在(zai)工業(ye)應用(yong)(yong)中，測量點一般在(zai)現場(chang)，而顯示設備或(huo)者(zhe)控制(zhi)(zhi)設備一般都在(zai)控制(zhi)(zhi)室或(huo)控制(zhi)(zhi)柜上。兩(liang)者(zhe)之間(jian)距離可能數十(shi)至(zhi)數百(bai)米(mi)。按一百(bai)米(mi)距離計算，省去2根信號傳(chuan)輸 導(dao)(dao)線(xian)(xian)(xian)(xian)意味著成本降低近百(bai)元！另(ling)外四線(xian)(xian)(xian)(xian)制(zhi)(zhi)變送器和三線(xian)(xian)(xian)(xian)制(zhi)(zhi)變送器因(yin)導(dao)(dao)線(xian)(xian)(xian)(xian)內電(dian)流不對稱必須使用(yong)(yong)昂(ang)貴的屏蔽線(xian)(xian)(xian)(xian),而兩(liang)線(xian)(xian)(xian)(xian)制(zhi)(zhi)變送器可使用(yong)(yong)非常便(bian)宜的的雙絞線(xian)(xian)(xian)(xian)導(dao)(dao)線(xian)(xian)(xian)(xian),因(yin)此 在(zai)應用(yong)(yong)中兩(liang)線(xian)(xian)(xian)(xian)制(zhi)(zhi)變送器必然是首選。

 ;   電流(liu)輸出型將物理量(liang)轉(zhuan)換成4~ 20mA電流(liu)輸出，必然要有(you)外電源(yuan)為(wei)(wei)其供電。最典型的是變送器(qi)需要兩根電源(yuan)線，加上兩根電流(liu)輸出線，總共(gong)要接4根線，稱之為(wei)(wei)四線制變送器(qi)。

　　電流輸出可以與電源公用一根線（公用VCC或者GND），可節省一根線，稱之為三線制變送器。
其實大家可能(neng)注意到， 4-20mA電流本(ben)身就可以為(wei)(wei)變(bian)(bian)(bian)送(song)器(qi)(qi)供(gong)電，變(bian)(bian)(bian)送(song)器(qi)(qi)在(zai)電路中相(xiang)當于一(yi)個(ge)特殊(shu)的負載(zai)，特殊(shu)之處在(zai)于變(bian)(bian)(bian)送(song)器(qi)(qi)的耗電電流在(zai)4~20mA之間根據傳感器(qi)(qi)輸(shu)出而(er)變(bian)(bian)(bian)化。 顯示儀表只需(xu)(xu)要串在(zai)電路中即可。這(zhe)種變(bian)(bian)(bian)送(song)器(qi)(qi)只需(xu)(xu)外接2根線，因而(er)被(bei)稱為(wei)(wei)兩(liang)線制變(bian)(bian)(bian)送(song)器(qi)(qi)。工業電流環標準下限為(wei)(wei)4mA，因此只要在(zai)量程范圍內，變(bian)(bian)(bian)送(song)器(qi)(qi)至少有 4mA供(gong)電。這(zhe)使得兩(liang)線制傳感器(qi)(qi)的設計成(cheng)為(wei)(wei)可能(neng)。

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