(1) Principle of LED illumination
The light-emitting diode is made of a III-IV compound such as GaAs (gallium arsenide), GaP (gallium phosphide), GaAsP (phosphorus gallium arsenide) and the like, and its core is a PN junction. Therefore, it has the I-N characteristics of a general P-N junction, that is, forward conduction, reverse cutoff, and breakdown characteristics. In addition, it has luminescent properties under certain conditions. At the forward voltage, electrons are injected into the P region from the N region, and holes are injected into the N region from the P region. A part of the minority carriers (small children) entering the other area is combined with the majority carriers (multiple sub-groups) to emit light, as shown in Fig. 1.
Assuming that luminescence occurs in the P region, the injected electrons directly composite with the valence band holes to emit light, or are first captured by the luminescent center and then condensed with the holes. In addition to this luminescent composite, some electrons are captured by the non-luminous center (this center is near the middle of the conduction band and the intermediate band), and then recombined with the holes, and the energy released each time is not large, and visible light cannot be formed. The greater the ratio of the amount of luminescent composite to the amount of non-luminescent composite, the higher the photon efficiency. Since the recombination emits light in the minority carrier diffusion region, light is generated only within a few μm of the PN junction surface.
Theory and practice prove that the peak wavelength λ of light and the semiconductor material forbidden bandwidth in the light-emitting region
Degree Eg, ie
The unit of Eg in the formula is electron volt (eV). If visible light (wavelength between 380 nm and 780 nm red) is produced, the Eg of the semiconductor material should be between 3.26 and 1.63 eV. Light longer than the wavelength of red light is infrared light. There are now infrared, red, yellow, green and blue light-emitting diodes, but the blue-light diodes are expensive and expensive, and are not commonly used.
(two) the characteristics of LED
1. Meaning of limit parameters
(1) Allowable power consumption Pm: The maximum value of the product of the forward DC voltage applied across the LED and the current flowing through it. Above this value, the LED is hot and damaged.
(2) Maximum forward DC current IFm: The maximum forward DC current allowed to be applied. Exceeding this value can damage the diode.
(3) Maximum reverse voltage VRm: The maximum reverse voltage allowed to be applied. Above this value, the LED may be damaged by breakdown.
(4) Working environment topm: The ambient temperature range in which the LED can work normally. Below or above this temperature range, the LED will not work properly and the efficiency will be greatly reduced.
2. Meaning of electrical parameters
(1) Spectral distribution and peak wavelength: The light emitted by a certain LED is not a single wavelength, and its wavelength is generally as shown in Fig. 2.
It can be seen from the figure that the light intensity of a certain wavelength λ0 in the light emitted by the light-emitting tube is the largest, and the wavelength is the peak wavelength.
(2) Luminous intensity IV: The luminous intensity of the light-emitting diode generally refers to the luminous intensity in the direction of the normal line (to the axis of the cylindrical luminous tube). If the radiation intensity in this direction is (1/683) W/sr, then the light can be 1 candela (symbol cd). Since the general LED has a small intensity of light emission, the intensity of the light is commonly used as a unit of candela (mcd).
(3) Spectral half-width Δλ: It indicates the spectral purity of the arc tube. It refers to the interval between the two wavelengths corresponding to the 1/2 peak intensity in Figure 3.
(4) Half-value angle θ 1/2 and viewing angle: θ 1/2 means an angle between the direction in which the luminous intensity value is half of the axial intensity value and the axial direction of the light emission (normal direction).
Two times the half value angle is the angle of view (or half power angle).
Figure 3 shows the angular distribution of the luminous intensity of two different types of LEDs. The coordinates of the mid-perpendicular line (normal) AO are the relative luminous intensities (ie, the ratio of the luminous intensity to the maximum luminous intensity). Obviously, the relative luminous intensity in the normal direction is 1, and the larger the angle from the normal direction, the smaller the relative luminous intensity. From this graph, a half value angle or a viewing angle value can be obtained.
(5) Forward working current If: It refers to the forward current value when the LED is normally illuminated. In actual use, the IF should be selected below 0.6·IFm as needed.
(6) Forward working voltage VF: The operating voltage given in the parameter table is obtained at a given forward current. It is generally measured at IF = 20 mA. The forward working voltage VF of the LED is 1.4~3V. As the outside temperature rises, the VF will drop.
(7) V-I characteristics: The relationship between the voltage and current of the light-emitting diode can be represented by FIG.
When the forward voltage is just below a certain value (called the threshold), the current is extremely small and does not emit light. When the voltage exceeds a certain value, the forward current rapidly increases with the voltage and illuminates. From the V-I curve, parameters such as forward voltage, reverse current and reverse voltage of the arc tube can be obtained. The forward LED reverse leakage current IR<10μA or less.
(three) classification of LED
1. According to the luminous color of the luminous tube
According to the color of the luminous tube, it can be divided into red, orange, green (subdivided into yellow-green, standard green and pure green), blue light and so on. In addition, some light-emitting diodes include chips of two or three colors.
The light-emitting diodes of the above various colors can be classified into four types of colored transparent, colorless transparent, colored scattering and colorless scattering according to whether the light-emitting diode is doped with or without a scattering agent, colored or colorless. Scatter-type LEDs are used for indicator lights.
2. According to the characteristics of the light-emitting surface of the luminous tube
According to the characteristics of the light-emitting surface of the light-emitting tube, the round lamp, the square lamp, the rectangular, the surface light-emitting tube, the lateral tube, the surface-mounted micro tube, and the like. The circular lamps are divided into φ2 mm, φ4.4 mm, φ5 mm, φ8 mm, φ10 mm, and φ20 mm according to the diameter. In foreign countries, a light-emitting diode of φ3 mm is generally referred to as T-1; a φ5 mm is referred to as T-1 (3/4); and a φ4.4 mm is referred to as T-1 (1/4).
The angular distribution of the circular luminous intensity can be estimated from the half value angle size. There are three categories from the angular distribution of luminous intensity:
Directivity. It is usually a tip epoxy package or a metal reflector package with no scattering agent. The half value angle is 5 ° ~ 20 ° or less, has a high directivity, can be used as a local illumination source, or combined with a light detector to form an automatic detection system.
(2) Standard type. Usually used as indicator light, its half value angle is 20 ° ~ 45 °.
(3) Scattering type. This is an indicator light with a large viewing angle, the half value angle is 45° to 90° or more, and the amount of the scattering agent is large.
3. According to the structure of the light-emitting diode
According to the structure of the light-emitting diode, there are structures such as a full epoxy encapsulation, a metal base epoxy package, a ceramic base epoxy package, and a glass package.
4. According to luminous intensity and working current
LEDs with ordinary brightness according to luminous intensity and working current (lighting intensity <10mcd); ultra-high brightness LEDs (lighting intensity >100mcd); high-intensity light-emitting diodes with luminous intensity between 10 and 100mcd.
Generally, the operating current of the LED is between ten mA and several tens of mA, and the operating current of the low current LED is below 2 mA (the brightness is the same as that of the ordinary luminous tube).
In addition to the above classification methods, there are methods for classifying by chip material and classifying by function.
(4) Application of LED
Since the color, size, shape, luminous intensity, and transparency of the light-emitting diode are different, the light-emitting diode should be appropriately selected according to actual needs.
Since the LED has a limitation of the maximum forward current IFm and the maximum reverse voltage VRm, it should be ensured that this value is not exceeded. For safety reasons, the actual current IF should be below 0.6 IFm; the possible reverse voltage VR < 0.6 VRm should be allowed.
LEDs are widely used in a variety of electronic and electronic devices and can be used as power indicators, level indicators or micro-light sources. Infrared light pipes are often used in remote controls for televisions, video recorders, and the like.
(1) A circuit for making a miniature flashlight using a high-brightness or ultra-high-brightness light-emitting diode is shown in FIG. 5. In the figure, the resistor R current limiting resistor should be such that the current of the LED should be lower than the maximum allowable current IFm.
(2) Figures 6(a), (b), and (c) show the DC power supply, the rectified power supply, and the AC power supply indicating circuit.
Resistance ≈(E-VF)/IF in Figure (a);
R≈(1.4Vi-VF)/IF in Figure (b);
R≈Vi/IF in Figure (c)
Where Vi is the rms value of the AC voltage.
(3) Single LED level indicating circuit. At the output of the amplifier, oscillator or pulse digital circuit, LEDs can be used to indicate whether the output signal is normal, as shown in Figure 7. R is a current limiting resistor. The LED may only illuminate when the output voltage is greater than the threshold voltage of the LED.
(4) Single LED can be used as a low voltage regulator. Since the LED is forward-conducting, the current changes very rapidly with voltage, and it has the characteristics of ordinary voltage regulator. The stable voltage of the LED is between 1.4 and 3V. VF should be selected as needed, as shown in Figure 8.
(5) Level meter. At present, LED level meters are widely used in audio equipment. It uses multiple light pipes to indicate the output signal level, that is, the number of LEDs that emit light is different, indicating the change of the output level. Fig. 9 is a level meter composed of five light emitting diodes. When the input signal level is low, it does not emit light at all. When the input signal level increases, LED1 first lights up, and then LED2 lights up.
(5) Detection of light-emitting diodes
1. Detection of ordinary light-emitting diodes
(1) Test with a multimeter. The use of a pointer type multimeter with a ×10kΩ block can roughly judge the quality of the LED. Normally, the forward resistance of the diode is tens to 200 kΩ, and the value of the reverse resistance is ∝. If the forward resistance value is 0 or ∞, and the reverse resistance value is small or 0, it is easily damaged. This kind of detection method can't see the illuminating condition of the illuminating tube in reality, because the ×10kΩ block can not provide a large forward current to the LED.
If there are two pointer multimeters (preferably the same model), it can better check the illumination of the LEDs. Use one wire to connect the “+” terminal of one of the multimeters to the “-” terminal of the other. The remaining "-" pen is connected to the positive pole (P zone) of the illuminated tube, and the remaining "+" pen is connected to the negative pole (N zone) of the illuminated tube. Both multimeters are set to ×10Ω. Under normal circumstances, it will glow normally after being turned on. If
The brightness is very low, even if it does not emit light. You can set both multimeters to ×1Ω. If it is still dark or even does not emit light, it indicates that the LED is poor or damaged. It should be noted that the two multimeters should not be placed at ×1 Ω at the beginning to avoid excessive current and damage to the LEDs.
(2) External power supply measurement. The optical and electrical characteristics of the LED can be accurately measured with a 3V regulated source or two series of dry cells and a multimeter (pointer or digital). To do this, connect the circuit as shown in Figure 10. If the measured VF is between 1.4 and 3V, and the brightness of the light is normal, it can be said that the light is normal. If VF=0 or VF≈3V is measured and does not emit light, the light-emitting tube is broken.
2. Infrared light emitting diode detection
Due to the infrared light-emitting diode, it emits infrared light of 1 to 3 μm, which is invisible to the human eye. Usually, the emission power of a single infrared light-emitting diode is only a few mW, and the angular distribution of the luminous intensity of different types of infrared LEDs is also different. The forward voltage drop of an infrared LED is generally 1.3 to 2.5V. Because the infrared light emitted by the human eye is invisible, the detection method of the visible light LED can only determine whether the positive and negative electrical characteristics of the PN junction are normal, and it is impossible to determine whether the illumination condition is normal or not. For this reason, it is best to prepare a photosensor (such as a 2CR, 2DR type silicon photocell) as a receiver. Use a multimeter to measure the change in voltage across the battery. To determine whether the infrared LED emits infrared light after adding an appropriate forward current. Its measurement circuit is shown in Figure 11.