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其實大家可能注意到(dao), 4-20mA電(dian)流(liu)本身就可以為變(bian)送器(qi)(qi)供電(dian)。變(bian)送器(qi)(qi)在(zai)電(dian)路中相當于一個特殊的(de)負載,特殊之(zhi)處(chu)在(zai)于變(bian)送器(qi)(qi)的(de)耗電(dian)電(dian)流(liu)在(zai)4~20mA之(zhi)間根(gen)據傳感器(qi)(qi)輸出而(er)變(bian)化。顯示儀表只需要(yao)串(chuan)在(zai)電(dian)路中即可。這(zhe)種變(bian)送器(qi)(qi)只需外接2根(gen)線(xian)(xian)(xian),因而(er)被稱為兩線(xian)(xian)(xian)制變(bian)送器(qi)(qi)。工業電(dian)流(liu)環(huan)標準下(xia)限為4mA,因此只要(yao)在(zai)量程范(fan)圍內,變(bian)送器(qi)(qi)至(zhi)少有4mA供電(dian)。這(zhe)使得兩線(xian)(xian)(xian)制傳感器(qi)(qi)的(de)設(she)計成為可能。
在工業(ye)應用中(zhong)(zhong),測(ce)量點一(yi)(yi)般在現場,而顯示設備(bei)或(huo)(huo)者控(kong)制(zhi)設備(bei)一(yi)(yi)般都(dou)在控(kong)制(zhi)室或(huo)(huo)控(kong)制(zhi)柜(ju)上。兩(liang)者之(zhi)間距(ju)離可能數十(shi)至(zhi)數百(bai)米(mi)。按一(yi)(yi)百(bai)米(mi)距(ju)離計算,省去2根(gen)信號傳輸導(dao)線(xian)意味著成(cheng)本降(jiang)低近(jin)百(bai)元!另外四線(xian)制(zhi)變(bian)(bian)送器(qi)(qi)和三(san)線(xian)制(zhi)變(bian)(bian)送器(qi)(qi)因(yin)導(dao)線(xian)內電流不對稱必須(xu)使用昂貴的(de)屏蔽(bi)線(xian),而兩(liang)線(xian)制(zhi)變(bian)(bian)送器(qi)(qi)可使用非常便宜的(de)的(de)雙絞線(xian)導(dao)線(xian),因(yin)此在應用中(zhong)(zhong)兩(liang)線(xian)制(zhi)變(bian)(bian)送器(qi)(qi)必然是(shi)