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其實(shi)大(da)家可(ke)能注意到(dao)， 4-20mA電(dian)(dian)(dian)流本身就可(ke)以為(wei)變(bian)送(song)(song)(song)器(qi)(qi)供電(dian)(dian)(dian)。變(bian)送(song)(song)(song)器(qi)(qi)在(zai)(zai)電(dian)(dian)(dian)路中相(xiang)當于(yu)一個特殊(shu)的(de)(de)負載，特殊(shu)之處在(zai)(zai)于(yu)變(bian)送(song)(song)(song)器(qi)(qi)的(de)(de)耗電(dian)(dian)(dian)電(dian)(dian)(dian)流在(zai)(zai)4~20mA之間根(gen)據傳感(gan)器(qi)(qi)輸出而變(bian)化。顯示儀表只需(xu)要(yao)串(chuan)在(zai)(zai)電(dian)(dian)(dian)路中即(ji)可(ke)。這(zhe)種變(bian)送(song)(song)(song)器(qi)(qi)只需(xu)外接2根(gen)線，因而被稱為(wei)兩(liang)線制變(bian)送(song)(song)(song)器(qi)(qi)。工業電(dian)(dian)(dian)流環標(biao)準下限為(wei)4mA，因此只要(yao)在(zai)(zai)量(liang)程范圍內，變(bian)送(song)(song)(song)器(qi)(qi)至少有4mA供電(dian)(dian)(dian)。這(zhe)使(shi)得兩(liang)線制傳感(gan)器(qi)(qi)的(de)(de)設計成為(wei)可(ke)能。

在工業應用(yong)(yong)中，測(ce)量點一般在現場，而(er)(er)顯示設備(bei)或者控(kong)(kong)制(zhi)設備(bei)一般都在控(kong)(kong)制(zhi)室或控(kong)(kong)制(zhi)柜上。兩者之間距離可能數十至數百米。按一百米距離計算，省去2根信號傳輸導線(xian)意(yi)味著成(cheng)本(ben)降低(di)近百元！另外四(si)線(xian)制(zhi)變(bian)(bian)送器和(he)三(san)線(xian)制(zhi)變(bian)(bian)送器因導線(xian)內電流不對稱必須(xu)使(shi)用(yong)(yong)昂(ang)貴的屏(ping)蔽線(xian),而(er)(er)兩線(xian)制(zhi)變(bian)(bian)送器可使(shi)用(yong)(yong)非(fei)常(chang)便宜的的雙絞(jiao)線(xian)導線(xian),因此在應用(yong)(yong)中兩線(xian)制(zhi)變(bian)(bian)送器必然是(shi)