PLC使用中,I/O端口分数字量和模拟量端口,对于输入端口,又分数字量输入端口和模拟量输入端口。对于PLC电流模拟量输入端口,还需注意过压保护。因为在电流模拟量I/O口中,有一个取样电阻,通常为高精度的250欧姆,当PLC输入电流比如4~20mA的电流信号时,电流通过I/O端口250欧姆电阻后,在PLC内部形成1~5V的电压信号。由于PLC的DC电源通常为24V,PLC测试或者在现场调试的时候,调试者很容易将24V电源接到PLC的模拟量输入I/O端口,如果没有保护的话,250欧姆取样电阻长时间接24V电源,在取样电阻上的功耗就会大量增加(由公式V2/R决定),这样取样电阻就会损坏。一个可能的保护解决方案就是电路中增加一个Zener管(或者TVS管)和取样电阻并联,由Zener管来保护取样电阻。但是随之而来的问题就是,Zener管(或者TVS管)它们是瞬态电压抑制器,对于瞬态电压,它们有良好的保护特性,但是对于长时的过压保护,它们也无能为力。TE connectivity的PPTC+Zener或者PolyZen可以提供一个完整的解决方案。
在出现持续的高功率状态(过压事件)时,由于PPTC和Zener器件之间无缝的热耦合性能,PolyZen器件中的PPTC可以感测Zener管的温度,当Zener管处于长时间过压而温度增加时,PPTC的温度也会升高,由于PPTC是一个正温度系数的温度敏感电阻,PPTC电阻会温度的增加迅速从低电阻状态转变到高电阻状态,限制了电流并保护了Zener----------------------------------------------------
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