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一种半导体PN结型二极管，GaA构造的二极管,有电流通过时会发光.当施加正向偏置时，将发出波长取决于其材料和掺杂的光。砷化镓(GaAs)的晶体结构允许不可见的红外(IR)发射。、在砷化镓 中加入磷形成磷砷化镓((GaAsP)，可使发射移向可见的红区。磷化镓(GaP)在红、黄、绿区发射。在GaAsP上的砷化铝镓(A1GaAsP)则可 产生更明亮(高强度)的红和黄光发射。在GaP上的GaP可发射高强度的绿光。
（一）LED发光原理
发光二极管是由Ⅲ-Ⅳ族化合物，如GaAs（砷化镓）、GaP（磷化镓）、GaAsP（磷砷化镓）等半导体制成的，其核心是PN结。因此它具有一般P-N结的I-N特性，即正向导通，反向截止、击穿特性。此外，在一定条件下，它还具有发光特性。在正向电压下，电子由N区注入P区，空穴由P区注入N区。进入对方区域的少数载流子（少子）一部分与多数载流子（多子）复合而发光，如图1所示。
 假设发光是在P区中发生的，那么注入的电子与价带空穴直接复合而发光，或者先被发光中心捕获后，再与空穴复合发光。除了这种发光复合外，还有些电子被非发光中心（这个中心介于导带、介带中间附近）捕获，而后再与空穴复合，每次释放的能量不大，不能形成可见光。发光的复合量相对于非发光复合量的比例越大，光量子效率越高。由于复合是在少子扩散区内发光的，所以光仅在靠近PN结面数μm以内产生。
理论和实践证明，光的峰值波长λ与发光区域的半导体材料禁带宽度Ｅg有关，即
λ≈1240/Eg（mm）
式中Eg的单位为电子伏特（eV）。若能产生可见光（波长在380nm紫光～780nm红光），半导体材料的Eg应在3.26～1.63eV之间。比红光波长长的光为红外光。现在已有红外、红、黄、绿及蓝光发光二极管，但其中蓝光二极管成本、价格很高，使用不普遍。
（二）LED的特性
1．极限参数的意义
（1）允许功耗Pm:允许加于LED两端正向直流电压与流过它的电流之积的最大值。超过此值，LED发热、损坏。
（2）最大正向直流电流IFm：允许加的最大的正向直流电流。超过此值可损坏二极管。
（3）最大反向电压VRm：所允许加的最大反向电压。超过此值，发光二极管可能被击穿损坏。
（4）工作环境topm:发光二极管可正常工作的环境温度范围。低于或高于此温度范围，发光二极管将不能正常工作，效率大大降低。
2．电参数的意义
（1）光谱分布和峰值波长：某一个发光二极管所发之光并非单一波长，其波长大体按图2所示。
 由图可见，该发光管所发之光中某一波长λ0的光强最大，该波长为峰值波长。
（2）发光强度IV：发光二极管的发光强度通常是指法线（对圆柱形发光管是指其轴线）方向上的发光强度。若在该方向上辐射强度为（1/683）W/sr时，则发光1坎德拉（符号为cd）。由于一般LED的发光二强度小，所以发光强度常用坎德拉(mcd)作单位。
（3）光谱半宽度Δλ:它表示发光管的光谱纯度.是指图3中1/2峰值光强所对应两波长之间隔.
（4）半值角θ1/2和视角：θ1/2是指发光强度值为轴向强度值一半的方向与发光轴向（法向）的夹角。
半值角的2倍为视角（或称半功率角）。
 图3给出的二只不同型号发光二极管发光强度角分布的情况。中垂线（法线）AO的坐标为相对发光强度（即发光强度与最大发光强度的之比）。显然，法线方向上南喽苑⒐馇慷任?，离开法线方向的角度越大，相对发光强度越小。由此图可以得到半值角或视角值。
（5）正向工作电流If：它是指发光二极管正常发光时的正向电流值。在实际使用中应根据需要选择IF在0.6·IFm以下。
（6）正向工作电压VF：参数表中给出的工作电压是在给定的正向电流下得到的。一般是在IF=20mA时测得的。发光二极管正向工作电压VF在1.4～3V。在外界温度升高时，VF将下降。
（7）V-I特性：发光二极管的电压与电流的关系可用图4表示。
 在正向电压正小于某一值（叫阈值）时，电流极小，不发光。当电压超过某一值后，正向电流随电压迅速增加，发光。由V-I曲线可以得出发光管的正向电压，反向电流及反向电压等参数。正向的发光管反向漏电流IR<10μA以下。
（三）LED的分类
1．按发光管发光颜色分
按发光管发光颜色分，可分成红色、橙色、绿色（又细分黄绿、标准绿和纯绿）、蓝光等。另外，有的发光二极管中包含二种或三种颜色的芯片。
根据发光二极管出光处掺或不掺散射剂、有色还是无色，上述各种颜色的发光二极管还可分成有色透明、无色透明、有色散射和无色散射四种类型。散射型发光二极管和达于做指示灯用。
2．按发光管出光面特征分
按发光管出光面特征分圆灯、方灯、矩形、面发光管、侧向管、表面安装用微型管等。圆形灯按直径分为φ2mm、φ4.4mm、φ5mm、φ8mm、φ10mm及φ20mm等。国外通常把φ3mm的发光二极管记作T-1；把φ5mm的记作T-1（3/4）；把φ4.4mm的记作T-1（1/4）。
由半值角大小可以估计圆形发光强度角分布情况。从发光强度角分布图来分有三类：
（1）高指向性。一般为尖头环氧封装，或是带金属反射腔封装，且不加散射剂。半值角为5°～20°或更小，具有很高的指向性，可作局部照明光源用，或与光检出器联用以组成自动检测系统。
（2）标准型。通常作指示灯用，其半值角为20°～45°。
（3）散射型。这是视角较大的指示灯，半值角为45°～90°或更大，散射剂的量较大。
3．按发光二极管的结构分
按发光二极管的结构分有全环氧包封、金属底座环氧封装、陶瓷底座环氧封装及玻璃封装等结构。
4．按发光强度和工作电流分
按发光强度和工作电流分有普通亮度的LED（发光强度<10mcd）；超高亮度的LED（发光强度>100mcd）；把发光强度在10～100mcd间的叫高亮度发光二极管。
一般LED的工作电流在十几mA至几十mA，而低电流LED的工作电流在2mA以下（亮度与普通发光管相同）。
除上述分类方法外，还有按芯片材料分类及按功能分类的方法。
（四）LED的应用
由于发光二极管的颜色、尺寸、形状、发光强度及透明情况等不同，所以使用发光二极管时应根据实际需要进行恰当选择。
由于发光二极管具有最大正向电流IFm、最大反向电压VRm的限制，使用时，应保证不超过此值。为安全起见，实际电流IF应在0.6IFm以下；应让可能出现的反向电压VR<0。6VRm。
LED被广泛用于种电子仪器和电子设备中，可作为电源指示灯、电平指示或微光源之用。红外发光管常被用于电视机、录像机等的遥控器中。
（1）利用高亮度或超高亮度发光二极管制作微型手电的电路如图5所示。图中电阻R限流电阻，其值应保证电源电压最高时应使LED的电流小于最大允许电流IFm。
 （2）图6(a)、(b)、(c)分别为直流电源、整流电源及交流电源指示电路。
图(a)中的电阻≈（E-VF）/IF；
图(b)中的R≈（1.4Vi-VF）/IF;
图(c)中的R≈Vi/IF
式中，Vi——交流电压有效值。
 （3）单LED电平指示电路。在放大器、振荡器或脉冲数字电路的输出端，可用LED表示输出信号是否正常，如图7所示。R为限流电阻。只有当输出电压大于LED的阈值电压时，LED才可能发光。
 （4）单LED可充作低压稳压管用。由于LED正向导通后，电流随电压变化非常快，具有普通稳压管稳压特性。发光二极管的稳定电压在1.4～3V间，应根据需要进行选择VF，如图8所示。
（5）电平表。目前，在音响设备中大量使用LED电平表。它是利用多只发光管指示输出信号电平的，即发光的LED数目不同，则表示输出电平的变化。图9是由5只发光二极管构成的电平表。当输入信号电平很低时，全不发光。输入信号电平增大时，首先LED1亮，再增大LED2亮……。
 （五）发光二极管的检测
1．普通发光二极管的检测
（1）用万用表检测。利用具有×10kΩ挡的指针式万用表可以大致判断发光二极管的好坏。正常时，二极管正向电阻阻值为几十至200kΩ,反向电阻的值为∝。如果正向电阻值为0或为∞，反向电阻值很小或为0，则易损坏。这种检测方法，不能实地看到发光管的发光情况，因为×10kΩ挡不能向LED提供较大正向电流。
如果有两块指针万用表（最好同型号）可以较好地检查发光二极管的发光情况。用一根导线将其中一块万用表的“+”接线柱与另一块表的“-”接线柱连接。余下的“-”笔接被测发光管的正极（P区），余下的“+”笔接被测发光管的负极（N区）。两块万用表均置×10Ω挡。正常情况下，接通后就能正常发光。若亮度很低，甚至不发光，可将两块万用表均拨至×1Ω若，若仍很暗，甚至不发光，则说明该发光二极管性能不良或损坏。应注意，不能一开始测量就将两块万用表置于×1Ω，以免电流过大，损坏发光二极管。
（2）外接电源测量。用3V稳压源或两节串联的干电池及万用表（指针式或数字式皆可）可以较准确测量发光二极管的光、电特性。为此可按图10所示连接电路即可。如果测得VF在1.4～3V之间，且发光亮度正常，可以说明发光正常。如果测得VF=0或VF≈3V，且不发光，说明发光管已坏。
 2．红外发光二极管的检测
由于红外发光二极管，它发射1～3μm的红外光，人眼看不到。通常单只红外发光二极管发射功率只有数mW，不同型号的红外LED发光强度角分布也不相同。红外LED的正向压降一般为1.3～2.5V。正是由于其发射的红外光人眼看不见，所以利用上述可见光LED的检测法只能判定其PN结正、反向电学特性是否正常，而无法判定其发光情况正常否。为此，最好准备一只光敏器件（如2CR、2DR型硅光电池）作接收器。用万用表测光电池两端电压的变化情况。来判断红外LED加上适当正向电流后是否发射红外光。其测量电路如图11所示
 LED显示器结构及分类
通过发光二极管芯片的适当连接（包括串联和并联）和适当的光学结构。可构成发光显示器的发光段或发光点。由这些发光段或发光点可以组成数码管、符号管、米字管、矩阵管、电平显示器管等等。通常把数码管、符号管、米字管共称笔画显示器，而把笔画显示器和矩阵管统称为字符显示器。
（一）LED显示器结构
基本的半导体数码管是由七个条状发光二极管芯片按图12排列而成的。可实现0～9的显示。其具体结构有“反射罩式”、“条形七段式”及“单片集成式多位数字式”等。
 （1）反射罩式数码管一般用白色塑料做成带反射腔的七段式外壳，将单个LED贴在与反射罩的七个反射腔互相对位的印刷电路板上，每个反射腔底部的中心位置就是LED芯片。在装反射罩前，用压焊方法在芯片和印刷电路上相应金属条之间连好φ30μm的硅铝丝或金属引线，在反射罩内滴入环氧树脂，再把带有芯片的印刷电路板与反射罩对位粘合，然后固化。
反射罩式数码管的封装方式有空封和实封两种。实封方式采用散射剂和染料的环氧树脂，较多地用于一位或双位器件。空封方式是在上方盖上滤波片和匀光膜，为提高器件的可靠性，必须在芯片和底板上涂以透明绝缘胶，这还可以提高光效率。这种方式一般用于四位以上的数字显示（或符号显示）。
（2）条形七段式数码管属于混合封装形式。它是把做好管芯的磷化镓或磷化镓圆片，划成内含一只或数只LED发光条，然后把同样的七条粘在日字形“可伐”框上，用压焊工艺连好内引线，再用环氧树脂包封起来。
（3）单片集成式多位数字显示器是在发光材料基片上（大圆片），利用集成电路工艺制作出大量七段数字显示图形，通过划片把合格芯片选出，对位贴在印刷电路板上，用压焊工艺引出引线，再在上面盖上“鱼眼透镜”外壳。它们适用于小型数字仪表中。
（4）符号管、米字管的制作方式与数码管类似。
（5）矩阵管（发光二极管点阵）也可采用类似于单片集成式多位数字显示器工艺方法制作。
（二）LED显示器分类
（1）按字高分：笔画显示器字高最小有1mm（单片集成式多位数码管字高一般在2～3mm）。其他类型笔画显示器最高可达12.7mm（0.5英寸）甚至达数百mm。
（2）按颜色分有红、橙、黄、绿等数种。
（3）按结构分，有反射罩式、单条七段式及单片集成式。
（4）从各发光段电极连接方式分有共阳极和共阴极两种。
所谓共阳方式是指笔画显示器各段发光管的阳极（即P区）是公共的，而阴极互相隔离。
所谓共阴方式是笔画显示器各段发光管的阴极（即N区）是公共的，而阳极是互相隔离的。如图13所示。
图13
（三）LED显示器的参数
由于LED显示器是以LED为基础的，所以它的光、电特性及极限参数意义大部分与发光二极管的相同。但由于LED显示器内含多个发光二极管，所以需有如下特殊参数：
1．发光强度比
由于数码管各段在同样的驱动电压时，各段正向电流不相同，所以各段发光强度不同。所有段的发光强度值中最大值与最小值之比为发光强度比。比值可以在1.5～2.3间，最大不能超过2.5。
2．脉冲正向电流
若笔画显示器每段典型正向直流工作电流为IF，则在脉冲下，正向电流可以远大于IF。脉冲占空比越小，脉冲正向电流可以越大。
（四）LED显示器的应用指南
1．七段数码显示器
（1）如果数码宇航局为共阳极形式，那么它的驱动级应为集电极开路（OC）结构，如图14（a）所示。
如果数码管为共阴极形式，它的驱动级应为射极输出或源极输出电路，如图14(b)所示。
例如国产TTL集成电路CT1049、CT4049为集电极开路形式七段字形译码驱动电路；而CMOS集成电路CC4511为源极输出七段锁存、译码驱动电路。
（2）控制数码管驱动级的控制电路（也称驱动电路）有静态式和动态式两类。
①静态驱动：静态驱动也称直流驱动。静态驱动是指每个数码管各用一个笔画译码器（如BCD码二-十进制译码器）译码驱动。图15是一位数码管的静态驱动之例。图集成电路TC5002BP内含有射极输出驱动级，所以采用共阴极数码管。A、B、C、D端为BCD码（二-十进制的8421码）输入端，BL为数码管熄灭及显示状态控制端，R为外接电阻。
图16为N位数字静态驱动显示电路。
②动态驱动：动态驱动是将所有数码管使用一个专门的译码驱动器，使各位数码管逐个轮流受控显示，这就是动态驱动。由于扫描速度极快。显示效果与静态驱动相同。图17是一种四位数字动态驱动（脉搏冲驱动）方法的线路。图中只用了一个译码驱动电路TC5002BP。
图17
TC4508BP内含两个锁存器，每个锁存器可锁存四位二进BCD码，对应于四位十进制数的四组BCD码分别输入到四个锁存器，四个锁存器，四组BCD码由四个锁存器分时轮流输出进入译码器，译码后进入数码管驱动级集成电路TD62505P（输入端I1～I7与输出端Q1～Q7一一对应）。Q1～Q7分别加到四个数码管的a～g七个阳极上。数字驱动电路TD62003P是由达林顿构成的阵列电路，Q1～Q4中哪一端接地，由输入端I1～I4的四师长“使能”信号DS1～DS4控制。由于四个锁存器的轮换输出也是受“使能”信号DS1～DS4控制。所以四个数码管轮流通电显示。由于轮流显示频率较高，故显示的数字不呈闪烁现象。
2．米字管、符号管显示器
米字管和符号管的结构原理相机，所以其驱动方式也基本相同，只是译码电路的译码过程与七段译码器不同。
米字管可以显示包括英文字母在内的多种符号。符号管主要是用来显示+、-或±号等。
3．LED点阵式显示器
LED点阵式显示器与由单个发光二极管连成的显示器相比，具有焊点少、连线少，所有亮点在同平面、亮度均匀、外形美观等优点。
点阵管根据其内部LED尺寸的大小、数量的多少及发光强度、颜色等可分为多种规格。图18所示是具有代表性的P2057A和P2157A两种φ5高亮度橙红色5×7点阵组件。采用双列直插14脚封装，两种显示器的差别是LED极性不同，如图18所示。
该显示器用扫描驱动方式，选择较大峰值电流和窄脉冲作驱动源，每个LED的平均电流不应超过20mA。
LED点阵管可以代替数码管、符号管和米字管。不仅可以显示数字，也可显示所有西文字母和符号。如果将多块组合，可以构成大屏幕显示屏，用于汉字、图形、图表等等的显示。被广泛用于机场、车站、码头、银行及许多公共场所的指示、说明、广告等场合。
图19是一个LED点阵显示器驱动电路之例。
A LED is a 半导体 device that emits incoherent narrow-spectrum light when electrically biased in the forward direction. This effect is a form of electroluminescence. The color of the emitted light depends on the chemical composition of the semiconducting material used, and can be near-ultraviolet, visible or infrared. Nick Holonyak Jr. (born 1928) of the General Electric Company developed the first practical visible-spectrum LED in 1962.[1]
LED技术
物理功能
An LED is a special type of 半导体 二极管. Like a normal diode, it consists of a chip of semiconducting material impregnated, or doped, with impurities to create a structure called a p-n junction. As in other diodes, current flows easily from the p-side, or anode to the n-side, or cathode, but not in the reverse direction. Charge-carriers - electrons and holes flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon as it does so.
The 波长 of the light emitted, and therefore its color, depends on the bandgap energy of the materials forming the pn junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect bandgap materials. The materials used for an LED have a direct bandgap with energies corresponding to near-infrared, visible or near-ultraviolet light.
LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with ever shorter wavelengths, producing light in a variety of colors.
Conventional LEDs are made from a variety of inorganic minerals, producing the following colors:
蓝色和白色LED
Commercially viable blue LEDs based on the wide bandgap semiconductor gallium nitride and indium gallium nitride were invented by Shuji Nakamura while working in Japan at Nichia Corporation in 1993 and became widely available in the late 1990s. They can be added to existing red and green LEDs to produce white light, though white LEDs today rarely use this principle.
Most "white" LEDs in production today use a 450 nm – 470 nm blue GaN (gallium nitride) LED covered by a yellowish phosphor coating usually made of cerium-doped yttrium aluminium garnet (Ce3+:YAG) crystals which have been powdered and bound in a type of viscous adhesive. The LED chip emits blue light, part of which is efficiently converted to a broad spectrum centered at about 580 nm (yellow) by the Ce3+:YAG. The single crystal form of Ce3+:YAG is actually considered a scintillator rather than a phosphor. Since yellow light stimulates the red and green receptors of the eye, the resulting mix of blue and yellow light gives the appearance of white, the resulting shade often called "lunar white". This approach was developed by Nichia and was used by them from 1996 for manufacturing of white LEDs.
The pale yellow emission of the Ce3+:YAG can be tuned by substituting the cerium with other rare earth elements such as terbium and gadolinium and can even be further adjusted by substituting some or all of the aluminium in the YAG with gallium. Due to the spectral characteristics of the diode, the red and green colors of objects in its blue+yellow light are not as vivid as in broad-spectrum light. Manufacturing variations make the LEDs produce light with different color temperatures, from warm yellowish to cold bluish; the LEDs have to be sorted during manufacture by their actual characteristics.
White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium based red and blue emitting phosphors plus green emitting copper and aluminium doped zinc sulfide (ZnS:Cu,Al). This is a method analogous to the way fluorescent lamps work. However the ultraviolet light causes photodegradation to the epoxy resin and many other materials used in LED packaging, causing manufacturing challenges and shorter lifetimes. This method is less efficient than the blue LED with YAG:Ce phosphor, as the Stokes shift is larger and more energy is therefore converted to heat, but yields light with better spectral characteristics, which renders colors better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both approaches offer comparable brightness.
The newest method used to produce white light LEDs uses no phosphors at all and is based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate which simultaneously emits blue light from its active region and yellow light from the substrate.
A new technique just developed by Michael Bowers, a graduate student at Vanderbilt University in Nashville, involves coating a blue LED with quantum dots that glow white in response to the blue light from the LED. This technique produces a warm, yellowish-white light similar to that produced by incandescent bulbs.
其它颜色
Recent color developments include pink and purple. They consist of one or two phosphor layers over a blue LED chip. The first phosphor layer of a pink LED is a yellow glowing one, and the second phosphor layer is either red or orange glowing. Purple LEDs are blue LEDs with an orange glowing phosphor over the chip. Some pink LEDs have run into issues. For example, some are blue LEDs painted with fluorescent paint or fingernail polish that can wear off, and some are white LEDs with a pink phosphor or dye that unfortunately fades after a short time.
Ultraviolet, blue, pure green, white, pink and purple LEDs are relatively expensive compared to the more common reds, oranges, greens, yellows and infrareds and are thus less commonly used in commercial applications.
The semiconducting chip is encased in a solid plastic lens, which is much tougher than the glass envelope of a traditional light bulb or tube. The plastic may be colored, but this is only for cosmetic reasons or to improve the contrast ratio; the color of the packaging does not substantially affect the color of the light emitted.
有机发光二极管(OLEDs)
If the emissive layer material of an LED is an organic compound, it is known as an Organic Light Emitting Diode (OLED). To function as a semiconductor, the organic emissive material must have conjugated pi bonds. The emissive material can be a small organic molecule in a crystalline phase, or a polymer. Polymer materials can be flexible; such LEDs are known as PLEDs or FLEDs.
Compared with regular LEDs, OLEDs are lighter and polymer LEDs can have the added benefit of being flexible. Some possible future applications of OLEDs could be:
工作参数和效率
Most typical LEDs are designed to operate with no more than 30-60 milliwatts of electrical power. Around 1999, commercial LEDs capable of continuous use at one watt of input power were introduced. These LEDs used much larger semiconductor die sizes to handle the large power input. As well, the semiconductor dies were mounted to metal slugs to allow for heat removal from the LED die. In 2002, 5-watt LEDs were available with efficiencies of 18-22 lumens per watt. It is projected that by 2005, 10-watt units will be available with efficiencies of 60 lumens per watt. These devices will produce about as much light as a common 50-watt incandescent bulb, and will facilitate use of LEDs for general illumination needs.
In September 2003 a new type of blue LED was demonstrated by the company Cree, Inc. to have 35% efficiency at 20 mA. This produced a commercially packaged white light having 65 lumens per watt at 20 mA, becoming the brightest white LED commercially available at the time. In 2005 they have demonstrated a prototype with a record white LED efficiency of 70 lumens per watt at 350 mA [2].
Today, OLEDs operate at substantially lower efficiency than inorganic (crystaline) LEDs. The best efficiency of an OLED so far is about 10%. These promise to be much cheaper to fabricate than inorganic LEDs, and large arrays of them can be deposited on a screen using simple printing methods to create a color graphic display so there are compensating benefits.
使用考虑
Unlike incandescent light bulbs, which light up regardless of the electrical polarity, LEDs will only light with positive electrical polarity. When the voltage across the pn junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. LEDs can be operated on an AC voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.
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发光二极管的正确极性通常如下规定：
It should be noted that looking the inside of the LED is not an accurate way of determining polarity. While in most LEDs the large part is the "-", in some it is the + terminal. The flat tab or the short pin are more accurate ways of determining polarity.
Because the voltage versus current characteristics of an LED are much like any diode, they can be destroyed by connecting them to a voltage source higher than their turn-on voltage. Most LEDs have low reverse breakdown voltage ratings, so they will also so be damaged by an applied reverse voltage of more than a few volts. Since some manufacuters don‘t follow the indicator standards above, if possible the data sheet should be consulted before hooking up an LED, or the LED may be tested in series with a resistor.
Because of the risk of excess voltage damaging the device, a good LED driver circuit is a constant current source. If high efficiency is not required, an approximation to a current source made by connecting the LED in series with a current limiting resistor to a voltage source may be substituted. To increase efficiency, the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human flicker fusion threshold, the LED will appear to be continuously lit.
Parallel operation is generally problematic. The LEDs have to be of the same type in order to have a similar forward voltage. Even then, variations in the manufacturing process can make the odds of satisfactory operation low. For more information see Nichia Application Note.
Some LED units contain two diodes, one in each direction (that is, two diodes in inverse parallel) and each a different color (typically red and green), allowing two-color operation or a range of apparent colors to be created by altering the percentage of time the voltage is in each polarity. Other LED units contain two or more diodes (of different colors) arranged in either a common anode or common cathode configuration. These can be driven to different colors without reversing the polarity.
LED units may have an integrated multivibrator circuit that makes the LED flash.
使用LED的优点
LEDs light up very quickly. An LED will achieve full brightness in approximately 0.01 seconds, 10 times faster than an Incandescent light bulb (0.1 second), and many times faster than a compact fluorescent lamp, which starts to come on after 0.5 seconds or 1 second, but does not achieve full brightness for 30 seconds or more.
LED的应用 LED的应用列表
Some of these applications are further elaborated upon in the following text.
LEDs are used as informative indicators in various types of embedded systems:
LEDs may also be used to transmit digital information:
LEDs find further application in safety devices, where high brightness and reliability are critical:
LEDs are also used for illumination:
As a replacement for incandescent and fluorescent bulbs in home and office lighting, an application known as Solid State Lighting (SSL).
Finally, LEDs have additional applications not categorized above:
显示应用
LEDs used as a replacement for incandescent bulbs and fluorescent lamps are known as Solid State Lighting (SSL) LEDs. SSL LEDs are packaged as a cluster of white LEDs grouped together to form a light source. LEDs are moderately efficient: the average commercial LED currently outputs 32 lumens per watt (lm/W), and new technologies promise to deliver up to 80 lm/W. They are also more mechanically robust than incandescent light bulbs and fluorescent tubes. LEDs today are not sold in many places, require power source conversion in household applications, and are relatively expensive, although their costs are decreasing.
Incandescent bulbs are much less expensive but also less efficient, generating from about 16 lm/W for a domestic tungsten bulb to 22 lm/W for a halogen bulb. Fluorescent tubes are more efficient, providing 50 to 100 lm/W for domestic tubes (average 60 lm/W), but are bulky and fragile and require starter or ballast circuits that sometimes buzz audibly. Compact fluorescent light bulbs, which include a quiet integrated ballast, are relatively robust and efficient, fit in standard light bulb sockets, and are currently the best choice for efficient household lighting.
Proponents of LEDs expect that technological advances will reduce costs such that SSL can be introduced into most homes by 2020. However, they are still not commercially viable for general lighting applications, and so LEDs are found today in illumination applications where their special characteristics provide a distinct advantage.
Due to their monochromatic nature, LED lights have great power advantages over white lights when a specific color is required. Unlike traditional white lights, the LED does not need a coating or diffuser that can absorb much of the emitted light. LED lights are inherently colored, and are available in a wide range of colors. One of the most recently introduced colors is the emerald green (bluish green, about 500 nm) that meets the legal requirements for traffic signals and navigation lights.
There are applications that specifically require light without any blue component. Examples are photographic darkroom safe lights, illumination in laboratories where certain photo-sensitive chemicals are used, and situations where dark adaptation (night vision) must be preserved, such as cockpit and bridge illumination, observatories, etc. Yellow LED lights are a good choice to meet these special requirements because the human eye is more sensitive to yellow light (about 500 lm/watt emitted) than that emitted by the other LEDs.
LED显示屏
There are two types of LED panels: conventional, using discrete LEDs, and Surface Mounted Device (SMD) panels. Most outdoor screens and some indoor screens are built around discrete LEDs, also known as individually mounted LEDs. A cluster of red, green, and blue diodes is driven together to form a full-color pixel, usually square in shape. These pixels are spaced evenly apart and are measured from center to center for absolute pixel resolution. The largest LED display in the world is 36 metres high (118 feet), at Times Square, Manhattan.
Most indoor screens on the market are built using SMD technology — a trend that is now extending to the outdoor market. An SMD pixel consists of red, green, and blue diodes mounted on a chipset, which is then mounted on the driver PC board. The individual diodes are smaller than a pin and are set very close together. The difference is that minimum viewing distance is reduced by 25% from the discrete diode screen with the same resolution.
Indoor use generally requires a screen that is based on SMD technology and has a minimum brightness of 600 candelas per square metre (unofficially called nits). This will usually be more than sufficient for corporate and retail applications, but under high ambient-brightness conditions, higher brightness may be required for visibility. Fashion and auto shows are two examples of high-brightness stage lighting that may require higher LED brightness. Conversely, when a screen may appear in a shot on a television show, the requirement will often be for lower brightness levels with lower color temperatures (common displays have a white point of 6500-9000K, which is much bluer than the common lighting on a television production set).
For outdoor use, at least 2,000 nits are required for most situations, whereas higher brightness types of up to 5,000 nits cope even better with direct sunlight on the screen. Until recently, only discrete diode screens could achieve that brightness level. (The brightness of LED panels can be reduced from the designed maximum, if required.)
Suitable locations for large display panels are identified by factors such as sight lines, local authority planning requirements (if the installation is to become semi-permanent), vehicular access (trucks carrying the screen, truck-mounted screens, or cranes), cable runs for power and video (accounting for both distance and health and safety requirements), power, suitability of the ground for the location of the screen (check to make sure there are no pipes, shallow drains, caves, or tunnels that may not be able to support heavy loads), and overhead obstructions.
参考
参见其它
LED是通过将电压加在LED的PN结两端，使PN结本身形成一个能级（实际上，是一系列的能级），然后电子在这个能级上跃变并产生光子来发光的。所以，LED需要加在PN结两端的电压来驱使其发光，即LED驱动。
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