摘要 ：恒流源顾名思义，是一种能够向负载提供稳定电流的电源。恒流源在许多情况下都能应用而且是必不可少的，应用范围非常广泛。它既可以作为其有源负载又可为稳定静态工作点的各种放大电路提供偏流来提高其放大倍数。并且在脉冲产生电路、差动放大电路中得到了广泛应用。24334
本文设计的是数控直流恒流源，应用单片机AT89S51单片机最小系统来实现对恒流源的控制。通过按键来增减预设电流的大小，步进电流等级可达1mA。系统初始化是数码管显示的电流值设为64，然后按压按键我们可以上下增减电流，得到不同的电流值，这个电流在数码管上显示。单片机根据键盘设定然后计算出恒流的数值，然后送到PCF8591的D/A装换器，D/A转换器的输出电压和恒流电路是一定比例，所以通过计算就能够得到恒流的大小。实际实验测试结果表明，本系统只能效应用于小功率恒流源领域。
毕业论文关键字： 数控直流恒流源；D/A转换器；单片机；恒流源；PCF8591
DC current source design of the train car
Abstract：Constant current source, as the name implies, is a kind of can provide stable current for the load of power supply. In many cases, constant current source, can be applied and is essential, application scope is very broad. It can be used as the active load and can provide various amplifier circuit to stabilize the quiescent point drift to improve its magnification. And in the pulse generating circuit, differential amplifier circuit has been widely applied. 
The designed in this paper the numerical control dc constant current source. To realize the control of constant current source wo use application of single chip microcomputer AT89S51 minimum system.Through the button to increase or decrease the size of the preset current. Step into the current level can be up to 1 mA. System initialization  digital tube display the current value is set to 64. Then press the button up and down we can increase or decrease in current, the different current value. The electric current display on the digital tube. SCM according to the keyboard set then the value of constant current is calculated, and then sent to the PCF8591 D/A converter. D/A converter of output voltage and constant current circuit is A certain proportion, so the size of the constant current is obtained by calculation .Actual test results show that the system can only effect for small power constant current source.
 目录
1 绪论    1
1.1 课题的目的和意义    1
1.2 国内外研究现状与水平    1
1.2.1 国外发展现状    1
1.2.2国内发展现状    2
1.3 发展趋势    2
1.4 恒流源应用    3
1.4.1  在半导体器件参数的测量上的应用    3
1.4.2 在计量领域中的应用    3
1.4.3 在核物理试验中的应用    3
1.5 本文主要工作与内容安排    4
2 恒流源的总体方案与设计理论    5
2.1 恒流源综述    5
2.2 总体方案选取及性能指标    5
2.2.1 系统总体设计    5
2.2.2 性能指标    6
2.3 方案论证    6
2.3.1 主控器    6
2.3.2 供电电源    7
2.3.3 恒流源    7
2.3.4  ADC和DAC方案论证    8
2.3.5  控制单元方案选择    9
2.3.6  显示    9
3  系统硬件设计    10
3.1 单片机功能介绍    10
3.1.1  主要功能    10
3.1.2  引脚功能    11 51单片机列车车载的直流恒流源的设计+电路图+源程序:http://www.youerw.com/tongxin/lunwen_17799.html