我(wo)們用(yong)什(shen)么方式來傳(chuan)輸模(mo)擬(ni)信號呢？工業(ye)(ye)上(shang)普遍需要測量(liang)各類(lei)非電物理(li)量(liang)，例如(ru)溫度、壓力、速(su)度、角(jiao)度等，這些都需要轉換成模(mo)擬(ni)量(liang)電信號才能傳(chuan)輸到幾百米外的控(kong)制室或顯示設備(bei)上(shang)。工業(ye)(ye)上(shang)廣泛(fan)采用(yong)的是用(yong)4~20mA電流來傳(chuan)輸模(mo)擬(ni)量(liang)。

采用電(dian)(dian)(dian)流信(xin)號的(de)(de)(de)原因是(shi)不容易受干擾(rao)，因為工(gong)業現場(chang)的(de)(de)(de)噪(zao)(zao)聲(sheng)電(dian)(dian)(dian)壓的(de)(de)(de)幅度可(ke)能達(da)到數(shu)V，但是(shi)噪(zao)(zao)聲(sheng)的(de)(de)(de)功率很弱，所以(yi)噪(zao)(zao)聲(sheng)電(dian)(dian)(dian)流通常小于n*別，因此給4－20mA傳(chuan)輸帶(dai)來的(de)(de)(de)誤(wu)差非常小；電(dian)(dian)(dian)流源內(nei)(nei)阻(zu)趨于無窮大(da)，導線(xian)電(dian)(dian)(dian)阻(zu)串聯(lian)在回路中不影(ying)響(xiang)精度，因此在普通雙絞(jiao)線(xian)上(shang)可(ke)以(yi)傳(chuan)輸數(shu)百米；由于電(dian)(dian)(dian)流源的(de)(de)(de)大(da)內(nei)(nei)阻(zu)和恒流輸出，在接收端我們只(zhi)(zhi)需放(fang)置一個250歐姆(mu)到地的(de)(de)(de)電(dian)(dian)(dian)阻(zu)就可(ke)以(yi)獲得0－5V的(de)(de)(de)電(dian)(dian)(dian)壓，低(di)輸入阻(zu)抗的(de)(de)(de)接收器的(de)(de)(de)好處是(shi)n*的(de)(de)(de)輸入電(dian)(dian)(dian)流噪(zao)(zao)聲(sheng)只(zhi)(zhi)產生非常微(wei)弱的(de)(de)(de)電(dian)(dian)(dian)壓噪(zao)(zao)聲(sheng)。

上限取(qu)(qu)20mA是(shi)(shi)因(yin)(yin)為(wei)(wei)防爆(bao)的(de)(de)(de)(de)要求(qiu)：20mA的(de)(de)(de)(de)電(dian)(dian)流(liu)(liu)(liu)通斷(duan)(duan)(duan)引起的(de)(de)(de)(de)火花能(neng)量不足(zu)以(yi)引燃瓦斯。下(xia)限沒(mei)有(you)取(qu)(qu)0mA的(de)(de)(de)(de)原因(yin)(yin)是(shi)(shi)為(wei)(wei)了能(neng)檢測斷(duan)(duan)(duan)線(xian)：正常(chang)工作時不會低于(yu)4mA，當傳輸(shu)線(xian)因(yin)(yin)故障斷(duan)(duan)(duan)路，環(huan)路電(dian)(dian)流(liu)(liu)(liu)降(jiang)為(wei)(wei)0。常(chang)取(qu)(qu)2mA作為(wei)(wei)斷(duan)(duan)(duan)線(xian)報警值(zhi)。電(dian)(dian)流(liu)(liu)(liu)型(xing)變(bian)送(song)(song)器(qi)將物理量轉換(huan)成4~20mA電(dian)(dian)流(liu)(liu)(liu)輸(shu)出(chu)，必然(ran)要有(you)外(wai)電(dian)(dian)源(yuan)(yuan)(yuan)為(wei)(wei)其(qi)供(gong)(gong)電(dian)(dian)。典型(xing)的(de)(de)(de)(de)是(shi)(shi)變(bian)送(song)(song)器(qi)需要兩(liang)根電(dian)(dian)源(yuan)(yuan)(yuan)線(xian)，加(jia)上兩(liang)根電(dian)(dian)流(liu)(liu)(liu)輸(shu)出(chu)線(xian)，總共要接4根線(xian)，稱之為(wei)(wei)四(si)線(xian)制變(bian)送(song)(song)器(qi)。當然(ran)，電(dian)(dian)流(liu)(liu)(liu)輸(shu)出(chu)可(ke)以(yi)與(yu)電(dian)(dian)源(yuan)(yuan)(yuan)公(gong)用(yong)一(yi)根線(xian)（公(gong)用(yong)VCC或(huo)者GND），可(ke)節(jie)省一(yi)根線(xian)，所以(yi)現在(zai)基本上將四(si)線(xian)制變(bian)送(song)(song)器(qi)稱之為(wei)(wei)三線(xian)制變(bian)送(song)(song)器(qi)。其(qi)實大家可(ke)能(neng)注意到， 4-20mA電(dian)(dian)流(liu)(liu)(liu)本身(shen)就可(ke)以(yi)為(wei)(wei)變(bian)送(song)(song)器(qi)供(gong)(gong)電(dian)(dian)，變(bian)送(song)(song)器(qi)在(zai)電(dian)(dian)路中相當于(yu)一(yi)個特殊的(de)(de)(de)(de)負(fu)載(zai)，這種變(bian)送(song)(song)器(qi)只需外(wai)接2根線(xian)，因(yin)(yin)而被稱為(wei)(wei)兩(liang)線(xian)制變(bian)送(song)(song)器(qi)。工業電(dian)(dian)流(liu)(liu)(liu)環(huan)標準(zhun)下(xia)限為(wei)(wei)4mA，因(yin)(yin)此在(zai)量程范(fan)圍內，變(bian)送(song)(song)器(qi)通常(chang)只有(you)24V，4mA供(gong)(gong)電(dian)(dian)（因(yin)(yin)此，在(zai)輕(qing)負(fu)載(zai)條件(jian)下(xia)率的(de)(de)(de)(de)DC/DC電(dian)(dian)源(yuan)(yuan)(yuan)（TPS54331,TPS54160），低功(gong)耗的(de)(de)(de)(de)傳感(gan)(gan)器(qi)和(he)信號鏈(lian)產品、以(yi)及(ji)低功(gong)耗的(de)(de)(de)(de)處理器(qi)（如MSP430）對于(yu)兩(liang)線(xian)制的(de)(de)(de)(de)4-20mA收發非常(chang)重要）。這使得兩(liang)線(xian)制傳感(gan)(gan)器(qi)的(de)(de)(de)(de)設計(ji)成為(wei)(wei)可(ke)能(neng)而又富(fu)有(you)挑戰。

一般需(xu)要設(she)計一個VI轉換器，輸入0-3.3v，輸出(chu)4mA-20mA，可采用(yong)運放LM358，供電+12v。

我們系統地來看看模擬量設備為什么都偏愛用4~20mA傳輸信號~

4-20mA. DC(1-5V.DC)信(xin)號(hao)(hao)制(zhi)是電工委員(yuan)會( IEC )過程控制(zhi)系統(tong)采(cai)用(yong)的(de)模擬信(xin)號(hao)(hao)傳輸(shu)標準。我國也采(cai)用(yong)這一標準信(xin)號(hao)(hao)制(zhi)，儀表(biao)傳輸(shu)信(xin)號(hao)(hao)采(cai)用(yong)4-20mA.DC，接收信(xin)號(hao)(hao)采(cai)用(yong)1-5V.DC，即采(cai)用(yong)電流傳輸(shu)、電壓接收的(de)信(xin)號(hao)(hao)系統(tong)。

一(yi)般儀(yi)器儀(yi)表的(de)(de)信(xin)(xin)號電(dian)流都為(wei)(wei)4-20mA，指(zhi)小電(dian)流為(wei)(wei)4mA,大電(dian)流為(wei)(wei)20mA 。傳(chuan)(chuan)(chuan)輸(shu)(shu)信(xin)(xin)號時候(hou)，因為(wei)(wei)導(dao)線上也有電(dian)阻，如果(guo)用電(dian)壓傳(chuan)(chuan)(chuan)輸(shu)(shu)則會在導(dao)線內產生一(yi)定(ding)的(de)(de)壓降，那接收端(duan)的(de)(de)信(xin)(xin)號就會產生一(yi)定(ding)的(de)(de)誤(wu)差了，所以一(yi)般使用電(dian)流信(xin)(xin)號作為(wei)(wei)變(bian)送器的(de)(de)標準(zhun)傳(chuan)(chuan)(chuan)輸(shu)(shu)。

一、什么是4~20mA.DC（1~5V.DC）信號制？

4~20mA.DC（1~5V.DC）信號(hao)(hao)(hao)制(zhi)是(shi)電(dian)(dian)(dian)工委員會（IEC）：過程控制(zhi)系統用模擬信號(hao)(hao)(hao)標準。我國從DDZ-Ⅲ型電(dian)(dian)(dian)動儀表開始(shi)采(cai)用這(zhe)一標準信號(hao)(hao)(hao)制(zhi)，儀表傳輸信號(hao)(hao)(hao)采(cai)用4~20mA.DC，聯絡信號(hao)(hao)(hao)采(cai)用1~5V.DC，即采(cai)用電(dian)(dian)(dian)流(liu)傳輸、電(dian)(dian)(dian)壓接收(shou)的信號(hao)(hao)(hao)系統。

4~20mA電流環工作原理：

在(zai)工業(ye)現(xian)場，用一(yi)個(ge)儀表(biao)放大器來(lai)完成信(xin)(xin)(xin)號(hao)(hao)的(de)調理并進行長(chang)線傳輸(shu)，會(hui)產生以下問(wen)題：，由(you)于傳輸(shu)的(de)信(xin)(xin)(xin)號(hao)(hao)是(shi)電(dian)(dian)壓(ya)信(xin)(xin)(xin)號(hao)(hao)，傳輸(shu)線會(hui)受到噪(zao)聲的(de)干擾(rao)；第二，傳輸(shu)線的(de)分布電(dian)(dian)阻會(hui)產生電(dian)(dian)壓(ya)降；第三，現(xian)場如(ru)何(he)提供儀表(biao)放大器的(de)工作電(dian)(dian)壓(ya)也是(shi)個(ge)問(wen)題。為了解決上(shang)述問(wen)題和避開相(xiang)關噪(zao)聲的(de)影(ying)響，我們用電(dian)(dian)流來(lai)傳輸(shu)信(xin)(xin)(xin)號(hao)(hao)，因為電(dian)(dian)流對(dui)噪(zao)聲并不敏感。4～20mA的(de)電(dian)(dian)流環便是(shi)用4mA表(biao)示(shi)零信(xin)(xin)(xin)號(hao)(hao)，用20mA表(biao)示(shi)信(xin)(xin)(xin)號(hao)(hao)的(de)滿刻度，而(er)低于4mA高于20mA的(de)信(xin)(xin)(xin)號(hao)(hao)用于各(ge)種故障的(de)報警。

二、4~20mA.DC（1~5V.DC）信號制的優點？

現(xian)(xian)場(chang)儀(yi)表可實現(xian)(xian)兩(liang)(liang)線制，所(suo)謂兩(liang)(liang)線制即電源(yuan)、負(fu)載串(chuan)聯在一起(qi)，有一公共點(dian)(dian)，而現(xian)(xian)場(chang)變(bian)送器與控制室儀(yi)表之前的信號聯絡及供電僅用兩(liang)(liang)根(gen)電線。因為信號起(qi)點(dian)(dian)電流為4mA.DC，為變(bian)送器提(ti)供了靜態工作電流，同時儀(yi)表電氣零(ling)點(dian)(dian)為4mA.DC，不與機械零(ling)點(dian)(dian)重(zhong)合，這種“活零(ling)點(dian)(dian)”有利于識別斷電和斷線等故障。而且兩(liang)(liang)線制還便于使用安全柵，利于安全防爆。

控制(zhi)室儀表(biao)采用(yong)電壓并聯(lian)信(xin)號(hao)傳(chuan)輸，同(tong)一個控制(zhi)系統所屬的儀表(biao)之間(jian)有公共端，便(bian)于檢測儀表(biao)、調節儀表(biao)、計算機、報(bao)警裝(zhuang)置配用(yong)，并方(fang)便(bian)接線。

現場(chang)儀(yi)表(biao)與控(kong)制室儀(yi)表(biao)之間(jian)的(de)聯絡(luo)信(xin)號采用(yong)4~20mA.DC的(de)理(li)由是：因為現場(chang)與控(kong)制室之間(jian)的(de)距離較遠(yuan)，連接電(dian)(dian)(dian)線的(de)電(dian)(dian)(dian)阻(zu)較大(da)(da)，如果(guo)用(yong)電(dian)(dian)(dian)壓信(xin)號遠(yuan)傳(chuan)，優于電(dian)(dian)(dian)線電(dian)(dian)(dian)阻(zu)與接收儀(yi)表(biao)輸入電(dian)(dian)(dian)阻(zu)的(de)分壓，將產(chan)生較大(da)(da)的(de)誤差，而(er)用(yong)恒流(liu)源(yuan)信(xin)號作(zuo)為遠(yuan)傳(chuan)，只(zhi)要傳(chuan)送(song)回路不出現分支，回路中的(de)電(dian)(dian)(dian)流(liu)就不會隨電(dian)(dian)(dian)線長短而(er)改(gai)變，從而(er)保證了傳(chuan)送(song)的(de)精度。

控(kong)制室儀(yi)(yi)表(biao)之間(jian)的聯絡信(xin)(xin)號(hao)(hao)采(cai)用1~5V.DC理由是(shi)：為了便(bian)于(yu)多(duo)臺(tai)儀(yi)(yi)表(biao)共(gong)同(tong)(tong)接收(shou)同(tong)(tong)一(yi)個信(xin)(xin)號(hao)(hao)，并(bing)有利于(yu)接線和構成各種復雜的控(kong)制系統。如果用電流源作聯絡信(xin)(xin)號(hao)(hao)，當多(duo)臺(tai)儀(yi)(yi)表(biao)共(gong)同(tong)(tong)接收(shou)同(tong)(tong)一(yi)個信(xin)(xin)號(hao)(hao)時(shi)，它們的輸入(ru)電阻(zu)必(bi)須(xu)串聯起(qi)來，這(zhe)會(hui)使大負載(zai)電阻(zu)超過(guo)變送儀(yi)(yi)表(biao)的負載(zai)能力(li)，而且各接收(shou)儀(yi)(yi)表(biao)的信(xin)(xin)號(hao)(hao)負端電位各不(bu)相同(tong)(tong)，會(hui)引入(ru)干擾，而且不(bu)能做(zuo)到單(dan)一(yi)集中(zhong)供電。

采用電(dian)(dian)壓源信號聯(lian)絡，與現(xian)場(chang)儀表(biao)的聯(lian)絡用的電(dian)(dian)流(liu)信號必須轉換(huan)為(wei)電(dian)(dian)壓信號，簡單的辦法就是(shi)：在電(dian)(dian)流(liu)傳送回路(lu)中串聯(lian)一個250Ω的標準(zhun)電(dian)(dian)阻(zu)，把4~20mA.DC轉換(huan)為(wei)1~5V.DC，通常由(you)配(pei)電(dian)(dian)器來(lai)完成這一任務。

三、為什么變送器選擇4~20mA.DC作傳送信號？

1、首先是從現場應用的安全考慮

安(an)全重(zhong)點是以防爆安(an)全火花(hua)型儀表來考(kao)(kao)慮(lv)(lv)的(de)(de)，并以控制儀表能量為前提，把維(wei)持儀表正常(chang)工作的(de)(de)靜態和動(dong)態功(gong)耗降低(di)到低(di)限度。輸出(chu)4~20mA.DC標(biao)準(zhun)信號的(de)(de)變送器，其(qi)電(dian)源電(dian)壓(ya)通常(chang)采(cai)用(yong)24V.DC，采(cai)用(yong)直流(liu)電(dian)壓(ya)的(de)(de)主要原(yuan)因是可以不用(yong)大(da)容(rong)(rong)量的(de)(de)電(dian)容(rong)(rong)器及電(dian)感(gan)器，就只需考(kao)(kao)慮(lv)(lv)變送器與控制室儀表連接(jie)導線的(de)(de)分布電(dian)容(rong)(rong)及電(dian)感(gan)，如(ru)2mm2 的(de)(de)導線其(qi)分布電(dian)容(rong)(rong)為0.05μ/km左右(you)；對(dui)于(yu)單線的(de)(de)電(dian)感(gan)為0.4mH/km左右(you)；大(da)大(da)低(di)于(yu)引爆氫氣(qi)的(de)(de)數值，顯然這(zhe)對(dui)防爆是非常(chang)有利的(de)(de)。

2、傳送信號用電流源優于電壓源

因(yin)為(wei)現(xian)場與(yu)控制室之(zhi)間的距離較(jiao)遠，連接(jie)(jie)電(dian)(dian)線的電(dian)(dian)阻(zu)較(jiao)大時，如果用電(dian)(dian)壓(ya)源(yuan)信(xin)號遠傳，由于電(dian)(dian)線電(dian)(dian)阻(zu)與(yu)接(jie)(jie)收儀表輸入電(dian)(dian)阻(zu)的分壓(ya)，將產(chan)生較(jiao)大的誤差(cha)，如果用電(dian)(dian)流(liu)源(yuan)信(xin)號作為(wei)遠傳，只要(yao)傳送回路不出現(xian)分支，回路中的電(dian)(dian)流(liu)就不會隨電(dian)(dian)線長短而改(gai)變(bian)，從(cong)而保證了傳送的精度。

3、信號大電流選擇20mA的原因

大電(dian)流(liu)20mA的(de)(de)選(xuan)擇是基于安全、實用、功耗、成本的(de)(de)考慮。安全火花儀(yi)表只(zhi)能采用低(di)電(dian)壓、低(di)電(dian)流(liu)，4~20mA電(dian)流(liu)和24V.DC對(dui)易燃氫氣也是安全的(de)(de)，對(dui)于24V.DC氫氣的(de)(de)引爆電(dian)流(liu)為200mA，遠在20mA以上，此外還(huan)要綜合(he)考慮生(sheng)產現場(chang)儀(yi)表之間的(de)(de)連(lian)接距離，所帶(dai)負載等因(yin)素；還(huan)有(you)功耗及成本問題，對(dui)電(dian)子元(yuan)件的(de)(de)要求(qiu)，供電(dian)功率的(de)(de)要求(qiu)等因(yin)素。

4、信號起點電流選擇4mA的原因

輸出(chu)為(wei)4~20mA的(de)(de)變(bian)送(song)器(qi)以兩(liang)線制的(de)(de)居(ju)多，兩(liang)線制即(ji)電(dian)(dian)源、負載串聯(lian)在一起，有(you)一公共點(dian)，而現場變(bian)送(song)器(qi)與(yu)控制室儀(yi)表之(zhi)間的(de)(de)信號(hao)聯(lian)絡及(ji)供電(dian)(dian)僅用兩(liang)根電(dian)(dian)線。為(wei)什么起點(dian)信號(hao)不是0mA？這(zhe)是基于(yu)(yu)兩(liang)點(dian)：一是變(bian)送(song)器(qi)電(dian)(dian)路(lu)沒有(you)靜態工作(zuo)電(dian)(dian)流將(jiang)無法工作(zuo)，信號(hao)起點(dian)電(dian)(dian)流4mA.DC，不與(yu)機械(xie)零(ling)點(dian)重(zhong)合(he)，這(zhe)種“活零(ling)點(dian)”有(you)利于(yu)(yu)識別斷電(dian)(dian)和斷線等故障。

四、4~20mA傳感器的由來？

采用電流信號的原(yuan)因是不(bu)容易受干擾、并且(qie)電流源(yuan)內阻無窮(qiong)大(da)，導線(xian)電阻串聯在回路中(zhong)不(bu)影響(xiang)精(jing)度，在普(pu)通雙絞線(xian)上可以傳輸數百米。

采用電(dian)流(liu)信號的(de)(de)原因是(shi)不容易受干擾，因為工業現場的(de)(de)噪(zao)聲電(dian)壓(ya)的(de)(de)幅度可(ke)(ke)能達到(dao)數V，但是(shi)噪(zao)聲的(de)(de)功率很弱，所以噪(zao)聲電(dian)流(liu)通常(chang)小(xiao)于n*別，因此給4－20mA傳(chuan)輸(shu)帶(dai)來(lai)的(de)(de)誤差非(fei)常(chang)小(xiao)；電(dian)流(liu)源內阻(zu)趨于無(wu)窮大，導線(xian)電(dian)阻(zu)串聯(lian)在回路中(zhong)不影響精度，因此在普通雙絞線(xian)上可(ke)(ke)以傳(chuan)輸(shu)數百米；由于電(dian)流(liu)源的(de)(de)大內阻(zu)和恒流(liu)輸(shu)出，在接收端我(wo)們只需(xu)放置一個250歐(ou)姆(mu)到(dao)地的(de)(de)電(dian)阻(zu)就可(ke)(ke)以獲得(de)0－5V的(de)(de)電(dian)壓(ya)，低輸(shu)入(ru)阻(zu)抗(kang)的(de)(de)接收器的(de)(de)好(hao)處是(shi)n*的(de)(de)輸(shu)入(ru)電(dian)流(liu)噪(zao)聲只產生非(fei)常(chang)微弱的(de)(de)電(dian)壓(ya)噪(zao)聲。

上限(xian)(xian)取(qu)(qu)20mA是因(yin)為(wei)(wei)(wei)(wei)(wei)防(fang)爆的(de)(de)要(yao)(yao)求：20mA的(de)(de)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)通斷引(yin)起的(de)(de)火花能量(liang)不足以(yi)引(yin)燃瓦(wa)斯。下(xia)限(xian)(xian)沒有取(qu)(qu)0mA的(de)(de)原因(yin)是為(wei)(wei)(wei)(wei)(wei)了能檢測斷線(xian)(xian)(xian)(xian)：正常(chang)工作時不會低于(yu)4mA，當(dang)傳輸(shu)線(xian)(xian)(xian)(xian)因(yin)故(gu)障斷路，環路電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)降為(wei)(wei)(wei)(wei)(wei)0。常(chang)取(qu)(qu)2mA作為(wei)(wei)(wei)(wei)(wei)斷線(xian)(xian)(xian)(xian)報警值(zhi)。電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)型變(bian)送器(qi)(qi)(qi)(qi)將物理量(liang)轉換成4~20mA電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)輸(shu)出，必然(ran)要(yao)(yao)有外電(dian)(dian)(dian)(dian)源(yuan)為(wei)(wei)(wei)(wei)(wei)其供電(dian)(dian)(dian)(dian)。典型的(de)(de)是變(bian)送器(qi)(qi)(qi)(qi)需(xu)要(yao)(yao)兩根(gen)(gen)(gen)電(dian)(dian)(dian)(dian)源(yuan)線(xian)(xian)(xian)(xian)，加上兩根(gen)(gen)(gen)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)輸(shu)出線(xian)(xian)(xian)(xian)，總共要(yao)(yao)接4根(gen)(gen)(gen)線(xian)(xian)(xian)(xian)，稱之(zhi)為(wei)(wei)(wei)(wei)(wei)四線(xian)(xian)(xian)(xian)制變(bian)送器(qi)(qi)(qi)(qi)。當(dang)然(ran)，電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)輸(shu)出可(ke)(ke)以(yi)與(yu)電(dian)(dian)(dian)(dian)源(yuan)公(gong)用一(yi)根(gen)(gen)(gen)線(xian)(xian)(xian)(xian)公(gong)用VCC或者GND，可(ke)(ke)節省一(yi)根(gen)(gen)(gen)線(xian)(xian)(xian)(xian)，稱之(zhi)為(wei)(wei)(wei)(wei)(wei)三線(xian)(xian)(xian)(xian)制變(bian)送器(qi)(qi)(qi)(qi)。其實大(da)家可(ke)(ke)能注意到，4-20mA電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)本身就(jiu)可(ke)(ke)以(yi)為(wei)(wei)(wei)(wei)(wei)變(bian)送器(qi)(qi)(qi)(qi)供電(dian)(dian)(dian)(dian)。變(bian)送器(qi)(qi)(qi)(qi)在(zai)(zai)電(dian)(dian)(dian)(dian)路中相(xiang)當(dang)于(yu)一(yi)個特(te)殊(shu)的(de)(de)負載，特(te)殊(shu)之(zhi)處(chu)在(zai)(zai)于(yu)變(bian)送器(qi)(qi)(qi)(qi)的(de)(de)耗電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)在(zai)(zai)4~20mA之(zhi)間(jian)根(gen)(gen)(gen)據傳感器(qi)(qi)(qi)(qi)輸(shu)出而(er)變(bian)化。顯示儀(yi)表只需(xu)要(yao)(yao)串在(zai)(zai)電(dian)(dian)(dian)(dian)路中即可(ke)(ke)。這種變(bian)送器(qi)(qi)(qi)(qi)只需(xu)外接2根(gen)(gen)(gen)線(xian)(xian)(xian)(xian)，因(yin)而(er)被稱為(wei)(wei)(wei)(wei)(wei)兩線(xian)(xian)(xian)(xian)制變(bian)送器(qi)(qi)(qi)(qi)。工業電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)(liu)環標準下(xia)限(xian)(xian)為(wei)(wei)(wei)(wei)(wei)4mA，因(yin)此(ci)只要(yao)(yao)在(zai)(zai)量(liang)程(cheng)范圍內，變(bian)送器(qi)(qi)(qi)(qi)至少(shao)有4mA供電(dian)(dian)(dian)(dian)。

因此、4-20mA的信(xin)(xin)號(hao)輸(shu)(shu)出一般不(bu)容(rong)易受干擾而且(qie)安(an)全可(ke)靠(kao)、所以工業上普遍(bian)使(shi)用的都是(shi)二(er)線制4-20mA的電(dian)源輸(shu)(shu)出信(xin)(xin)號(hao)。但為了能更好的處(chu)理傳感器的信(xin)(xin)號(hao)、目(mu)前(qian)還有更多其(qi)它形式的輸(shu)(shu)出信(xin)(xin)號(hao)：3.33MV/V；2MV/V；0-5V; 0-10V等。