Howland电流源电路测试与分析
Howland电流源电路测试与分析
Howland电流源电路是一种可以为负载提供交流恒流输出的电路。本文将通过实际测试，详细探讨其基本特性。
一、前言
近期，作者看到一个有趣的恒流电路，被称为Howland电流源电路。它可以为负载提供交流恒流输出电源。下面，通过手边的元器件，测试一下它的基本特性。
图1.1.1 Howland电流源电路
二、电路设计
设计测试电路，通过P-IN端口，输入控制交流电压信号。P-Load端口，连接外部的负载。电路工作电源为正负12V，铺设单面PCB，一分钟之后得到测试电路板，电路板制作的非常完美，下面进行焊接调试。
图1.2.1 测试电路原理图
焊接电路。清洗之后进行测试。电路中所使用的电阻为千分之一高精度电阻。
三、测量结果
利用QR10可编程电阻箱作为负载，将其调整到1k欧姆。这样，输入电压与负载电压的比值，根据电路参数应该是20:1。输入一个峰峰值为5V，频率为1kHz的正弦波，它的均值为0V。上面黄色为输入电压波形，下面青色为负载电压波形。它们是同相的。电压量程比值为20:1，所以，它们的幅度比值符合电路中的参数。
图1.3.1 负载1k欧姆对应的输入输出电压波形
改变负载电阻，当负载电阻为5k欧姆的时候，输出电流已经失真，当负载电阻降低，输出电压随之降低。为了验证输出是恒定的，下面测量不同电阻下，输出的电压是多少。
图1.3.2 负载为5k时对应的电压波形
通过编程，设置QR10负载电阻从50欧姆变化到2k欧姆，测量不同电阻下负载电压。理想情况下，负载电压应该与电阻成正比，测量曲线显示出电压与电阻之间近似为直线。但是，输出电压似乎随着电阻的增加，呈现减缓的趋势。这说明所搭建的电路输出并不是一个严格的电流源。
图1.3.3 不同电阻下对应的输出电阻
from headm import *
import headm
from tsmodule.tsvisa import *
import serial
from _ast import Or
from serial.serialutil import SerialException
qr10 = serial.Serial()
qr10.baudrate = 115200
qr10.timeout = 0.05
try:
qr10.port = 'COM6'
except:
printf('Set qr10 port COM6 error. ')
try:
qr10.open()
except serial.serialutil.SerialException:
printf('Open qr10 port COM6 error.')
else:
printf('Open qr10 port COM6 Ok.')
rdim = linspace(50, 2000, 100)
vdim = []
dm3068open()
for r in rdim:
qr10.write(b'AT+USER.SP=%5.1f\n'%r)
time.sleep(1.5)
v = dm3068vac()
vdim.append(v)
tspsave("qr10", rdim=rdim, vdim=vdim)
printff(r, v)
plt.plot(rdim, vdim, lw=3)
plt.xlabel("Resistor(Ohm)")
plt.ylabel("Voltage(V)")
plt.grid(True)
plt.tight_layout()
plt.show()
rdim=[50.0000,69.6970,89.3939,109.0909,128.7879,148.4848,168.1818,187.8788,207.5758,227.2727,246.9697,266.6667,286.3636,306.0606,325.7576,345.4545,365.1515,384.8485,404.5455,424.2424,443.9394,463.6364,483.3333,503.0303,522.7273,542.4242,562.1212,581.8182,601.5152,621.2121,640.9091,660.6061,680.3030,700.0000,719.6970,739.3939,759.0909,778.7879,798.4848,818.1818,837.8788,857.5758,877.2727,896.9697,916.6667,936.3636,956.0606,975.7576,995.4545,1015.1515,1034.8485,1054.5455,1074.2424,1093.9394,1113.6364,1133.3333,1153.0303,1172.7273,1192.4242,1212.1212,1231.8182,1251.5152,1271.2121,1290.9091,1310.6061,1330.3030,1350.0000,1369.6970,1389.3939,1409.0909,1428.7879,1448.4848,1468.1818,1487.8788,1507.5758,1527.2727,1546.9697,1566.6667,1586.3636,1606.0606,1625.7576,1645.4545,1665.1515,1684.8485,1704.5455,1724.2424,1743.9394,1763.6364,1783.3333,1803.0303,1822.7273,1842.4242,1862.1212,1881.8182,1901.5152,1921.2121,1940.9091,1960.6061,1980.3030,2000.0000]
vdim=[0.0048,0.0068,0.0087,0.0106,0.0121,0.0139,0.0157,0.0175,0.0192,0.0207,0.0224,0.0241,0.0258,0.0274,0.0291,0.0306,0.0323,0.0339,0.0356,0.0372,0.0388,0.0404,0.0421,0.0437,0.0453,0.0469,0.0485,0.0501,0.0517,0.0533,0.0549,0.0564,0.0580,0.0596,0.0612,0.0627,0.0643,0.0658,0.0674,0.0689,0.0705,0.0721,0.0735,0.0750,0.0766,0.0781,0.0796,0.0812,0.0827,0.0842,0.0857,0.0872,0.0887,0.0902,0.0917,0.0932,0.0947,0.0962,0.0977,0.0992,0.1007,0.1022,0.1036,0.1051,0.1066,0.1080,0.1095,0.1110,0.1124,0.1138,0.1153,0.1167,0.1182,0.1196,0.1210,0.1224,0.1238,0.1252,0.1267,0.1281,0.1295,0.1309,0.1323,0.1337,0.1351,0.1365,0.1379,0.1392,0.1406,0.1420,0.1434,0.1447,0.1461,0.1475,0.1488,0.1502,0.1516,0.1529,0.1543,0.1556]
图1.3.4 线性拟合的结果
下面假设电路输出内阻为R0，可以推导出输出电压与负载电阻之间的关系。下面使用测量输出电压数据来估计R0数值。利用拟合算法，可以求出在R0估计值为12.7k欧姆的时候，拟合误差最小。因此，现在所搭建的电路等效的输出内阻只有12.7k欧姆，距离恒流源还相差很多。
图1.3.5 内阻拟合结果
总结
本文初步测试了Howland交流电流源电路。利用QR10可编程电阻箱测量了电路的等效内阻，只有12.7k欧姆，也许，这与所使用的运放以及电阻精度有关系。后面，再通过增加匹配电阻，来提高电路输出等效电阻。
相关文献链接:
- AN-1515 A Comprehensive Study of the Howland Current Pump
相关图表链接:
- 图1.1.1 Howland电流源电路
- 图1.2.1 测试电路原理图
- 图1.3.1 负载1k欧姆对应的输入输出电压波形
- 图1.3.2 负载为5k时对应的电压波形
- 图1.3.3 不同电阻下对应的输出电阻
- 图1.3.4 线性拟合的结果
- 图1.3.5 内阻拟合结果