional lighting source in the future.
LED’S quality control and quality difference
Direct current (DC) should be used
In order to lower the cost, some manufactures supply power to LED by resistance and lowering voltage which has a bad impact on LED’s life span. Providing electricity to LED by specialized switching power supply (constant power supply is best) will not have any impact on product’s using period, however the production cost is relatively high.
Measures against static electricity must be done well
In the LED production process, measures against static electricity must be adopted, for example: the work bench is earthed; workers must wear anti-static clothing, anti-static wrist strap and anti-static gloves etc; Electrostatic Ionizing air blower is better to install if condition allows. At the same time, the humidity in the plant must keep in about 65%, in case static will be produced for the air is too dry. Especially the green LED is easier to be damaged by static. In addition, the different quality of LED, its anti-static capacity is different. Better quality LED has better anti-static capacity.
Pay attention that the increase in temperature leads to decrease of LED internal resistance
When the environmental temperature rises, LED internal resistance decreases. If it is provided electricity by constant pressure electrical source, LED’s working current will rise. When the working current exceeds its rated current, LED’s using period will be affected, in severity LED’s light source will be burned out. So it is best to choose constant current electrical source to make sure that LED’s working current is not affected by the outer temperature.
The seal of LED
No matter what kinds of LED products, as long as it is used in the outdoor, it will face the seal problems like water proof or moisture proof. If the seal problems are not well treated, LED’s life span will be affected directly. At present, a few manufacturers which attach much importance to product quality seal LED in the way of “irrigation” with traditional epoxy resin. However, this method is not so easy to handle with; also it is not suitable to be used for big size LED products, which will result in the increase on product weight.
1. luminance
Different LED luminance, its price is different. LED using in LED luminaire should meet the standard of Laser Class I.
2. anti-static capacity
LED with strong anti-static capacity has long life span, so its price is high. Only the LED with anti-static over 700V can be used as LED lamp decoration
3. Wavelength
LED is monochromatic when wavelength is the same. Monochromatic LED’s price is high. Manufacturers who don’t have ……….. have the difficulty to produce pure monochromatic LED.
4. Creepage
LED is a kind of luminophor of one way electric conductivity. When there is any countercurrent, it is creepage. LED with large creepage current is short in life span and low in price.
5. Luminous angle
The luminous angle is different according to the purpose of LED. The price for special luminous angle is higher, for example full diffuse angle.
6. Life span
The key to judge quality is by life span which is determined by luminous decay. The small luminous decay is, long is the life span, thus price is high.
7. Wafer
LED’s luminophor is a wafer. There is a wide price range according to different kinds of wafers. Wafers made in Japan, USA are more expensive, while those homemade or in Taiwan are cheaper.
8. Wafer size
Side length represents the size of a wafer. The quality of LED of big wafer is better than that of small wafer. Price is proportional to the size of wafer.
9. Colloid
The colloid of normal LED is epoxy resin. Price is higher for LED providing UV protection and fireproofing, which the high quality LED lamps should be.
It is pretty difficult for common consumers to judge LED by its safety using time. The reliability of design and choice of components both have great relation to price. For products using in engineering, never choose those of low price and bad quality. For instance, price varies according to different wires. Some manufacturers produce LED lamps by old electron components dismantled from old lamps, recycled plastic or recycled metal. These inferior LED lamps’s price is cheaper as much as over 50% than normal LED lamp.
question and answers
1. Most products in the market are low power at present. Although they are claimed that the life span is over 50000hours, some even is said to be between 80000-100000hours, the luminance for most of them decays in less than two years.
Answer: at present the luminous decay of low power 5MM LED is relatively big, many decay to less than 50% within 1000~2000hours. Prudent attitude is needed when choosing LED products according to self demand.
2. When white LED can catch up with fluorescent lamp in luminous efficiency and has luminous efficiency of 100lm/W came true?
This long-term problem particular concern to lighting industry finally had the solution. Since June 2006, Nichia began to make white LED samples with luminous efficiency to 100lm/W and put into mass production in December. This 100lm/W luminous efficiency is even equal to that of some varieties which luminous efficiency is relatively high among fluorescent lamps. Although white LED with luminous efficiency over 100lm/W had been reported in all kinds of seminars, it was the first time that it appeared in the market. Nichia was not the only manufacturer to realize 100lm/W luminous efficiency. According to the development blue print of Citizen, 90lm/W LED was going to appear in 2006 and 100lm/W would be in 2007.
1. Why does 12V LED burn out or not work when it is connected to 12V electric transformer?
In order to save cost, the actual voltage of products produced by some manufacturers is between 6~7V, less than the so called 12V.
