Ⅰ. Brief introduction of LED
The basic knowledge that semiconductor materials can generate light has been recognized as early as 50 years ago. In 1962, Nick Holonyak Jr. of General Electric successfully developed the first visible light-emitting diode used in practice. LED is the abbreviation of light emitting diode. Its basic structure is a piece of electroluminescent semiconductor material, which is placed in a shelf with leads, and then sealed with epoxy resin around it, that is, solids are encapsulated. Therefore, it can protect the inner core wire, so that the LED has good shock resistance.
Initially, LED was used as indicating light sources for instruments and meters. Later, LED of various light colors were widely used in traffic lights and large-area display screens, and produced good economic and social benefits. Take the 12-inch red traffic light as an example. In the United States, a long-life, low-efficiency 140-watt incandescent lamp was originally used as a light source, which produces 2000 lumens of white light. After passing through the red filter, 90% of the light is lost, leaving only 200 lumens of red light. In the newly designed lamp, Lumileds uses 18 red LED light sources, which consume a total of 14 watts of power including circuit loss, to produce the same light effect. Automobile signal lights are also an important field of application of LED light sources.
Ⅱ. LED Colors
As we all know, LED of different colors are available in the market. The color of the emitted light is determined by the wavelength of the emitted light, which in turn is determined by the actual semiconductor compound used to form the PN junction during fabrication. Each semiconductor material has a different depletion region with a different forward voltage. Below is a table of different colored LED.
Ⅲ. Package of LED
There are many packaging forms for LED chips. According to different usage requirements and different optoelectronic characteristic requirements, we have various packaging forms, which can be summed up in the following common forms:
1. Chip package
We bond LED chips on tiny lead frames. After the electrode leads are welded, they are injection molded, and the light-emitting surface is generally encapsulated with epoxy resin.
2. Power package
There are also many packaging forms for power LED. It is characterized in that the bottom cavity of the bonded chip is large, and has specular reflection ability, high thermal conductivity, and low enough thermal resistance, so that the heat in the chip is quickly drawn out of the device, and the chip and the ambient temperature keep the temperature difference low.
3. Dual in-line package
We fix the chip with a copper lead frame similar to an IC package, and encapsulate it with transparent epoxy after welding the electrode leads. There are various "piranha" packages and super piranha packages with different bottom cavities that are common. This kind of package chip has better heat dissipation and low thermal resistance. The input power of LED can reach 0.1W~0.5W, which is larger than that of pin-type devices, but the cost is higher.
4. Soft package
The chip is directly bonded on a specific PCB printed board, connected into a specific character or display form through a welding wire, and the LED chip and the welding wire are protected with a transparent resin and assembled in a specific casing. This kind of flexible packaging is often used in products with digital display, character display or dot display.
5. Leaded package
The common leaded package has the LED chip fixed on the 2000 series lead frame. After the electrode leads are welded, we encapsulate them into a certain transparent shape with epoxy resin to become a single LED device. This pin package can be divided into φ3 and φ5 diameter packages according to the different dimensions. The feature of this type of package is that it can control the distance from the chip to the light-emitting surface, and obtain various light-emitting angles: 15°, 30°, 45°, 60°, 90°, 120°, etc.
Light-emitting diode can be divided into blinking LED, ordinary monochrome LED, color-changing LED, infrared LED, voltage-controlled LED, high brightness monochrome LED, and negative resistance LED.
1. Blinking LED
Blinking LED is a special light-emitting device composed of CMOS integrated circuits and light-emitting diode. It can be used for alarm indication and undervoltage and overpressure indication. The blinking LED does not need to be connected to other components when it is used, as long as an appropriate DC working voltage (5V) is applied to both ends of its pins, it can blink and emit light.
2. Ordinary monochromatic LED
Ordinary monochromatic LED has the advantages of small size, low operating voltage, low operating current, uniform and stable emission, fast response speed, and long lifespan. It can be powered on by various DC, AC, pulse and other power sources. It belongs to current controlled semiconductor devices, and suitable current limiting resistors need to be connected in series when used. The emission color of ordinary monochromatic LED is related to the wavelength of the emission, which in turn depends on the semiconductor material used to manufacture the light-emitting diode. The wavelength of the red LED is generally 650-700nm; the wavelength of amber LED is generally 630-650nm; the wavelength of orange LED is generally around 610-630nm; the wavelength of the yellow LED is generally around 585nm; the wavelength of green LED is generally 555-570nm.
Commonly used domestic ordinary monochromatic LED includes BT (factory standard model) series, FG (ministry standard model) series and 2EF series. Commonly used imported ordinary monochromatic LED includes SLR series and SLC series.
3. Color-changing LED
Color-changing LED is light-emitting diodes that can change the color of their light. Color-changing LED can be divided into two-color LED, three-color LED and multi-color (red, blue, green, white four colors) LED. Color-changing LED can be divided into two-terminal color-changing LED, three-terminal color-changing LED, four-terminal color-changing LED and six-terminal color-changing LED according to the number of pins. Commonly used two-color LED is 2EF series and TB series; commonly used three-color LED is 2EF3022EF312, 2EF322 and other models.
4. Infrared LED
It can directly convert electric energy into infrared light (invisible light) and can radiate out the light-emitting device, which is mainly used in various light control and remote control transmitting circuits. The structure and principle of infrared LED is similar to ordinary LED, but the semiconductor materials used are different. Infrared LED usually use materials such as gallium arsenide (GaAs), gallium aluminide arsenide (GaAlAs), and are packaged in fully transparent or light blue or black resin. Commonly used infrared LED is S|R series, S|M series, PLT series, GL series, H|R series and HG series.
5. Voltage-controlled LED
Ordinary LED is current-controlled device, and a current-limiting resistor with an appropriate resistance value needs to be connected in series when used. The voltage-controlled light-emitting diode (BTV) is made by integrating the LED and the current-limiting resistor, and can be directly connected to both ends of the power supply when in use. Ordinary red LED can work at 3V-10V, such as YX503URC, YX304 URC, YX503BRC. Voltage-controlled LED provides engineers and developers with greater choices.
6. High brightness monochrome LED
High-brightness monochromatic LED and ultra-high-brightness monochromatic LED use different semiconductor materials from ordinary monochromatic LED, so the intensity of light is also different. Usually, materials such as GaAlAs are used for high-brightness monochrome light-emitting diodes; materials such as GaAsInP are used for ultra-high-brightness monochrome LED; and GaP or GaAsP are used for ordinary monochrome LED.
Ⅴ. Detection methods of LED
1. External power measurement
We can use a 3V regulated source or two dry batteries in series and a multimeter (both pointer and digital) to accurately measure the optical and electrical characteristics of LED. If the measured VF is between 1.4 and 3V and the brightness of the light is normal, it means that the light is normal. If VF=0 or VF≈3V is measured and there is no light, it means that the luminescent tube is broken.
2. Detection of infrared LED
Due to the inability of the human eye to see the infrared light emitted by infrared LED, a single infrared LED typically emits only a few mW of power. The angular distribution of infrared LED emission intensity varies among different models. The forward voltage drop of infrared LED is generally 1.3-2.5V. It is precisely because the infrared light emitted by it is invisible to the human eye that we can only determine whether the forward and reverse electrical characteristics of its PN junction are normal using the visible light LED detection method mentioned above, and cannot determine whether its luminescence is normal. For this reason, it is best to prepare a photosensitive device (such as a 2CR or 2DR silicon photocell) as a receiver, and use a multimeter to measure the voltage changes at both ends of the photocell, in order to determine whether the infrared LED emits infrared light after adding an appropriate forward current.
3. Check with a multimeter
Using a pointer multimeter with a ×10kΩ block, we can roughly judge whether the LED is good or bad. Normally, the forward resistance of the diode is tens to 200kΩ, and the reverse resistance is ∝. If the forward resistance value is 0 or ∞, and the reverse resistance value is very small or 0, the LED is easily damaged. This detection method cannot actually see the luminescence of the luminous tube, because the ×10kΩ block cannot provide a large forward current to the LED.
If there are two pointer multimeters (preferably the same model), we can better check the LED. We use a wire to connect the "+" post of one of the multimeters to the "-" post of the other meter. The remaining "-" pen is connected to the positive electrode (P area) of the luminous tube under test, and the remaining "+" pen is connected to the negative electrode (N area) of the luminous tube under test. Both multimeters are set to ×10Ω block. Under normal circumstances, the LED can emit light normally after it is turned on. If the brightness of the LED is very low or even does not emit light, we can dial both multimeters to ×1Ω. If the LED is still very dim or even does not emit light, it means that the light emitting diode is bad or damaged. It should be noted that we cannot place the two multimeters at ×1Ω at the beginning of the measurement, so as not to damage the LED due to excessive current.
Ⅵ. The advantages and disadvantages of LED
(1) Cycling: LED is well suited for applications where frequent on-off cycling occurs. This is different from incandescent and fluorescent lamps, which fail more quickly when cycled frequently, or high-intensity discharge lamps (HID lamps), which take a long time to restart.
(2) Color: LED can emit light of the desired color without using any color filters like traditional lighting methods. This is more efficient and reduces initial costs.
(3) Lifespan: LED has a relatively long lifespan. One report estimates that LED has a lifetime of 35,000 to 50,000 hours, but the time to complete failure can be shorter or longer. Fluorescent tubes are usually rated for about 10,000 to 25,000 hours, some LED depending on usage conditions; incandescent bulbs are rated for 1,000 to 2,000 hours.
(4) Size: LED can be very small and easily attached to a printed circuit board.
(5) Turn-on time: The LED lights up extremely fast. A typical red LED reaches full brightness within one microsecond. LED used in communication equipment can have faster response times.
(6) Focusing: The solid package of an LED can be designed to focus its light. Incandescent and fluorescent light sources often require external reflectors to collect the light and direct it in a usable manner. For larger LED packages, we typically use total internal reflection (TIR) lenses to achieve the same effect. When a lot of light is needed, LED typically deploy many light sources. These light sources are difficult to focus or collimate on the same target.
(7) Slow failure: The failure of LED is mainly dimming over time, rather than the sudden failure of incandescent bulbs.
(1) Light pollution: Due to the fact that white LED emits more short wavelength light than high-pressure sodium vapor lamps and other light sources, the increased sensitivity of blue and green in dark vision means that white LED used in outdoor lighting generate more light pollution.
(2) Voltage sensitivity: The LED must be supplied with a voltage above its threshold voltage and a current below its rated value. Small changes in applied voltage can cause large changes in current and lifetime. Therefore, they require a current regulated power supply (usually just a series resistor for the LED indicator).
(3) Use in winter: Since LED lights do not emit much heat compared to incandescent lights. Therefore LED lights used for traffic control may be obscured by snow and cause accidents.
(4) Decrease in efficiency: The efficiency of LED decreases with the increase of current. The higher the current, the higher the heat generation, which affects the life of the LED. These effects place a practical limit on the current that can flow through the LED in high power applications.
(5) Temperature dependence: LED performance largely depends on the ambient temperature of the working environment. Overdriving LED at high ambient temperatures can cause overheating of the LED package and eventual device failure. It needs an adequate heatsink for a long lifespan. This is especially important in automotive, medical and military applications where devices must operate over a wide temperature range and require a low failure rate.
(6) Impact on wildlife: LED lights are more attractive to insects than sodium vapor lamps, so much so that people worry that LED lights may disrupt food webs. LED lighting near beaches, especially the strong blue and white colors, can disorient newly hatched turtles, causing them to wander inland.
Ⅶ. Advantages of LED in electronic display
Following are the main advantages of LED in electronic displays.
1. LED operates in a wide temperature range, such as 0° – 70°. Plus, it's extremely durable and can withstand shocks and changes.
2. The turn-on and turn-off time or switching time of the LED is less than 1 nanosecond. Therefore, LED is used in dynamic operation.
3. LED is small in size and can be stacked together to form numeric and alphanumeric displays in a high-density matrix.
4. LED can emit light in different colors, such as red, yellow, green and amber.
5. LED has high efficiency, but it requires moderate power to operate. Typically, full brightness requires 1.2V and 20mA. Therefore, it is used where there is less electricity.
6. LED is very economical and highly reliable because it is manufactured with the same technology as transistors.