工(gong)業上普通使用的(de)(de)壓(ya)力(li)傳(chuan)感器輸(shu)出(chu)值都是(shi)4-20mA的(de)(de)、這是(shi)常(chang)規的(de)(de)傳(chuan)感器輸(shu)出(chu)數(shu)值、但(dan)為什(shen)么會是(shi)4-20mA而不是(shi)其它的(de)(de)數(shu)值輸(shu)出(chu)呢(ni)？
*：采(cai)用電流(liu)(liu)信號的原因(yin)是不容易受干(gan)擾、并(bing)且(qie)電流(liu)(liu)源內阻無(wu)窮大，導線電阻串聯在回路中不影(ying)響精(jing)度(du)，在普通雙絞線上(shang)可(ke)以傳輸數百(bai)米(mi)。
上(shang)限(xian)取20mA是(shi)因(yin)為(wei)(wei)防(fang)爆的(de)要求(qiu)：20mA的(de)電(dian)(dian)流(liu)通斷引(yin)起的(de)火花能(neng)量不(bu)足(zu)以引(yin)燃瓦斯。下(xia)限(xian)沒有取0mA的(de)原因(yin)是(shi)為(wei)(wei)了(le)能(neng)檢測斷線(xian)(xian)(xian)(xian)(xian)：正常(chang)工作(zuo)時不(bu)會(hui)低于(yu)4mA，當傳(chuan)輸線(xian)(xian)(xian)(xian)(xian)因(yin)故障斷路，環路電(dian)(dian)流(liu)降為(wei)(wei)0。常(chang)取2mA作(zuo)為(wei)(wei)斷線(xian)(xian)(xian)(xian)(xian)報警值。電(dian)(dian)流(liu)型變(bian)(bian)送器(qi)(qi)(qi)(qi)將物理量轉換成4~20mA電(dian)(dian)流(liu)輸出，必然要有外(wai)電(dian)(dian)源(yuan)為(wei)(wei)其供電(dian)(dian)。zui典(dian)型的(de)是(shi)變(bian)(bian)送器(qi)(qi)(qi)(qi)需(xu)要兩(liang)根(gen)(gen)電(dian)(dian)源(yuan)線(xian)(xian)(xian)(xian)(xian)，加(jia)上(shang)兩(liang)根(gen)(gen)電(dian)(dian)流(liu)輸出線(xian)(xian)(xian)(xian)(xian)，總共要接4根(gen)(gen)線(xian)(xian)(xian)(xian)(xian)，稱之為(wei)(wei)四線(xian)(xian)(xian)(xian)(xian)制變(bian)(bian)送器(qi)(qi)(qi)(qi)。當然，電(dian)(dian)流(liu)輸出可以與電(dian)(dian)源(yuan)公用(yong)一(yi)根(gen)(gen)線(xian)(xian)(xian)(xian)(xian)公用(yong)VCC或(huo)者(zhe)GND，可節省一(yi)根(gen)(gen)線(xian)(xian)(xian)(xian)(xian)，稱之為(wei)(wei)三線(xian)(xian)(xian)(xian)(xian)制變(bian)(bian)送器(qi)(qi)(qi)(qi)。其實大家可能(neng)注意到，4-20mA電(dian)(dian)流(liu)本身(shen)就可以為(wei)(wei)變(bian)(bian)送器(qi)(qi)(qi)(qi)供電(dian)(dian)。變(bian)(bian)送器(qi)(qi)(qi)(qi)在(zai)電(dian)(dian)路中(zhong)相當于(yu)一(yi)個特殊的(de)負載，特殊之處在(zai)于(yu)變(bian)(bian)送器(qi)(qi)(qi)(qi)的(de)耗電(dian)(dian)電(dian)(dian)流(liu)在(zai)4~20mA之間根(gen)(gen)據傳(chuan)感器(qi)(qi)(qi)(qi)輸出而變(bian)(bian)化。顯示儀表(biao)只需(xu)要串在(zai)電(dian)(dian)路中(zhong)即可。這種變(bian)(bian)送器(qi)(qi)(qi)(qi)只需(xu)外(wai)接2根(gen)(gen)線(xian)(xian)(xian)(xian)(xian)，因(yin)而被(bei)稱為(wei)(wei)兩(liang)線(xian)(xian)(xian)(xian)(xian)制變(bian)(bian)送器(qi)(qi)(qi)(qi)。工業電(dian)(dian)流(liu)環標準下(xia)限(xian)為(wei)(wei)4mA，因(yin)此只要在(zai)量程范圍(wei)內(nei)，變(bian)(bian)送器(qi)(qi)(qi)(qi)至少有4mA供電(dian)(dian)。
因此、4-20mA的信(xin)(xin)號(hao)輸出一般不(bu)容(rong)易受干擾而且安全可靠、所(suo)以工業(ye)上普遍使用的都是二線制(zhi)4-20mA的電源(yuan)輸出信(xin)(xin)號(hao)。但為了能(neng)更好的處理傳(chuan)感(gan)器的信(xin)(xin)號(hao)、目前(qian)還有更多其它形式的輸出信(xin)(xin)號(hao)：3.33MV/V；2MV/V；0-5V; 0-10V等。
