1783-MS10T
1783-MS10T
1783-MS10T
光电耦合器之所以在传输信号的同时能有效地抑制尖脉冲和各种噪声干扰,使通道上的信号噪声比大为提高,主要有以下几方面的原因: (1) 光电耦合器的输入阻抗很小,只有几百欧姆,而干扰源的阻抗较大,通常为105~106Ω。据分压原理可知,即使干扰电压的幅度较大,但馈送到光电耦合器输入端的噪声电压会很小,只能形成很微弱的电流,由于没有足够的能量而不能使二极管发光,从而被抑制掉了。(2) 光电耦合器的输入回路与输出回路之间没有电气联系,也没有共地;之间的分布电容极小,而绝缘电阻又很大,因此回路一边的各种干扰噪声都很难通过光电耦合器馈送到另一边去,避免了共阻抗耦合的干扰信号的产生。
由于光电耦合器或者磁电耦合器均是一个集成电路,而集成电路内部空间小,所以隔离电压有限。我们常用的光电耦合器,采用的是标准DIP6或者DIP8封装,整个外形尺寸不到8×8mm,内部硅片的尺寸更加小,隔离电压最高只能到2500V。磁电耦合器也是一片集成电路,指标号称6000V隔离电压,但是实际隔离电压不见得比光电耦合器强多少。如果想增加隔离电压,就不能够使用光电耦合器或者磁电耦合器。隔离电压,也称为放电电压或者击穿电压,是指相互隔离的双方加上一个电压差,当这个电压差大到某个数值时,隔离的双方会产生放电或者击穿而不再隔离。 在均匀电场,气压为1个大气压、温度为20℃ 的条件下,空气击穿电压与极间距离关系图。距离越大,则隔离电压越大,基本上是成正比例关系,大约为30kV/cm。图中可查出,1cm的击穿电压为30kV,5cm的击穿电压为150kV。
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