1. Main features of light-emitting diode LED
(1) Working under the conditions of low voltage (1.5～2.5V) and small current (5～30mA), sufficient brightness can be obtained.
(2) The luminous response speed is fast (10-7～10-9 s), the high frequency characteristic is good, and the pulse information can be displayed.
(3) Good monochromaticity, common colors are red, green, yellow, orange, etc.
(4) Small size. The shape of the light-emitting surface is divided into circle, rectangle, and special shape (triangle, etc.). Among them, the outer diameter of the round tube has φ1, φ2, φ3, φ4, φ5, φ8, φ10, φ12, φ15, φ20 (mm) and other specifications, and the diameter of 1mm belongs to the ultra-micro LED.
(5) Good vibration and impact resistance, low power consumption and long life. Since the PN junction of the LED works in the forward conduction state, the power consumption of the original radio is low. As long as the necessary current limiting measures are added, it can be used for a long time, and the life span is more than 100,000 hours, even up to 1 million hours.
(6) Flexible use, it can be made into digital tube, character tube, level display, dot matrix display, solid light-emitting board, LED flat-polar TV screen, etc. according to needs.
(7) It is easy to match with digital integrated circuit.
2. The principle of light-emitting diodes
Inside the light-emitting diode is a PN junction with light-emitting characteristics. When the PN junction is turned on, it relies on the injection of minority carriers and subsequent recombination to emit light. The appearance, symbol and volt-ampere characteristics of ordinary light-emitting diodes are shown in Figure 1. The forward volt-ampere characteristic curve of the LED is relatively steep, and there is almost current before the forward conduction. When the voltage exceeds the turn-on voltage, the current rises sharply. Therefore, LED is a current-controlled semiconductor device, and its luminous brightness L (in cd/m2, read as candela per square meter) is approximately a positive double with the forward current IF, and there is a formula
L =K IFm
In the formula, K is the proportional coefficient, in the small current range (IF=1～10mA), m=1.3～1.5. When IF>10mA, m=1, formula (5.10.1) is simplified to L =K IF
means that the brightness is proportional to the forward current. Taking phosphor gallium arsenide yellow LED as an example, the relationship between relative luminous intensity and forward current is shown in Figure 2. The forward voltage of the LED is related to the forward current and the semiconductor material of the die. When using, you should select the appropriate IF value according to the required display brightness (usually about 10mA, for high-brightness LEDs, you can choose 1~2mA) to ensure that the brightness is moderate, and the LED will not be damaged. If the current is too large, it will burn the PN junction of the LED. In addition, the life span of the LED will be shortened.
Due to the low power consumption, small size, bright colors, fast response speed, and long life of light-emitting diodes, they are often used as level indicators, tuning indicators, power indicators, etc. for radios, radios and electronic instruments. The light-emitting diode has a certain voltage stabilizing effect when the forward conduction is conducted, and it can also be used as a voltage stabilizing diode in a DC voltage stabilizer to provide a reference voltage and double as a power indicator. At present, there is a second light emitting tube (such as BT104-B2, BT102-F) with reflecting cavity and fixing device on the market, which is easy to fix on the instrument panel.
The output spectrum of the LED determines its luminous color and light radiation purity, and also reflects the characteristics of semiconductor materials. Common die materials are gallium phosphide (GaP), gallium arsenide (GaAsP), gallium arsenide phosphorous (GaAlAs), and gallium aluminum arsenide (GaN) gallium nitride can emit blue light.
3. Precautions for use
(1) The polarity of the tube must not be reversed. Generally speaking, the longer lead is the positive electrode, and the shorter lead is the negative electrode.
(2) Each parameter in use shall not exceed the specified limit value. The forward current IF is not allowed to exceed the limit working current IFM value, and as the ambient temperature rises, it must be used for derating. The long-term use temperature should not exceed 75℃.
(3) The soldering time should be as short as possible, and the solder joint should not be at the root of the pin. When soldering, you should use tweezers to clamp the root of the pin to dissipate heat. It is advisable to use a neutral flux (rosin) or use rosin solder wire.
(4) Soaking or cleaning with organic solution is strictly prohibited.
(5) Current limiting resistors must be added to the LED drive circuit, generally one hundred ohms to several hundred ohms, depending on the power supply voltage.
(6) When the brightness of the light is basically unchanged, the use of pulse voltage driving can save power consumption. For the LED dot matrix display, the use of scanning display mode can greatly reduce the power consumption of the whole machine.
4. Check the quality of light-emitting diodes
The light-emitting diode has unidirectional conductivity, and its forward and reverse resistance can be measured using the R×10k file. Generally, the forward resistance should be less than 30k ohms, and the reverse resistance should be greater than 1M ohms. If the forward and reverse resistances are both zero, it indicates an internal breakdown short circuit. If the forward and reverse resistances are both infinite, the internal open circuit is proved.
The types and main parameters of common light-emitting diodes are shown in Table 2. Two points need to be explained: First, for the die of the same material, the light-emitting color is different due to the different dopants; second, the LED is a current-controlled device, VF varies with IF, and the marked VF value is only for reference.
In addition, the positive and negative electrodes of the light-emitting diode can also be distinguished according to the shape. The tube produced in the early stage had a metal tube seat, an optical lens was covered on it, and there was a protrusion on the side of the tube, and the positive electrode was close to the protrusion. The LEDs currently produced are all encapsulated with transparent or translucent epoxy resin, and the epoxy resin is used to form a lens for magnification and focusing. The longer lead of this type of tube is the positive electrode.
