数字电路课程设计报告_简易数字电容测试仪+原创

数电课程设计报告题目简易数字式电容测试仪简易数字电容C测量仪前言电子制作中需要用到各种各样的电容器，它们在电路中分别起着不同的作用。与电阻器相似，通常简称其为电容，用字母C表示。顾名思义，电容器就是“储存电荷的容器”。尽管电容器品种繁多，但它们的基本结构和原理是相同的。两片相距很近的金属中间被某物质（固体、气体或液体）所隔开，就构成了电容器。两片金属称为的极板，中间的物质叫做介质。电容器也分为容量固定的与容量可变的。但常见的是固定容量的电容，最多见的是电解电容和瓷片电容。不同的电容器储存电荷的能力也不相同。规定把电容器外加1伏特直流电压时所储存的电荷量称为该电容器的电容量。电容的基本单位为法拉（F）。但实际上，法拉是一个很不常用的单位，因为电容器的容量往往比1法拉小得多，常用微法（μF）、纳法（nF）、皮法（pF）（皮法又称微微法）等，它们的关系是：1法拉（F）=1000000微法（μF）1微法（μF）=1000纳法（nF）=1000000皮法（pF）。电容器在电子线路中得到广泛的应用，它的容量大小对电路的性能有重要的影响，本课题就是用数字显示方式对电容进行测量。本设计报告共分三章。第一章介绍系统设计；第二章介绍主要电路及其分析；第三章为总结部分。摘要：由于单稳态触发器的输出脉宽tW与电容C成正比，把电容C转换成宽度为tW的矩形脉冲，然后将其作为闸门信号控制计数器计标准频率脉冲的个数，并送锁存--译码--显示系统就可以得到电容量的数据。关键词：闸门信号标准频率脉冲目录第一章系统设计···························································································2一、设计目的·····························································································2二、设计内容要求·····················································································2三、设计技术指标·····················································································2四、方案比较··························································································2五、方案论证··························································································31、总体思路····················································································32、设计方案····················································································3第二章主要电路设计与说明···········································································4一、芯片简介···························································································41、555定时器···················································································42、单稳态触发器74121·······································································43、4位二进制加法计数器47161····························································54、4位集成寄存器74LSl75芯片···························································65、七段译码器74LS47-BCD芯片·························································7二、总电路图及分析·················································································71、总图····························································································72、参数选择及仪表调试······································································93、产品使用说明···············································································94、以测待测电容Cx的电容量为例说明电路工作过程及测容原理··················9三、各单元电路的设计与分析·······································································91、基准脉冲发生器············································································92、启动脉冲发生器··········································································103、Cx转化为Tw宽度的矩形脉冲························································104、计数器·······················································································105、寄存—译码—显示系统·································································10第三章总结······························································································11参考文献····································································································11附录········································································································11附录1元器件清单················································································11附录2用集成元件代分立元件电路···························································12评语·········································································································13第一章系统设计一、设计目的1掌握电容数字测量仪的设计、组装与调试方法。2熟悉相应的中大规模集成电路的使用方法，并掌握其工作原理。二、设计内容要求1设计电容数字测量仪电路。2组装、调试电容数字测量仪单元电路和整机系统。3画出电容数字测量仪的电路图，写出设计报告。三、设计技术指标1测量电容容量范围为100pF~100μF。2应设计3个以上的测量量程。3用四位数码管显示测量结果。4用红、绿色发光二极管表示单位。四、方案比较目前，测量电子元件集中参数R、L、C的仪表种类较多，方法也各不相同，这些方法都有其优缺点。方案一：像测量R一样，测量电容C的最典型的方法是电桥法，如图1所示。只是电容C要用交流电桥测量。电桥的平衡条件为12()()12nxjjnxZZeZZe图1通过调节阻抗Z1、Z2使电桥平衡，这时电表读数为零。根据平衡条件以及一些已知的电路参数就可以求出被测参数。用这种测量方法，参数的值还可以通过联立方程求解，调节电阻值一般只能手动，电桥的平衡判别亦难用简单电路实现。这样，电桥法不易实现自动测量。方案二：把电容量通过电路转换成电压量，然后把电压量经模数转换器转换成数字量进行显示。可由555集成定时器构成单稳态触发器、多谐振荡器等电路，当稳态触发器输出电压的脉宽为：tw=RCln3≈1.1RC。从式可以看到，当R固定时，改变电容C则输出脉宽tW跟着改变，由tW的宽度就可求出电容的大小。把单稳态触发器的输出电压V0取平均值，由于电容量的不同，tW的宽度也不同，则V0的平均值也不同，由V0的平均值大小可得到电容C的大小。如果把平均值送到位A/D转换器，经显示器显示的数据就是电容量的大小。但是我们对A/D转换器的掌握程度还不够充分设计有一些困难。方案三：用阻抗法测R、L、C有两种实现方法：用恒流源供电，然后测元件电压；用恒压源供电，然后测元件电流。由于很难实现理想的恒流源和恒压源，所以它们适用的测量范围很窄。方案四：万用（Q）表是用谐振法来测量C值如图2。它可以在工作频率上进行测量，使测量的条件更接近使用情况。但是，这种测量方法要求频率连续可调，直至谐振。因此它对振荡器的要求较高，另外，和电桥法一样，调节和平衡判别很难实现智能化。图2方案五：标准频率比较法。很多仪表都是把较难测量的物理量转变成精度较高且较容易测量的物理量。基于此思想，我们把电容C转换成频率信号f,转换的原理是555震荡器的震荡周期T=0.639(Ra+Rb)C,周期T与电容的电容量C成正比，通过闸门控制电路控制计数器，对闸门内的脉冲进行计数，并由LED数码管显示出电容量。其原理框图如图3图3方案六：相对于方案五，我们把电容C转换成宽度为Tw的矩形脉冲，然后将其作为闸门信号控制计数器计数，计数后再运算求出C的值，并送显示，转换的原理是由于单稳态触发器的输出脉宽tW与电容C成正比，可利用数字频率计的知识，把此脉冲作闸门时间和标准频率脉冲相“与”，得到计数脉冲，该计数脉冲送计数-锁存-译码显示系统就可以得到电容量的数据。其实，这种转换就是把模拟量近似地转化为数字量，频率f是数字电路很容易处理的数字量，这种数字化处理一方面便于使仪表实现智能化，另一方面也避免了由指针读数引起的误差。因此本次设计我们采用此方案。五、方案论证（一）设计思路本设计中用555震荡器产生一定周期的矩形脉冲作为计数器的CP脉冲，也就是标准频率。同时把待测电容C转换成宽度为tw的矩形脉冲，转换的原理是单稳态触发器的输出脉宽tW与电容C成正比。用这个宽度的矩形脉冲作为闸门信号控制计数器计数，合理处理计