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Light-emitting Diode Basics Tutorial

Author: Apogeeweb
Date: 21 Jun 2019
led properties


Ⅰ Introduction

1.1 Terminology

1.2 LED Material

Ⅱ Working Principle

Ⅲ LED Characteristics

3.1 Unidirectional Connectivity

3.2 Polarity

3.3 Efficacy

3.4 Usability

3.5 Stability

3.6 Response Time

3.7 Environmental pollution

3.8 Color

Ⅳ LED Parameters

Ⅴ LED Types

Ⅵ LED Development

6.1 Light Efficiency Development

6.2 White-light LED

6.3 Development Trend

Ⅶ LED Application

7.1 LED Display Screen

7.2 Traffic Lamps

7.3 Auto Lamp

7.4 LCD Backlight

7.5 Decorative Lighting

7.6 Lighting Source

Ⅷ Performance Requirement

Ⅸ Problem: Causes of Light Fading

Ⅰ Introduction

A Video Tutorial on the Basics of Using Light Emitting Diodes (LED): Polarity, Forward Voltage and Current are Discussed.

1.1 Terminology

The light emitting diode is simply referred to as an LED. It is made of a compound containing gallium (Ga), arsenic (As), phosphorus (P), nitrogen (N), or the like.

When electrons and holes recombine, they can radiate visible light, and thus can be used to make light-emitting diodes. It used as an indicator light in circuits and instruments, or as a text or digital display. The gallium arsenide diode emits red light, the gallium phosphide diode emits green light, the silicon carbide diode emits yellow light, and the gallium nitride diode emits blue light. In addition, organic light-emitting diodes OLED and inorganic light-emitting diode LEDs are further classified by chemical properties.

1.2 LED Material

The five major raw materials of LED are: wafer, bracket, silver glue, gold wire, epoxy resin.

LED symbol

LED Symbol

Ⅱ Working Principle

It is a type of semiconductor diode that converts electrical energy into light energy. Like most ordinary diodes, LEDs consist of a PN junction and also have unidirectional conductivity. When a forward voltage is applied to the light-emitting diode, holes injected from the P region into the N region and electrons injected into the P region from the N region are separated from the electrons of the N region and the P region by a few micrometers in the vicinity of the PN junction. The hole complexes to produce fluorescence of spontaneous radiation. The energy states of electrons and holes in different semiconductor materials are different. When the energy released by the recombination of electrons and holes is different, the more energy is released, the shorter the wavelength of the emitted light. Commonly used diodes that are emitting red, green or yellow light. And the reverse breakdown voltage of the LED is greater than 5 volts. Its forward volt-ampere characteristic curve is very steep, and a current limiting resistor must be connected in series to control the current through the diode. The current limiting resistor value R can be calculated by:


Where E is the supply voltage, UF is the forward voltage drop of the LED, and IF is the normal operating current of the LED. The core of the LED is a wafer consisting of a P-type semiconductor and an N-type semiconductor, and there is a transition layer between the P-type semiconductor and the N-type semiconductor, called a PN junction. In some PN junctions of semiconductor materials, the injected minority carriers recombine with the majority carriers to release excess energy in the form of light, thereby directly converting electrical energy into light energy. The PN junction is added with a reverse voltage, and minority carriers are difficult to inject, so they do not emit light. This is the basic lighting principle of LED. When it is in the forward working state (ie, the forward voltage is applied to both ends), and the current flows from the anode of the LED to the cathode, the semiconductor crystal emits light of different colors from ultraviolet to infrared, and the intensity of the light is related to the current.


Ⅲ LED Characteristics

Compared with incandescent bulbs and xenon lamps, LEDs are characterized by low operating voltages (some only a few volts), low operating currents (some can only be illuminated at a few mAh), better impact and shock resistance, high reliability, long life, and the intensity of the light can be easily modulated by the current passing through the modulation. Due to these characteristics, light-emitting diodes are used as light sources in some photoelectric control devices and as signal displays in many electronic devices.

3.1 Unidirectional Connectivity

The LED can only be turned on in one direction (energized), called forward bias. When current flows, electrons and holes recombine to emit monochromatic light. This is called electroluminescence effect. The wavelength and color of the light are related to the type of semiconductor material used and the impurity impurities incorporated. It has the advantages of high efficiency, long life, not easy to break, high switching speed, high reliability and so on. The luminous efficiency of white LEDs has been significantly improved in recent years. At the same time, the purchase price per thousand lumens has also dropped significantly due to the competition from manufacturers entering the market.

3.2 Polarity

The longer of the two leads of the LED is the positive pole and should be connected to the positive pole of the power supply. In addition, some of the LEDs have the same length of the two leads, but the tube has a raised small tongue, and the lead near the small tongue is the positive.

3.3 Efficacy

The energy consumption is reduced by about 80% compared with the incandescent lamp with the same lighting efficiency, and is reduced by about 40% compared with the energy-saving lamp.

3.4 Usability

The volume is small, and each unit LED chip is a square of 3-5 mm, so it can be fabricated into various shapes of devices and is suitable for a variable environment.

3.5 Stability

100,000 hours, the light decay is the initial 50%.

3.6 Response Time

The response time of its incandescent lamp is millisecond, and the response time of the LED lamp is nanosecond.

3.7 Environmental pollution

Contains no harmful metal mercury, etc.

3.8 Color

The light-emitting diode is conveniently adjusted by chemical modification method to adjust the energy band structure and the forbidden band width of the material to realize multi-color luminescence of red, yellow, green, blue and orange. The operating voltage of the red light pipe is small, and the operating voltages of the red, orange, yellow, green and blue light-emitting diodes of different colors are sequentially increased.

led lamp

Ⅳ LED Parameters

Important aspects of the optical parameters of LEDs are: luminous flux, luminous efficiency, luminous intensity, light intensity distribution, and wavelength.

  • Luminous efficiency and luminous flux

Luminous efficiency is the ratio of luminous flux to electrical power, its unit is lm/W. Luminous efficiency represents the energy-saving characteristics of the light source, which is an important indicator for measuring the performance of modern light sources.

  • Luminous intensity and its distribution

The intensity of LED illumination is indicative of its intensity in a certain direction. Since the LEDs have a large difference in light intensity at different spatial angles, we have studied the intensity distribution characteristics of LEDs. This parameter has great practical significance and directly affects the minimum viewing angle of the LED display device. For example, the LED large-scale color display of the stadium, if the LED single tube distribution range is narrow, the audience facing the display at a large angle will see the distorted image. And traffic signs also require a large range of people to identify.

  • Wavelength

For the spectral characteristics of LEDs, its monochromaticity is the point, that is the main colors such as red, yellow, blue, green and white are pure or not. Because in many occasions, such as traffic lights, the color requirements are more stringent, wrong color display will affect the vision of drivers. From this phenomenon, we specifically study the spectral characteristics of LEDs, which is necessary and meaningful.


Ⅴ LED Types

Light-emitting diodes can be divided into ordinary single-color light-emitting diodes, high-brightness light-emitting diodes, ultra-high-brightness light-emitting diodes, color-changing light-emitting diodes, flashing light-emitting diodes, voltage-controlled light-emitting diodes, infrared light-emitting diodes, and negative-resistance light-emitting diodes.

LED control modes are constant current and constant voltage. There are various dimming methods, such as analog dimming and PWM dimming. Most LEDs use constant current control, which can keep the LED current stable and extend the LED life cycle.

  • Ordinary monochromatic LED

Ordinary monochromatic light-emitting diodes have the advantages of small volume, low operating voltage, small operating current, uniform and stable illumination, fast response, long life, etc., and can be driven by various DC, AC, pulse and other power sources. It belongs to a current-controlled semiconductor device and needs to be connected in series with a suitable current limiting resistor.

The color of the illumination of a conventional monochromatic LED is related to the wavelength of the illumination, and the wavelength depends on the semiconductor material used to fabricate the LED. The wavelength of the red LED is generally 650~700nm, amber LED is generally 630~650 nm, orange LED is generally 610~630nm, yellow LED is about 585nm, and green LED is typically 555~570 nm.

  • High-light monochromatic LED

The semiconductor materials of high-light monochromatic LEDs and ultra-high-brightness monochromatic LED are different from ordinary monochromatic LEDs, thus the intensity of light emission is also different.

Generally, high-light monochromatic LEDs use gallium arsenide (GaAlAs), ultra-high-brightness monochromatic light-emitting diodes use phosphorus indium gallium arsenide (GaAsInP), and ordinary monochromatic light-emitting diodes use gallium phosphide (GaP). ) or phosphorus gallium arsenide (GaAsP).

  • Various-colour LEDs

The various-colour LED is a light-emitting diode that can change the color of the light. It can be divided into two-color LEDs, three-color LEDs, and multi-color (red, blue, green, and white) LEDs.

The color-changing LEDs can be divided into two-terminal color-changing LEDs, three-terminal color-changing LEDs, four-terminal color-changing LEDs and six-terminal color-changing LEDs according to the number of pins.

  • Flashing LED

A flashing light-emitting diode is a special light-emitting device consisting of a CMOS integrated circuit and a light-emitting diode. It can be used for alarm indication and undervoltage and overvoltage indication. When using it, no external components need to be connected, adding appropriate DC working voltage (5V) to both ends of the pin to flash.

  • Voltage controlled LED

The voltage-controlled light-emitting diode integrates the light-emitting diode and the current-limiting resistor, and can be directly connected to both ends of the power supply when used.

  • infrared LED

red light emitting diode

The infrared light-emitting diode is made of a material with high infrared radiation efficiency (commonly used as gallium arsenide GaAs), and a forward bias is applied to inject a current into the PN junction to excite infrared light. The spectral power distribution has a center wavelength of 830 to 950 nm and a half-peak bandwidth of about 40 nm. Its biggest advantage is that it can be completely red-free, (using 940 ~ 950nm wavelength infrared tube) or only weak red storm (red storm is visible red light) to extend the service life. What we see in the human eye is called visible light. The wavelength range of visible light is 380nm-780nm, according to the wavelength of visible light, it is divided into red, orange, yellow, green, cyan, blue and violet light from long to short. The wavelength is shorter than violet light and longer than the red light is called infrared light.

Infrared emitter wavelengths are typically used: 850nm, 870nm, 880nm, 940nm, 980nm.

The relationship between power and infrared emission tube wavelength: 850nm>880nm>940nm.

  • Peak wavelength

The energy distribution measured by the illuminant or object on the spectrometer, in other words, the wavelength λp corresponding to its peak position.

  • Radiation intensity (POWER)

It indicates the infrared radiation energy of the infrared tube (IRLED), and its unit is mW ∕ sr.

The radiation intensity is proportional to the input current (If), and it is inversely proportional to the emission distance. Infrared radiation intensity is the amount of optical power radiated by the unit solid angle (sr) of the infrared light emitted by the transmitting tube. Increasing the power of the infrared emission tube can only temporarily expand the illumination distance, and finally the wafer attenuation is accelerated, causing the infrared light to become darker and the night vision to become less and less clear.

  • Electrical performance

3mm or 5mm in diameter is low-power infrared emitting tube, and 8mm and 10mm is the medium power and high power launch tube, respectively.

Low power transmitter tube forward voltage: 1.1-1.5V, current 20mA.

Medium power transmitter tube forward voltage: 1.4-1.65V, current 50-100mA.

High power transmitter tube forward voltage: 1.5-1.9V, current 200-350mA.

light emitting diodes

  • Directional characteristics

The emission intensity of an infrared light emitting diode varies depending on the direction of emission. When the direction angle is zero degree, the radiation intensity is defined as 100%. When the direction angle is larger, the radiation intensity is relatively decreased. When the emission intensity is half of the direction angle of the optical axis, the value is half of the peak value. This angle is called the direction half-value angle. The smaller the angle, the more sensitive the directivity of the component. In addition, infrared light-emitting diodes are generally used with lenses to make them more directional.

  • Distance properties

The radiation intensity of the infrared light-emitting diode varies depending on the distance on the optical axis, and also varies depending on the light-receiving element. It is a change in the amount of incident light of the light receiving element and a distance from the infrared light emitting tube. Basically, the light metric is inversely proportional to the square of the distance and is related to the characteristics of the light-receiving element.

When infrared rays are emitted to control the corresponding device, the distance controlled is proportional to the transmitted power. In order to increase the infrared control distance, the infrared light-emitting diode operates in a pulse state, because the effective transmission distance of the pulsating light (modulated light) is proportional to the peak current of the pulse, and it is only necessary to increase the peak value Ip to increase the emission distance of the infrared light. The method of increasing Ip is to reduce the pulse duty ratio, that is, the width T of the compression pulse, generally, the applying frequency is below 300KHz.

  • Receiving method

There are two ways to transmit and receive infrared rays, one is direct type, and the other is the reflective. Direct-type refers to the light-emitting tube and the receiving tube are relatively placed at the two ends of the emission and the controlled object, and the distance between them is a certain distance; the reflective-type means light-emitting tube and the receiving tube to juxtapose together, and the receiving tube always has no light, and only the infrared light emitted from the light-emitting tube when the light encounters the reflector, the receiving tube receives the reflected infrared light to work.

  • Transmitting circuit

The double-tube infrared transmitting circuit can increase the transmitting power and the working distance of the infrared emission.


Ⅵ LED Development

6.1 Light Efficiency Development

The earliest application of the LED light source made by the principle of semiconductor P-N junction light was introduced in the early 1960s. The material used at that time was GaAsP, which emitted red light (λp = 650 nm). When the driving current was 20 mA, the luminous flux was only a few thousandths of a lumen, and the corresponding luminous efficiency was about 0.1 lm/W.

In the mid-1970s, the elements In and N were introduced to produce green light (λp = 555 nm), yellow light (λp = 590 nm) and orange light (λp = 610 nm), and the luminous efficiency was also increased to 1 lm/W.

In the early 1980s, the LED light source of GaAlAs appeared, making the red LED's luminous efficiency reach 10 lm/W.

In the early 1990s, the development of two new materials, GaAlInP, which emits red light and yellow light, and GaInN, which emits green and blue light, have greatly improved the light efficiency of LEDs. In 2000, the former made LEDs in red and orange (λp = 615 nm) with a luminous efficacy of 100 lm/W, while the latter made LEDs with a luminous efficacy of 50 lm/W in the green (λp = 530 nm).

6.2 White-light LED

white light emitting diode

In 1993, Shuji Naka mura, who worked at Nichia Corporation in Japan, invented commercial applications based on wide-bandgap semiconductor materials such as gallium nitride (GaN) and indium nitride (InGaN). Blue LEDs, which were widely used in the late 1990s. In theory, blue LEDs can produce white light in combination with the original red and green LEDs, but white LEDs are rarely made in this way.

Most of the white LEDs produced today are made by coating a layer of light yellow phosphor on a blue LED (near-UV, wavelength 450nm to 470nm). This yellow phosphor is usually made by mixing erbium. Aluminum garnet (Ce3+:YAG) crystals are ground into a powder and mixed in a dense binder. When the LED chip emits blue light, part of the blue light is efficiently converted by the crystal into a predominantly yellow light with a broad spectrum (about 580 nm in the center of the spectrum). In fact, single crystal Ce-doped YAG is considered to be a scintillator more than a phosphor. Since yellow light stimulates the red and green receptors in the naked eye, the blue light of the LED itself is mixed, making it look like white light. And its color is often called "white of moonlight."

This method of making white LEDs was developed by Nichia Corporation and has been used since 1996 to produce white LEDs. To adjust the color of light yellow, other rare earth metal lanthanum or cerium can be used to replace the cerium (Ce) incorporated in Ce3+:YAG, and it can even be done by replacing some or all of the aluminum in YAG. Based on the characteristics of color spectrum, red and green objects will not look as sharp as the broad spectrum light source when illuminated by this LED. In addition, due to variations in production conditions, the color temperature of the finished product of this LED is not uniform, so it will be distinguished by its characteristics during the production process, from warm yellow to cold blue.

Another method of making white LEDs is a bit like a fluorescent lamp. LEDs that emit near-ultraviolet light are coated with a mixture of two phosphors: europium emits red and blue light, and copper and aluminum doped with zinc sulfide (ZnS) emits green light. However, since the ultraviolet rays cause cracking and deterioration of the epoxy resin in the adhesive, the production is difficult and the life cycle is short. Compared with the first method, it is less efficient and produces more heat, but its spectrum are better, therefore, the light produced is better. Although the LED power of the ultraviolet light is higher in this way, the efficiency is lower than that of the first method. The brightness made by these ways is similar.

The latest method of making white LEDs does not use phosphors. A new approach is to grow an epitaxial layer of zinc selenide on a zinc selenide (ZnSe) substrate. When it is energized, its active zone will emit blue light and the substrate will glow yellow. When mixed, it will be white light.

6.3 Development Trend

With the great leap of technology breakthrough, LED light efficiency is constantly improving, and prices continue to fall. The emergence of new modular dies has also led to an increase in the power of individual LED tubes. Through the continuous efforts of the industry, the breakthrough of new optical design, the development of new lights, and the single product situation are expected to be further reversed. Improvements in control software also make LED lighting more convenient to use. These gradual changes reflect the broad prospects of LED light-emitting diodes in lighting applications.

LED is called the fourth generation light source, which has the characteristics of energy saving, environmental protection, safety, long life, low power consumption, low heat, high brightness, waterproof, small size, shockproof, easy dimming, concentrated beam, easy maintenance, and so on. Thus it widely used in various indications, displays, decorations, backlights, general lighting and other fields.

  • LED advantages

1. small size

The LED is basically a small chip that is encapsulated in epoxy, so it is very small and very light.

2. low voltage

LED power consumption is quite low, generally the operating voltage of the LED is 2-3.6V. Only a very weak current is required to illuminate normally.

3. long life cycle

At the right current and voltage, LEDs life span can last up to 100,000 hours.

4. high brightness, low heat

LEDs use cold-lighting technology and generate much less heat than ordinary lighting fixtures of the same power.

5. eco-friendly

LEDs are made of non-toxic materials, unlike mercury-containing fluorescent lamps that can cause pollution, and LEDs can be recycled.

  • LED shortcomings

High starting cost, poor color rendering, low efficiency of high power LED, constant current drive (requires typical drive circuit).

  • Incandescent lamp

Low conversion efficiency of electro-optical light (about 10%), short life (about 1000 hours), high generation temperature, single color and low color temperature.

  • Fluorescent lamp

Electro-optical conversion efficiency is not high (about 30%), harm to the environment (including harmful elements such as mercury, about 3.5-5mg / only), non-adjustable brightness (low voltage can not be illuminated), ultraviolet radiation, flicker phenomenon, low start-up, large size, and repeated switching affects life. 

High-pressure gas discharge lamp: large power consumption, unsafe, short life, heat dissipation problems, and mostly used for outdoor lighting.

blue light emitting diode


Ⅶ LED Application

With the development of high brightness and multi-color LEDs, its application field is expanding. From the lower luminous flux indicator to the display, outdoor display to the medium luminous flux signal and the special illumination white light source, it finally developed into the upper right corner of the high-light flux general illumination source.

7.1 LED Display Screen

Since the mid-1980s, monochrome and multi-color displays have made, initially as text screens or animated screens. In the early 1990s, the development of electronic computer technology and integrated circuit technology enabled the video technology of LED displays to be realized. The TV image was directly on the screen, especially in the mid-1990s, the blue and green ultra-high brightness LED was successfully developed and put into production quickly, which greatly expanded the application of the outdoor screen. At present, LED display is in the stadium, squares, conference halls, even streets and shopping malls have been widely used. In addition, applications in securities market screens, bank exchange rate screens, interest rate screens, etc. also account for a large proportion; there are also large developments in the information screens of expressways and elevated roads. In a word, the application of LEDs in this field has become a scale.

7.2 Traffic Lamps

LEDs have been used as light sources for many years on the main roads. LED lights not only have high brightness, but also because of the good color purity of LEDs, they are particularly vivid and easy to identify signals. The current work is to improve and improve. Road traffic lights have made great progress in recent years, technology development is fast, application development is rapid, according to the use effect, long life, power saving and maintenance-free effect is obvious. At present, the illuminating peak wavelengths of LEDs are 630nm in red, 590nm in yellow, and  505nm in green. The problem that should be noted now is that the drive current should not be too large, otherwise the high temperature conditions in the summer sun will affect the life of the LED.

7.3 Auto Lamp

Super bright LEDs can be used as brake lights, tail lights and directional lights for cars. They can also be used for instrument lighting and interior lighting. They have obvious advantages over incandescent lamps in terms of vibration resistance, power saving and long life. When used as brake lights, its response time is 60ns, which is much shorter than the 140ms of incandescent lamps.In addition, on a typical highway, it will increase the safety distance of 4-6m.

7.4 LCD Backlight

As a backlight for liquid crystal display, LED can be used not only as a green, red, blue, white backlight, but also as a color-changing backlight.

7.5 Decorative Lighting

Due to the increase in brightness and falling price of LEDs, coupled with long life, power saving, easy driving and control, and it not only can flash, but also change color, so that monochromatic, multi-color and even discoloration made of ultra-high brightness LED is equipped with light-emitting units of other shapes to decorate the tall buildings, bridges, streets and plazas. The landscape works well, showing a colorful, star-studded and colorful scene.

7.6 Lighting Source

Because the LED light source has no infrared radiation, it is easy to conceal, and it also has the advantages of vibration resistance, battery power supply, structural solidification and portability, etc. It will have a great development in special lighting sources. Lamps and buried lights have been produced on a large scale. They are also used as microscope field illumination, flashlights, surgeons' headlights, lighting for museums or exhibitions, and reading lamps. They have gradually developed into the world of general lighting.


Ⅷ Performance Requirement

1. High reliability is very important for LED street lamps. They are installed at high altitude, thus it is inconvenient to maintain, and the cost is also large.

2. High-efficiency LEDs are energy-efficient products that drive power supplies with high efficiency. It is especially important for the structure in which the power supply is installed in the luminaire. Since the luminous efficiency of the LED decreases as the temperature of the LED increases, the heat dissipation of the LED is very important. The power source has high efficiency, its power consumption is small, and the heat generated in the lamp is small, which reduces the temperature rise of the lamp. It is beneficial to delay the light decay of LEDs.

3. The high-power factor is the grid's load requirements. Generally, there are no mandatory indicators for electrical appliances below 70 watts. Although the power factor of a single power electrical equipment with a small power has little effect on the power grid, but a large area is energized at night, the similar load is too concentrated, which will cause serious pollution to the power grid.

4. There are two types of driving modes: one is a constant voltage source for multiple constant current sources, and each constant current source supplies power to each LED separately. In this way, the combination is flexible, and all the LED faults do not affect the work of other LEDs, but the cost will be slightly higher. The other is direct constant current supply, with LEDs running in series or in parallel. Its advantage is that the cost is lower, but the flexibility is poor, and it is necessary to solve a certain LED failure without affecting the operation of other LEDs.

5. The ability of surge protection LEDs to resist surges is relatively poor, especially against reverse voltage capability. It is also important to strengthen protection in this area. Some LED lights are installed outdoors, such as LED street lights. Due to the start of the grid load and the induction of lightning strikes, various surges will be invaded from the grid system, and some surges will cause LED damage. Therefore, the LED driver power supply must have the ability to suppress the intrusion of surges and protect the LEDs from damage.

6. The spectral power distribution measurement of the LED is designed to grasp the spectral characteristics and chromaticity of the LED, and is to correct the measured metric of the LED.

When measuring the spectral power distribution of LEDs, the following points should be noted. One is because the spectral intensity of the standard lamp is much stronger than that of the LED when compared with the standard spectral irradiance. In order to avoid this problem, it is better to be in front of the standard lamp, adding a neutral filter to bring its spectral intensity closer to the LED.

The spectral width of the LED is very narrow. In order to accurately depict the spectral distribution profile of the LED, it is better to measure with a narrow-band wavelength-width monochromator with a wavelength interval of 1 nm.

7. Although protection function power supply the conventional protection function, it is better to increase the LED temperature negative feedback in the constant current output to prevent the LED temperature from being too high.

8. For the protection aspect of the lamp, the power supply structure should be waterproof and moisture-proof, and the outer casing should be light-resistant.

9. The life of the drive power supply should match the life of the LED.

10. It is important to comply with safety and electromagnetic compatibility requirements when using LEDs.


Ⅸ Problem: Causes of Light Fading

1. quality problem

1) The LED chip quality is not good, and the brightness is attenuated faster.

2) There are defects in the production process, and the heat dissipation of the LED chip cannot be well derived from the PIN pin, resulting in an excessive temperature of the LED chip, which intensifies the chip attenuation.

2. Working condition

1) The LED is driven by a constant current, and some of the LEDs are driven by voltage to cause the LED to decay too fast.

2) The drive current is greater than the rated drive condition.

Three factors affecting the quality of LED lamps

Firstly, what kind of LED white light to choose?

Secondly, LED lamp bead working environment temperature.

Finaly, the design of the working parameters of the LED lamp bead.

LED Image


Frequently Asked Questions about Light-emitting Diode Basics

1. What does light emitting diode mean?
In the simplest terms, a light-emitting diode (LED) is a semiconductor device that emits light when an electric current is passed through it. Light is produced when the particles that carry the current (known as electrons and holes) combine together within the semiconductor material.


2. What is a light emitting diode and how does it work?
A light-emitting diode (LED) is a semiconductor light source that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. ... Recent developments have produced high-output white light LEDs suitable for room and outdoor area lighting.


3. What is basic principle of LED?
LED is a semiconductor optoelectronic device. ... The basic principle is that electrons and holes in semiconductors recombine and emit photons under forward bias.


4. Can you use an LED as a diode?
Yes, an LED works as a photo diode (as do most diodes) but are always packaged so as to admit light. So if you need a rather poor photo-diode, you can use an LED, and if it is good enough for the application, then it will likely be cheaper than a "real" photo diode that is made in much lower quantities.


5. Why LED is forward biased?
When Light Emitting Diode (LED) is forward biased, free electrons in the conduction band recombines with the holes in the valence band and releases energy in the form of light. ... In normal p-n junction diodes, silicon is most widely used because it is less sensitive to the temperature.


6. What is the difference between laser diode and LED?
LASERs (also known as laser diodes or LD) and LEDs (light emitting diode) have different characteristics in the way in which they emit light. While a LASER emits converged light, the output of an LED is highly diverged. ... The spectral width of an LED is bigger than that of a LD.


7. Do LEDs have resistance?
LEDs do not have a linear relationship between current and voltage so they cannot be modeled as simply as a resistor using Ohm's Law, V = IR . An LED can be approximated as a resistor with a fixed voltage source.


8. What happens if you wire an LED backwards?
LEDs, being diodes, will only allow current to flow in one direction. And when there's no current-flow, there's no light. Luckily, this also means that you can't break an LED by plugging it in backwards. ... A reversed LED can keep an entire circuit from operating properly by blocking current flow.


9. Why is led not made of silicon or germanium?
LEDs are p-n junction devices constructed of gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), or gallium phosphide (GaP). Silicon and germanium are not suitable because those junctions produce heat and no appreciable IR or visible light. ... An exposed semiconductor surface can then emit light.


10. Why do LEDs not obey Ohm's law?
A light bulb is a simple example; the filament undergoes huge changes in temperature when current passesthrough it. Therefore, the resistance of the filament is not constant, it increaseswith increased current. non-ohmic resistor does not obey the ohm's law.


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