Characteristics and Significance of Light Emitting Diodes
Semiconductor light-emitting devices include semiconductor light-emitting diodes (LEDs for short), digital tubes, symbol tubes, rice-shaped tubes, and dot-matrix display screens. In fact, each light emitting unit in the digital tube, symbol tube, rice tube and matrix tube is a light emitting diode.
Meaning of limit parameter
(1) Allowable power consumption Pm: The maximum value of the product of the forward DC voltage across the LED and the current flowing through it. Beyond this value, the LED heats up and is damaged.
(2) Maximum forward DC current IFm: The maximum forward DC current allowed. Exceeding this value can damage the diode.
(3) Maximum reverse voltage VRm: the maximum reverse voltage allowed. Beyond this value, the LED may be damaged by breakdown.
(4) Working environment topm: The ambient temperature range in which the light-emitting diode can work normally. Below or above this temperature range, the light emitting diode will not work properly, and the efficiency will be greatly reduced.
2. The meaning of electrical parameters
(1) Spectral distribution and peak wavelength: the light emitted by a certain LED is not a single wavelength.
It can be seen from the figure that among the light emitted by the light-emitting tube, the light intensity at a certain wavelength λ0 is the largest, and this 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 (the axis of the cylindrical light-emitting tube). If the radiation intensity in this direction is (1/683) W / sr, 1 candela is emitted (symbol cd). Due to the small intensity of light emitting diodes in general, candela is commonly used as a unit of candela (mcd).
(3) Spectral half width Δλ: It represents the spectral purity of the arc tube.
(4) Half-value angle θ1 / 2 and viewing angle: θ1 / 2 refers to the angle between the direction in which the light emission intensity value is half of the axial intensity value and the light emission axis (normal direction). Two times the half value angle is the viewing angle (or half power angle).
Fig. 3 shows the light emission intensity distribution of two different types of light emitting diodes. The coordinates of the perpendicular (normal) AO are the relative luminous intensity (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 away from the normal direction, the smaller the relative luminous intensity.
(5) Forward working current If: It refers to the forward current value when the light emitting diode is normally emitting light. In actual use, the IF should be selected below 0.6 · IFm as required.
(6) Forward working voltage VF: The working voltage given in the parameter table is obtained under a given forward current. Generally measured at IF = 20mA. The forward working voltage VF of the light emitting diode is between 1.4 and 3V. When the outside temperature rises, VF will drop.
(7) V-I characteristics: the relationship between the voltage and current of the light-emitting diode.
When the forward voltage is less than 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 increases rapidly with the voltage and emits light. From the V-I curve, parameters such as forward voltage, reverse current, and reverse voltage of the arc tube can be obtained. The forward leakage current IR of the arc tube is less than 10μA.