咱們用這類模式來傳輸(shu)數(shu)據數(shu)據摸(mo)擬(ni)網(wang)訊號呢？工藝上(shang)普(pu)通必須要 在(zai)線測量當下非電物(wu)理化學量，隨(sui)后(hou)的溫度(du)、心理壓力、速(su)度(du)快(kuai)、方向角等，這類都必須要 轉型成摸(mo)擬(ni)網(wang)量電訊號可以傳輸(shu)數(shu)據數(shu)據到好幾百米(mi)外的操縱室或表(biao)示的設備上(shang)。工藝上(shang)很廣泛選取的是用4~20mA電流(liu)來傳輸(shu)模擬(ni)量。

適(shi)用電(dian)(dian)(dian)流(liu)電(dian)(dian)(dian)壓電(dian)(dian)(dian)流(liu)走勢(shi)的(de)緣由(you)是沒有(you)方便受電(dian)(dian)(dian)磁干擾，而是化工當場(chang)的(de)燥音(yin)電(dian)(dian)(dian)流(liu)電(dian)(dian)(dian)壓的(de)小(xiao)幅度(du)已經達(da)到(dao)數(shu)V，但是噪聲的(de)功(gong)率很弱，所以(yi)噪聲電(dian)(dian)(dian)流(liu)通常(chang)小(xiao)于(yu)nA級別，因(yin)此(ci)給4－20mA傳輸(shu)帶來的(de)誤(wu)差非常(chang)小(xiao)；電(dian)(dian)(dian)流(liu)源內阻趨(qu)于(yu)無窮大(da)，導線電(dian)(dian)(dian)阻串聯(lian)在回路(lu)中不(bu)影(ying)響精度(du)，因(yin)此(ci)在普通雙絞(jiao)線上可以(yi)傳輸(shu)數(shu)百(bai)米；由(you)于(yu)電(dian)(dian)(dian)流(liu)源的(de)大(da)內阻和恒流(liu)輸(shu)出，在接收端(duan)我們(men)只(zhi)需放置一個250歐姆到(dao)地的(de)電(dian)(dian)(dian)阻就可以(yi)獲(huo)得0－5V的(de)電(dian)(dian)(dian)壓，低輸(shu)入阻抗的(de)接收器的(de)好處是nA級的(de)輸(shu)入電(dian)(dian)(dian)流(liu)噪聲只(zhi)產生(sheng)非常(chang)微弱的(de)電(dian)(dian)(dian)壓噪聲。

累計取(qu)20mA是因(yin)(yin)為(wei)(wei)(wei)防(fang)爆的(de)(de)(de)(de)要求：20mA的(de)(de)(de)(de)電(dian)(dian)(dian)流(liu)(liu)通斷(duan)(duan)引起(qi)的(de)(de)(de)(de)火花能(neng)量(liang)(liang)不(bu)足以(yi)引燃瓦斯。下限(xian)沒有(you)取(qu)0mA的(de)(de)(de)(de)原(yuan)因(yin)(yin)是為(wei)(wei)(wei)了能(neng)檢測(ce)斷(duan)(duan)線(xian)(xian)：正常(chang)工作時不(bu)會(hui)低于4mA，當(dang)傳(chuan)(chuan)輸(shu)線(xian)(xian)因(yin)(yin)故障斷(duan)(duan)路(lu)，環路(lu)電(dian)(dian)(dian)流(liu)(liu)降(jiang)為(wei)(wei)(wei)0。常(chang)取(qu)2mA作為(wei)(wei)(wei)斷(duan)(duan)線(xian)(xian)報警值。電(dian)(dian)(dian)流(liu)(liu)型變(bian)送(song)(song)(song)器(qi)將物(wu)理量(liang)(liang)轉(zhuan)換成4~20mA電(dian)(dian)(dian)流(liu)(liu)輸(shu)出，必然(ran)(ran)要有(you)外電(dian)(dian)(dian)源(yuan)為(wei)(wei)(wei)其(qi)供(gong)電(dian)(dian)(dian)。最典型的(de)(de)(de)(de)是變(bian)送(song)(song)(song)器(qi)需要兩根(gen)電(dian)(dian)(dian)源(yuan)線(xian)(xian)，加上(shang)兩根(gen)電(dian)(dian)(dian)流(liu)(liu)輸(shu)出線(xian)(xian)，總共要接4根(gen)線(xian)(xian)，稱之為(wei)(wei)(wei)四線(xian)(xian)制(zhi)變(bian)送(song)(song)(song)器(qi)。當(dang)然(ran)(ran)，電(dian)(dian)(dian)流(liu)(liu)輸(shu)出可以(yi)與電(dian)(dian)(dian)源(yuan)公用一根(gen)線(xian)(xian)（公用VCC或者GND），可節省一根(gen)線(xian)(xian)，所(suo)以(yi)現在(zai)(zai)(zai)(zai)基本上(shang)將四線(xian)(xian)制(zhi)變(bian)送(song)(song)(song)器(qi)稱之為(wei)(wei)(wei)三線(xian)(xian)制(zhi)變(bian)送(song)(song)(song)器(qi)。其(qi)實(shi)大家可能(neng)注意到(dao)， 4-20mA電(dian)(dian)(dian)流(liu)(liu)本身就可以(yi)為(wei)(wei)(wei)變(bian)送(song)(song)(song)器(qi)供(gong)電(dian)(dian)(dian)，變(bian)送(song)(song)(song)器(qi)在(zai)(zai)(zai)(zai)電(dian)(dian)(dian)路(lu)中相當(dang)于一個特殊(shu)的(de)(de)(de)(de)負載，這種變(bian)送(song)(song)(song)器(qi)只需外接2根(gen)線(xian)(xian)，因(yin)(yin)而(er)被稱為(wei)(wei)(wei)兩線(xian)(xian)制(zhi)變(bian)送(song)(song)(song)器(qi)。工業電(dian)(dian)(dian)流(liu)(liu)環標準下限(xian)為(wei)(wei)(wei)4mA，因(yin)(yin)此在(zai)(zai)(zai)(zai)量(liang)(liang)程范圍內，變(bian)送(song)(song)(song)器(qi)通常(chang)只有(you)24V，4mA供(gong)電(dian)(dian)(dian)（因(yin)(yin)此，在(zai)(zai)(zai)(zai)輕負載條件下高(gao)效率的(de)(de)(de)(de)DC/DC電(dian)(dian)(dian)源(yuan)（TPS54331,TPS54160），低功耗(hao)的(de)(de)(de)(de)傳(chuan)(chuan)感器(qi)和信(xin)號鏈產品(pin)、以(yi)及低功耗(hao)的(de)(de)(de)(de)處理器(qi)（如MSP430）對于兩線(xian)(xian)制(zhi)的(de)(de)(de)(de)4-20mA收發非常(chang)重要）。這使得(de)兩線(xian)(xian)制(zhi)傳(chuan)(chuan)感器(qi)的(de)(de)(de)(de)設(she)計成為(wei)(wei)(wei)可能(neng)而(er)又富有(you)挑(tiao)戰。

    通(tong)常情況(kuang)需求(qiu)設汁是(shi)一個VI轉換器，輸入(ru)0-3.3v，輸出4mA-20mA，可采用運放LM358，供(gong)電+12v。

  .我整體地來講看仿真量主設備憑什么東西都(dou)喜歡用4~20mA傳輸信號~

     4-20mA. DC(1-5V.DC)預警制是國.際上電工作業常務專委會( IEC )工作操縱設備主要選取的養成預警輸送要求。目前我國也主要選取一項國.際上要求預警制，儀器儀表輸送預警主要選取4-20mA.DC，發收預警主要選取1-5V.DC，即主要選取工作電流輸送、額定電壓發收的預警設備。

一(yi)(yi)樣分析儀器(qi)設備的(de)(de)信(xin)號燈電(dian)流(liu)值都為4-20mA，指最(zui)小電(dian)流(liu)為4mA,最(zui)大(da)電(dian)流(liu)為20mA 。傳輸信(xin)號時候，因為導(dao)(dao)線上(shang)也有電(dian)阻(zu)，如果用電(dian)壓(ya)(ya)傳輸則會(hui)在導(dao)(dao)線內(nei)產(chan)生(sheng)一(yi)(yi)定的(de)(de)壓(ya)(ya)降，那接收端的(de)(de)信(xin)號就會(hui)產(chan)生(sheng)一(yi)(yi)定的(de)(de)誤差了，所以一(yi)(yi)般(ban)使用電(dian)流(liu)信(xin)號作為變送器(qi)的(de)(de)標準傳輸。

什么東西是4~20mA.DC（1~5V.DC）信號制？

        4~20mA.DC（1~5V.DC）數(shu)(shu)(shu)據(ju)警報(bao)(bao)制是(shi)亞(ya)(ya)太水電(dian)(dian)工(gong)專(zhuan)委會會（IEC）：歷程保持體(ti)系用摸擬數(shu)(shu)(shu)據(ju)警報(bao)(bao)條(tiao)件(jian)。東北地區從(cong)DDZ-Ⅲ型電(dian)(dian)動四輪設備開使適(shi)(shi)用這些(xie)亞(ya)(ya)太條(tiao)件(jian)數(shu)(shu)(shu)據(ju)警報(bao)(bao)制，設備視頻(pin)文件(jian)傳(chuan)輸數(shu)(shu)(shu)據(ju)警報(bao)(bao)適(shi)(shi)用4~20mA.DC，建立聯系數(shu)(shu)(shu)據(ju)警報(bao)(bao)適(shi)(shi)用1~5V.DC，即適(shi)(shi)用直流電(dian)(dian)阻(zu)視頻(pin)文件(jian)傳(chuan)輸、電(dian)(dian)阻(zu)發送的數(shu)(shu)(shu)據(ju)警報(bao)(bao)體(ti)系。

         4~20mA電壓環工作中道理：

        在制造業實(shi)地，用一(yi)(yi)(yi)位儀(yi)表板(ban)(ban)板(ban)(ban)縮(suo)放器來到(dao)位數(shu)(shu)(shu)據(ju)(ju)(ju)數(shu)(shu)(shu)據(ju)(ju)(ju)信息的廚衛(wei)并(bing)舉行長線炒股(gu)視(shi)頻(pin)數(shu)(shu)(shu)據(ju)(ju)(ju)文件互(hu)(hu)傳(chuan)，會存在以(yi)內(nei)現象：一(yi)(yi)(yi)，鑒于視(shi)頻(pin)數(shu)(shu)(shu)據(ju)(ju)(ju)文件互(hu)(hu)傳(chuan)的數(shu)(shu)(shu)據(ju)(ju)(ju)數(shu)(shu)(shu)據(ju)(ju)(ju)信息是交(jiao)流電(dian)流值數(shu)(shu)(shu)據(ju)(ju)(ju)數(shu)(shu)(shu)據(ju)(ju)(ju)信息，視(shi)頻(pin)數(shu)(shu)(shu)據(ju)(ju)(ju)文件互(hu)(hu)傳(chuan)線會受噪音分貝的騷(sao)擾；第(di)五(wu)，視(shi)頻(pin)數(shu)(shu)(shu)據(ju)(ju)(ju)文件互(hu)(hu)傳(chuan)線的分散電(dian)容會存在交(jiao)流電(dian)流值降；第(di)一(yi)(yi)(yi)，在實(shi)地怎(zen)么才能提供數(shu)(shu)(shu)據(ju)(ju)(ju)儀(yi)表板(ban)(ban)板(ban)(ban)縮(suo)放器的業務交(jiao)流電(dian)流值也(ye)是個(ge)現象。

    想要(yao)避免上述(shu)內容(rong)難題和躲開有(you)關的(de)背景(jing)噪(zao)聲源的(de)導致，.我民用交流(liu)電來互(hu)傳數(shu)據信息，可(ke)能交流(liu)電對背景(jing)噪(zao)聲源并(bing)不太敏感。4～20mA的(de)電流(liu)環便是用4mA表(biao)示零信號，用20mA表(biao)示信號的(de)滿刻度，而低(di)于(yu)(yu)4mA高于(yu)(yu)20mA的(de)信號用于(yu)(yu)各種故障的(de)報警(jing)。

     二(er)、4~20mA.DC（1~5V.DC）信號(hao)制的(de)優(you)點？

     現(xian)場(chang)(chang)視頻圖智能儀表板可確保兩條(tiao)線(xian)(xian)制，實(shi)際上兩條(tiao)線(xian)(xian)制即(ji)電(dian)源(yuan)模塊(kuai)、負載電(dian)阻串(chuan)聯(lian)(lian)和(he)并聯(lian)(lian)在一塊(kuai)，有(you)很(hen)大公共服(fu)務點(dian)，而現(xian)場(chang)(chang)視頻圖智能變送器(qi)與管控室(shi)智能儀表板前面(mian)的手機數字(zi)信號建立(li)聯(lian)(lian)系(xi)及用電(dian)僅用多根家裝電(dian)線(xian)(xian)。是由于(yu)手機數字(zi)信號起始點(dian)直流電(dian)為(wei)4mA.DC，為(wei)變送器(qi)提供了靜(jing)態(tai)工作電(dian)流，同時儀表電(dian)氣零(ling)(ling)點(dian)為(wei)4mA.DC，不與機械零(ling)(ling)點(dian)重(zhong)合，這種“活零(ling)(ling)點(dian)”有(you)利于(yu)識別斷(duan)電(dian)和(he)斷(duan)線(xian)(xian)等(deng)故障。而且兩線(xian)(xian)制還(huan)便(bian)于(yu)使用安(an)全柵(zha)，利于(yu)安(an)全防爆。

把(ba)控(kong)好室儀盤表(biao)(biao)板(ban)(ban)(ban)板(ban)(ban)(ban)板(ban)(ban)(ban)適(shi)用電壓值并(bing)接數據信號數據傳輸，同樣個把(ba)控(kong)好平臺所在區域的(de)儀盤表(biao)(biao)板(ban)(ban)(ban)板(ban)(ban)(ban)板(ban)(ban)(ban)范圍內有服務性端，容易探測儀盤表(biao)(biao)板(ban)(ban)(ban)板(ban)(ban)(ban)板(ban)(ban)(ban)、設定(ding)儀盤表(biao)(biao)板(ban)(ban)(ban)板(ban)(ban)(ban)板(ban)(ban)(ban)、算機(ji)、報案配置通用，并(bing)便捷接線方法。

  現場(chang)視(shi)頻(pin)儀容儀表(biao)盤(pan)與(yu)的(de)控(kong)制室儀容儀表(biao)盤(pan)左右的(de)信(xin)息溝通(tong)移動(dong)信(xin)號(hao)按照(zhao)4~20mA.DC的(de)理(li)由是：因為現場(chang)與(yu)控(kong)制室之間的(de)距離較(jiao)遠，連接(jie)電(dian)(dian)(dian)線的(de)電(dian)(dian)(dian)阻(zu)較(jiao)大(da)(da)，如果用(yong)電(dian)(dian)(dian)壓(ya)信(xin)號(hao)遠傳，優于電(dian)(dian)(dian)線電(dian)(dian)(dian)阻(zu)與(yu)接(jie)收儀表(biao)輸入電(dian)(dian)(dian)阻(zu)的(de)分壓(ya)，將(jiang)產(chan)生較(jiao)大(da)(da)的(de)誤(wu)差，而用(yong)恒(heng)流(liu)源信(xin)號(hao)作為遠傳，只要傳送(song)回路不出現分支(zhi)，回路中的(de)電(dian)(dian)(dian)流(liu)就(jiu)不會隨電(dian)(dian)(dian)線長短而改(gai)變(bian)，從而保證了傳送(song)的(de)精度。

 控制室儀(yi)容儀(yi)表(biao)(biao)(biao)兩(liang)者之(zhi)間的(de)信(xin)(xin)(xin)息(xi)溝(gou)通衛星信(xin)(xin)(xin)號選取1~5V.DC理由是：為(wei)了(le)便于多臺儀(yi)表(biao)(biao)(biao)共(gong)同(tong)(tong)接(jie)(jie)收同(tong)(tong)一個(ge)信(xin)(xin)(xin)號，并有利于接(jie)(jie)線和構成各種復雜的(de)控制系(xi)統。如果(guo)用電(dian)(dian)流源(yuan)作聯(lian)絡信(xin)(xin)(xin)號，當(dang)多臺儀(yi)表(biao)(biao)(biao)共(gong)同(tong)(tong)接(jie)(jie)收同(tong)(tong)一個(ge)信(xin)(xin)(xin)號時，它們的(de)輸(shu)入(ru)電(dian)(dian)阻必須串聯(lian)起來，這會(hui)使最大負(fu)載(zai)電(dian)(dian)阻超(chao)過變(bian)送儀(yi)表(biao)(biao)(biao)的(de)負(fu)載(zai)能(neng)力(li)，而(er)且(qie)各接(jie)(jie)收儀(yi)表(biao)(biao)(biao)的(de)信(xin)(xin)(xin)號負(fu)端電(dian)(dian)位各不(bu)相同(tong)(tong)，會(hui)引入(ru)干擾，而(er)且(qie)不(bu)能(neng)做到(dao)單一集中供電(dian)(dian)。

選取相(xiang)交(jiao)流(liu)電電壓(ya)源手(shou)(shou)機數(shu)據訊(xun)號(hao)交(jiao)流(liu)，與現場圖(tu)電子(zi)儀(yi)表的(de)(de)交(jiao)流(liu)用(yong)的(de)(de)直流(liu)電手(shou)(shou)機數(shu)據訊(xun)號(hao)一(yi)(yi)定要改換(huan)(huan)為相(xiang)交(jiao)流(liu)電電壓(ya)手(shou)(shou)機數(shu)據訊(xun)號(hao)，最比較簡單的(de)(de)依(yi)據可(ke)是：在直流(liu)電復(fu)制控制回(hui)路(lu)中關(guan)聯同一(yi)(yi)個250Ω的(de)(de)標(biao)準電阻，把(ba)4~20mA.DC轉換(huan)(huan)為1~5V.DC，通常由配電器(qi)來完成這一(yi)(yi)任務。

     三(san)、為(wei)甚么智(zhi)能變送(song)器選定(ding)4~20mA.DC作傳送(song)信號(hao)？

       1、率先是在實地應用的可靠綜合考慮

       安全(quan)防(fang)護管理核心是(shi)以防(fang)燃安全(quan)防(fang)護管理火光型(xing)電(dian)(dian)(dian)子(zi)儀器來要考慮到的，僅以保持電(dian)(dian)(dian)子(zi)儀器動能(neng)為(wei)的前提，把保持電(dian)(dian)(dian)子(zi)儀器常見事業(ye)的最新和最新傷(shang)害(hai)功率減少到最少幅(fu)度(du)。傷(shang)害(hai)4~20mA.DC標準信(xin)號的變送(song)(song)器，其電(dian)(dian)(dian)源電(dian)(dian)(dian)壓通常采用(yong)(yong)24V.DC，采用(yong)(yong)直流電(dian)(dian)(dian)壓的主(zhu)要原因是(shi)可以不(bu)用(yong)(yong)大(da)(da)容量的電(dian)(dian)(dian)容器及電(dian)(dian)(dian)感器，就(jiu)只需考慮變送(song)(song)器與控制室儀表連接導(dao)線的分布電(dian)(dian)(dian)容及電(dian)(dian)(dian)感，如(ru)2mm2 的導(dao)線其分布電(dian)(dian)(dian)容為(wei)0.05μ/km左右(you)；對于單線的電(dian)(dian)(dian)感為(wei)0.4mH/km左右(you)；大(da)(da)大(da)(da)低于引爆氫氣的數值，顯然這(zhe)對防(fang)爆是(shi)非(fei)常有利的。

     2、傳大信息耗電流源遠高于電阻源

因當場(chang)與(yu)管理室(shi)之前的(de)相(xiang)距近好，連(lian)接方式電(dian)(dian)(dian)源線的(de)功率電(dian)(dian)(dian)阻(zu)器(qi)值極(ji)大(da)時，假(jia)若工廠電(dian)(dian)(dian)費(fei)壓源網(wang)絡訊號遠傳(chuan)，根據電(dian)(dian)(dian)源線功率電(dian)(dian)(dian)阻(zu)器(qi)值與(yu)推送儀盤表讀(du)取功率電(dian)(dian)(dian)阻(zu)器(qi)值的(de)分壓，將形(xing)成(cheng)極(ji)大(da)的(de)出現(xian)偏差(cha)的(de)原因，假(jia)若工廠電(dian)(dian)(dian)費(fei)流源網(wang)絡訊號是 遠傳(chuan)，只需傳(chuan)輸(shu)漏電(dian)(dian)(dian)開關(guan)不出現(xian)支(zhi)系，漏電(dian)(dian)(dian)開關(guan)中的(de)感應電(dian)(dian)(dian)流就不懂隨(sui)電(dian)(dian)(dian)源線大(da)小(xiao)而變更，導致(zhi) 保(bao)障(zhang)了傳(chuan)輸(shu)的(de)要求。

      3、信號燈大工作電流取舍20mA的情況

  比(bi)較大(da)直流(liu)電(dian)20mA的(de)(de)選(xuan)擇是基于安(an)全、實(shi)用、功(gong)耗、成本(ben)的(de)(de)考慮(lv)。安(an)全火(huo)花儀表只(zhi)能采用低電(dian)壓、低電(dian)流(liu)，4~20mA電(dian)流(liu)和(he)24V.DC對(dui)易燃氫氣也是安(an)全的(de)(de)，對(dui)于24V.DC氫氣的(de)(de)引爆電(dian)流(liu)為200mA，遠(yuan)在20mA以上(shang)，此外還要綜合考慮(lv)生產(chan)現場(chang)儀表之間的(de)(de)連接距離，所帶負載等因(yin)(yin)素；還有功(gong)耗及成本(ben)問題，對(dui)電(dian)子元件的(de)(de)要求(qiu)(qiu)，供(gong)電(dian)功(gong)率的(de)(de)要求(qiu)(qiu)等因(yin)(yin)素。

    4、訊號出發點瞬時電流選用4mA的理由

  輸(shu)出的(de)為4~20mA的(de)變(bian)(bian)送器以兩(liang)線(xian)(xian)制的(de)居(ju)多(duo)，兩(liang)線(xian)(xian)制即電源、負(fu)載串聯在一(yi)起，有一(yi)公共(gong)點(dian)(dian)，而(er)現場變(bian)(bian)送器與(yu)控制室儀(yi)表之間(jian)的(de)信號(hao)(hao)聯絡及(ji)供電僅用兩(liang)根電線(xian)(xian)。為什么(me)起點(dian)(dian)信號(hao)(hao)不(bu)是(shi)0mA？這是(shi)基于(yu)兩(liang)點(dian)(dian)：一(yi)是(shi)變(bian)(bian)送器電路(lu)沒(mei)有靜態工(gong)作電流將無(wu)法工(gong)作，信號(hao)(hao)起點(dian)(dian)電流4mA.DC，不(bu)與(yu)機械零點(dian)(dian)重合，這種“活零點(dian)(dian)”有利于(yu)識別(bie)斷電和斷線(xian)(xian)等(deng)故(gu)障。

     四、4~20mA傳感器的(de)由來？

    運(yun)用感應電(dian)流大(da)小數據信號的情(qing)況就是會受干預、如(ru)果(guo)感應電(dian)流大(da)小源電(dian)容無窮(qiong)大(da)，電(dian)線電(dian)容結合在電(dian)路開關(guan)中(zhong)不不良(liang)影響精確，在普遍雙(shuang)銅編織線上能夠 傳遞數百人米。 

  分(fen)為(wei)直流(liu)(liu)(liu)額定(ding)電(dian)(dian)壓(ya)預警的(de)因素是(shi)不會加容(rong)易受干預，而是(shi)工業制造工地(di)的(de)躁聲(sheng)(sheng)額定(ding)電(dian)(dian)壓(ya)的(de)波幅可(ke)以高于(yu)數V，但是(shi)噪(zao)(zao)(zao)聲(sheng)(sheng)的(de)功率很(hen)弱，所以噪(zao)(zao)(zao)聲(sheng)(sheng)電(dian)(dian)流(liu)(liu)(liu)通常(chang)小于(yu)nA級(ji)別，因此(ci)給4－20mA傳(chuan)輸(shu)帶來(lai)的(de)誤差非常(chang)小；電(dian)(dian)流(liu)(liu)(liu)源內阻(zu)趨(qu)于(yu)無窮(qiong)大，導線電(dian)(dian)阻(zu)串聯在回路(lu)中不影(ying)響精度，因此(ci)在普通雙絞線上可(ke)以傳(chuan)輸(shu)數百米(mi)；由于(yu)電(dian)(dian)流(liu)(liu)(liu)源的(de)大內阻(zu)和恒流(liu)(liu)(liu)輸(shu)出，在接收端我(wo)們只需放置一個(ge)250歐姆到地(di)的(de)電(dian)(dian)阻(zu)就(jiu)可(ke)以獲得0－5V的(de)電(dian)(dian)壓(ya)，低(di)輸(shu)入阻(zu)抗的(de)接收器(qi)的(de)好處是(shi)nA級(ji)的(de)輸(shu)入電(dian)(dian)流(liu)(liu)(liu)噪(zao)(zao)(zao)聲(sheng)(sheng)只產(chan)生非常(chang)微(wei)弱的(de)電(dian)(dian)壓(ya)噪(zao)(zao)(zao)聲(sheng)(sheng)。

 累計(ji)取20mA是(shi)因(yin)(yin)(yin)為(wei)(wei)(wei)防爆的要求：20mA的電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)通斷引起的火花能(neng)量不(bu)足(zu)以(yi)引燃瓦斯(si)。下(xia)限沒有取0mA的原因(yin)(yin)(yin)是(shi)為(wei)(wei)(wei)了能(neng)檢測斷線(xian)(xian)：正常工作時不(bu)會低于4mA，當傳(chuan)輸(shu)(shu)線(xian)(xian)因(yin)(yin)(yin)故障(zhang)斷路，環路電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)降為(wei)(wei)(wei)0。常取2mA作為(wei)(wei)(wei)斷線(xian)(xian)報(bao)警值。電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)型(xing)變(bian)(bian)(bian)送(song)(song)器將物理量轉換成(cheng)4~20mA電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)輸(shu)(shu)出(chu)(chu)，必(bi)然要有外電(dian)(dian)(dian)(dian)(dian)源為(wei)(wei)(wei)其供電(dian)(dian)(dian)(dian)(dian)。最(zui)典型(xing)的是(shi)變(bian)(bian)(bian)送(song)(song)器需(xu)要兩根(gen)(gen)電(dian)(dian)(dian)(dian)(dian)源線(xian)(xian)，加(jia)上兩根(gen)(gen)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)輸(shu)(shu)出(chu)(chu)線(xian)(xian)，總共要接4根(gen)(gen)線(xian)(xian)，稱之為(wei)(wei)(wei)四線(xian)(xian)制變(bian)(bian)(bian)送(song)(song)器。當然，電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)輸(shu)(shu)出(chu)(chu)可(ke)以(yi)與電(dian)(dian)(dian)(dian)(dian)源公用(yong)一根(gen)(gen)線(xian)(xian)公用(yong)VCC或者(zhe)GND，可(ke)節省一根(gen)(gen)線(xian)(xian)，稱之為(wei)(wei)(wei)三線(xian)(xian)制變(bian)(bian)(bian)送(song)(song)器。其實大家(jia)可(ke)能(neng)注意到(dao)，4-20mA電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)本身就可(ke)以(yi)為(wei)(wei)(wei)變(bian)(bian)(bian)送(song)(song)器供電(dian)(dian)(dian)(dian)(dian)。變(bian)(bian)(bian)送(song)(song)器在(zai)電(dian)(dian)(dian)(dian)(dian)路中相(xiang)當于一個特(te)(te)殊的負(fu)載，特(te)(te)殊之處在(zai)于變(bian)(bian)(bian)送(song)(song)器的耗電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)在(zai)4~20mA之間根(gen)(gen)據(ju)傳(chuan)感器輸(shu)(shu)出(chu)(chu)而變(bian)(bian)(bian)化。顯示儀表只(zhi)需(xu)要串在(zai)電(dian)(dian)(dian)(dian)(dian)路中即(ji)可(ke)。這種(zhong)變(bian)(bian)(bian)送(song)(song)器只(zhi)需(xu)外接2根(gen)(gen)線(xian)(xian)，因(yin)(yin)(yin)而被(bei)稱為(wei)(wei)(wei)兩線(xian)(xian)制變(bian)(bian)(bian)送(song)(song)器。工業電(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)環標準下(xia)限為(wei)(wei)(wei)4mA，因(yin)(yin)(yin)此只(zhi)要在(zai)量程范圍內，變(bian)(bian)(bian)送(song)(song)器至少(shao)有4mA供電(dian)(dian)(dian)(dian)(dian)。

 那么、4-20mA的(de)(de)信號輸(shu)出一般不(bu)容(rong)易(yi)受干擾而且(qie)安全可(ke)靠(kao)、所(suo)以工業上普遍使用的(de)(de)都是二線制4-20mA的(de)(de)電源輸(shu)出信號。但為了能(neng)更(geng)(geng)好的(de)(de)處(chu)理傳(chuan)感器的(de)(de)信號、目前(qian)還有更(geng)(geng)多(duo)其(qi)它形式的(de)(de)輸(shu)出信號：3.33MV/V；2MV/V；0-5V; 0-10V等(deng)。

另附每張4到20mA轉(zhuan)電壓信號的簡單(dan)電路(lu)圖：

      這(zhe)張圖食用一(yi)兩個250歐(ou)姆的電阻將4到20mA的電流信(xin)號轉換成1到5V的電壓信(xin)號，然后使用一(yi)個RC濾波加一(yi)個二極(ji)管(guan)（原(yuan)諒我模擬電路不好，并不知道(dao)是(shi)什么(me)意思）接到單片機的AD轉換引腳。