Detailed explanation of the basic knowledge of LED light-emitting diodes

LED is an abbreviation of Light Emitting Diode, which is translated into "light emitting diode" in Chinese. As the name suggests, a light emitting diode is an electronic device that can convert electrical energy into light energy and has the characteristics of a diode. At present, different light emitting diodes can emit light of different wavelengths from infrared to blue. At present, light emitting diodes that emit purple and even ultraviolet light have also been born. In addition, there are white light LEDs that convert blue light into white light by coating phosphor on blue light LEDs .

LED color and technology:

Different materials used to make LEDs can produce photons with different energies , which can control the wavelength of light emitted by the LED, that is, the spectrum or color. The material used in the first LED in history was gallium arsenide (Ga), whose forward PN junction voltage drop (VF, which can be understood as the lighting or operating voltage) was 1.424V, and the light emitted was in the infrared spectrum. Another commonly used LED material is gallium phosphide (Ga), whose forward PN junction voltage drop is 2.261V, and the light emitted is green light.

Based on these two materials, the early LED industry used GaAs1-xPx material structure, which theoretically could produce LEDs with any wavelength from infrared light to green light. The subscript X represents the percentage of phosphorus replacing arsenic. Generally, the wavelength color of the LED can be determined by the PN junction voltage drop. Typical examples include GaAs0.6P0.4 red LED, GaAs0.35P0.65 orange LED, GaAs0.14P0.86 yellow LED, etc. Since gallium, arsenic and phosphorus are used in manufacturing, these LEDs are commonly known as three-element light-emitting tubes. GaN (gallium nitride) blue LED, GaP green LED and GaAs infrared LED are called two-element light-emitting tubes. The latest technology is the four-element LED made of AlGaInN, which is a four-element material that mixes aluminum (Al), calcium (Ca), indium (In) and nitrogen (N), which can cover all visible light and part of the ultraviolet spectrum.

LED luminous intensity :

The units of measurement for luminous intensity include illuminance (Lux), luminous flux ( Lumen), and luminous intensity (Candle power ).

1CD (candela) refers to the luminous intensity of a completely radiant object at the freezing point of platinum, per one-sixtieth square centimeter area. (Previously, it referred to a whale oil candle with a diameter of 2.2 cm and a mass of 75.5 grams, burning 7.78 grams per hour, with a flame height of 4.5 cm, and luminous intensity in the horizontal direction)

1L (lumen) refers to the luminous flux of 1 CD candlelight irradiated on a plane with a distance of 1 cm and an area of ​​1 square centimeter.

1Lux refers to the illumination when 1L of luminous flux is evenly distributed over an area of ​​1 square meter.

Generally, active light sources use the unit of luminous intensity, candlelight CD, such as incandescent lamps, LEDs, etc.; reflective or penetrating objects use the unit of luminous flux, lumen L, such as LCD projectors, etc.; and the unit of illumination, Lux, is generally used in photography and other fields. The three units of measurement are equivalent in value, but need to be understood from different perspectives. For example: if the brightness (luminous flux) of an LCD projector is 1600 lumens, and the size of its projection to the full reflection screen is 60 inches (1 square meter), then its illumination is 1600 lux. Assuming that its light outlet is 1 cm away from the light source and the area of ​​the light outlet is 1 square centimeter, the luminous intensity of the light outlet is 1600CD. However, due to the loss of light propagation, the loss of reflection or light-transmitting film, and the uneven distribution of light, the brightness of a real LCD projector will be greatly reduced, and generally 50% efficiency is good.

In actual use, light intensity calculation often uses data units that are easier to measure or use in reverse. For LED display screens, this kind of active light source generally uses CD/square meter as the unit of luminous intensity, and uses the observation angle as an auxiliary parameter, which is equivalent to the unit of illumination lux on the surface of the screen; multiply this value by the effective display area of ​​the screen to obtain the luminous intensity of the entire screen at the best viewing angle. Assuming that the luminous intensity of each pixel in the screen is constant in the corresponding space, this value can be considered as the luminous flux of the entire screen. Generally, outdoor LED display screens must reach a brightness of more than 4000 CD/square meter to have a relatively ideal display effect under sunlight. The maximum brightness of ordinary indoor LEDs is around 700 to 2000 CD/square meter.

The luminous intensity of a single LED is measured in CD:

The viewing angle parameter is also provided. The luminous intensity has nothing to do with the color of the LED. The luminous intensity of a single tube ranges from a few mCD to 5,000 mCD. The luminous intensity given by the LED manufacturer refers to the point where the luminous intensity is the highest at the best viewing angle and the center position when the LED is lit at a current of 20mA. The shape of the top lens when encapsulating the LED and the position of the LED chip from the top lens determine the LED viewing angle and light intensity distribution. Generally speaking, the larger the viewing angle of the same LED, the smaller the maximum luminous intensity, but the accumulated luminous flux on the entire three-dimensional hemisphere remains unchanged.

When multiple LEDs are arranged closely and regularly, their luminous spheres overlap each other, resulting in a relatively uniform distribution of luminous intensity across the entire luminous plane. When calculating the luminous intensity of the display screen , the maximum point luminous intensity value provided by the manufacturer should be multiplied by 30% to 90% based on the LED viewing angle and the LED emission density, and the result is the average luminous intensity of a single tube.

LED has a long luminous life:

Manufacturers generally indicate that the LED lifespan is more than 100,000 hours. In reality, attention should also be paid to the brightness decay cycle of the LED. For example, most UR red tubes used in car taillights will only be half as bright after being lit for more than ten to dozens of hours. The brightness decay cycle is closely related to the material process of LED production. Generally, if economic conditions permit, four-element LEDs with slower brightness decay should be selected.

Color matching, white balance:

White is a mixture of red, green and blue in proportion to their brightness. When the brightness of green is 69%, the brightness of red is 21% and the brightness of blue is 10%, the human eye perceives pure white after the mixture. However, the chromaticity coordinates of the red, green and blue colors of LED cannot achieve the effect of the full color spectrum due to the process and other reasons. Controlling the brightness of the primary colors, including the deviated primary colors, to obtain white light is called color matching.

When matching the colors for a full-color LED display, in order to achieve the best brightness and the lowest cost, you should try to choose LED devices with the three primary colors luminous intensity in a ratio of approximately 3:6:1 to form pixels.

White balance requires that the three primary colors still synthesize into pure white at the same gray value.

Primary colors, base colors:

Primary colors refer to the basic colors that can be synthesized into various colors. The primary colors in color light are red, green, and blue. The following figure is a spectrum chart, and the three vertices in the chart are ideal primary color wavelengths. If the primary colors are biased, the area that can be synthesized will be reduced, and the triangle in the spectrum chart will be smaller. From a visual perspective, the colors will not only be biased, but also less rich.

The red, green and blue light emitted by LED can be roughly divided into purple-red, pure red, orange-red, orange, orange-yellow, yellow, yellow-green, pure green, emerald green, blue-green, pure blue, blue-purple, etc. according to their different wavelength characteristics. Orange-red, yellow-green and blue-purple are much cheaper than pure red, pure green and pure blue. Green is the most important of the three primary colors because it occupies 69% of the brightness of white and is in the center of the horizontal color arrangement table. Therefore, when weighing the relationship between color purity and price, green is the object of focus.

2. LED anti-static requirements

Blue light, pure green light, blue-green light, white light, purple light, and purple-red light LEDs are first-level anti-static sensitive devices. Please take appropriate precautions during the hand-held and packaged production process. The following is for reference only:

Must be able to prevent static electricity: assemblers must wear anti-static clothing (such as anti-static clothes, hats, shoes, finger cots or gloves, etc.)

The static electricity on its surface or inside must be dissipated quickly: assembly operators need to wear anti-static wrist straps.

Can provide shielding protection against sudden electrostatic discharge or electric field shock:

① The assembly table (workbench) must use an anti-static mat and be grounded.

② Anti-static component boxes are required to hold LEDs .

③ The soldering iron, lead cutting machine, tin furnace (or automatic reflow soldering equipment) must all be grounded.

During the operation, try to avoid directly touching the pins of the light-emitting tube, and try to touch the colloid part when taking it or placing it.

Grounding measures should completely prevent the generation of static electricity. The grounding of the workbench, soldering iron, foot cutting machine, tin furnace (or automatic reflow soldering equipment) must be led into the soil with a thick iron wire, and a large iron block must be tied to the end of the iron wire and buried 1 meter below the ground surface. Each grounding wire must be connected to the main line. If the anti-static ring worn by the operator has a lead-out wire, the lead-out wire must also be connected to the buried ground wire.

Semi-finished product and finished product testing equipment also need to be grounded.

3. Precautions for using LED

(1) The characteristics of LED are close to those of a stable diode. When the operating voltage changes by 0.1V, the operating current may change by about 20mA. For safety reasons, a series current limiting resistor is usually used. The huge energy loss is obviously not suitable for solar lawn lights, and the LED brightness changes with the operating voltage. A boost circuit is a good solution, and a simple constant current circuit can also be used. In short, the current must be automatically limited, otherwise the LED will be damaged.

(2) Generally, the peak current of LED is 50~100mA, and the reverse voltage is about 6V. Be careful not to exceed this limit, especially when the solar cell is reversely connected or the battery is unloaded. When the peak voltage of the boost circuit is too high, it is likely to exceed this limit and damage the LED.

(3) LED has poor temperature characteristics. When the temperature rises by 5°C, the luminous flux decreases by 3%. Be more careful when using it in summer.

(4) The operating voltage is highly discrete. The operating voltages of LEDs of the same model and the same batch vary to a certain extent, so they are not suitable for parallel use. If they must be used in parallel, the current sharing should be fully considered.

(5) The color temperature of ultra-bright white LEDs is 6400~30000k. Currently, ultra-bright white LEDs with low color temperature have not yet entered the market. Therefore, the penetration ability of solar lawn lights made of ultra-bright white LEDs is relatively poor. This should be paid attention to in optical design.

(6) Static electricity has a great impact on ultra-bright white light LEDs. Anti-static facilities should be used during installation, and workers should wear anti-static wristbands. The damage to ultra-bright white light LEDs caused by static electricity may not be visible to the naked eye at the time, but their service life will be shortened.