大(da)家用甚么(me)的(de)方式來傳遞模(mo)(mo)仿(fang)仿(fang)真數據呢？工藝上(shang)通常須要測量(liang)(liang)方法各種非電力(li)學量(liang)(liang)，假如(ru)溫度因素、壓(ya)力(li)差、快速、層面等(deng)，這(zhe)樣都須要改變(bian)成模(mo)(mo)仿(fang)仿(fang)真量(liang)(liang)電數據才(cai)行傳遞到幾百元米外的(de)設(she)定室或顯(xian)視生(sheng)產設(she)備上(shang)。工藝上(shang)很廣泛主要包(bao)括的(de)是用4~20mA電流來傳輸模(mo)(mo)擬量(liang)(liang)。

運用(yong)電(dian)(dian)(dian)流(liu)大小的(de)(de)(de)信號的(de)(de)(de)因(yin)此不非常(chang)容易受騷擾，因(yin)此實業場地的(de)(de)(de)燥(zao)音電(dian)(dian)(dian)阻的(de)(de)(de)上升時間(jian)有機會(hui)到(dao)數V，但(dan)是噪聲的(de)(de)(de)功率很(hen)弱(ruo)(ruo)，所(suo)以噪聲電(dian)(dian)(dian)流(liu)通(tong)常(chang)小于nA級別，因(yin)此給4－20mA傳(chuan)(chuan)輸帶來的(de)(de)(de)誤差非常(chang)小；電(dian)(dian)(dian)流(liu)源內阻趨于無窮大，導(dao)線電(dian)(dian)(dian)阻串聯在回路(lu)中(zhong)不影響精(jing)度，因(yin)此在普通(tong)雙絞(jiao)線上可以傳(chuan)(chuan)輸數百米；由于電(dian)(dian)(dian)流(liu)源的(de)(de)(de)大內阻和恒流(liu)輸出，在接收端我們只需放置(zhi)一個250歐姆到(dao)地的(de)(de)(de)電(dian)(dian)(dian)阻就可以獲(huo)得(de)0－5V的(de)(de)(de)電(dian)(dian)(dian)壓，低輸入阻抗的(de)(de)(de)接收器(qi)的(de)(de)(de)好處是nA級的(de)(de)(de)輸入電(dian)(dian)(dian)流(liu)噪聲只產生非常(chang)微(wei)弱(ruo)(ruo)的(de)(de)(de)電(dian)(dian)(dian)壓噪聲。

最大(da)值取20mA是因為(wei)(wei)防(fang)爆的(de)(de)(de)(de)(de)要(yao)求：20mA的(de)(de)(de)(de)(de)電(dian)(dian)(dian)流(liu)(liu)通(tong)斷(duan)(duan)引(yin)(yin)起的(de)(de)(de)(de)(de)火花能(neng)(neng)量(liang)不足以(yi)引(yin)(yin)燃瓦斯。下(xia)限(xian)沒有(you)取0mA的(de)(de)(de)(de)(de)原因是為(wei)(wei)了能(neng)(neng)檢測斷(duan)(duan)線(xian)(xian)：正(zheng)常工作時不會低于4mA，當(dang)傳輸(shu)(shu)線(xian)(xian)因故障斷(duan)(duan)路，環路電(dian)(dian)(dian)流(liu)(liu)降為(wei)(wei)0。常取2mA作為(wei)(wei)斷(duan)(duan)線(xian)(xian)報(bao)警值。電(dian)(dian)(dian)流(liu)(liu)型變(bian)送(song)(song)器(qi)(qi)(qi)將(jiang)物(wu)理量(liang)轉換成4~20mA電(dian)(dian)(dian)流(liu)(liu)輸(shu)(shu)出，必然(ran)要(yao)有(you)外電(dian)(dian)(dian)源(yuan)為(wei)(wei)其供(gong)電(dian)(dian)(dian)。最典型的(de)(de)(de)(de)(de)是變(bian)送(song)(song)器(qi)(qi)(qi)需(xu)要(yao)兩(liang)(liang)根(gen)(gen)(gen)電(dian)(dian)(dian)源(yuan)線(xian)(xian)，加上(shang)兩(liang)(liang)根(gen)(gen)(gen)電(dian)(dian)(dian)流(liu)(liu)輸(shu)(shu)出線(xian)(xian)，總(zong)共要(yao)接4根(gen)(gen)(gen)線(xian)(xian)，稱之(zhi)為(wei)(wei)四(si)線(xian)(xian)制(zhi)變(bian)送(song)(song)器(qi)(qi)(qi)。當(dang)然(ran)，電(dian)(dian)(dian)流(liu)(liu)輸(shu)(shu)出可以(yi)與電(dian)(dian)(dian)源(yuan)公用(yong)一(yi)(yi)根(gen)(gen)(gen)線(xian)(xian)（公用(yong)VCC或者GND），可節省一(yi)(yi)根(gen)(gen)(gen)線(xian)(xian)，所以(yi)現(xian)在基本上(shang)將(jiang)四(si)線(xian)(xian)制(zhi)變(bian)送(song)(song)器(qi)(qi)(qi)稱之(zhi)為(wei)(wei)三(san)線(xian)(xian)制(zhi)變(bian)送(song)(song)器(qi)(qi)(qi)。其實大(da)家(jia)可能(neng)(neng)注意到(dao)， 4-20mA電(dian)(dian)(dian)流(liu)(liu)本身就(jiu)可以(yi)為(wei)(wei)變(bian)送(song)(song)器(qi)(qi)(qi)供(gong)電(dian)(dian)(dian)，變(bian)送(song)(song)器(qi)(qi)(qi)在電(dian)(dian)(dian)路中(zhong)相當(dang)于一(yi)(yi)個特殊的(de)(de)(de)(de)(de)負(fu)載，這種變(bian)送(song)(song)器(qi)(qi)(qi)只需(xu)外接2根(gen)(gen)(gen)線(xian)(xian)，因而被稱為(wei)(wei)兩(liang)(liang)線(xian)(xian)制(zhi)變(bian)送(song)(song)器(qi)(qi)(qi)。工業(ye)電(dian)(dian)(dian)流(liu)(liu)環標準下(xia)限(xian)為(wei)(wei)4mA，因此(ci)在量(liang)程范圍內，變(bian)送(song)(song)器(qi)(qi)(qi)通(tong)常只有(you)24V，4mA供(gong)電(dian)(dian)(dian)（因此(ci)，在輕(qing)負(fu)載條件(jian)下(xia)高效率的(de)(de)(de)(de)(de)DC/DC電(dian)(dian)(dian)源(yuan)（TPS54331,TPS54160），低功耗(hao)的(de)(de)(de)(de)(de)傳感器(qi)(qi)(qi)和信號(hao)鏈產品、以(yi)及低功耗(hao)的(de)(de)(de)(de)(de)處理器(qi)(qi)(qi)（如MSP430）對(dui)于兩(liang)(liang)線(xian)(xian)制(zhi)的(de)(de)(de)(de)(de)4-20mA收發非常重要(yao)）。這使(shi)得兩(liang)(liang)線(xian)(xian)制(zhi)傳感器(qi)(qi)(qi)的(de)(de)(de)(de)(de)設計成為(wei)(wei)可能(neng)(neng)而又富(fu)有(you)挑戰。

    一般的必(bi)須 來設(she)計有一個VI轉換器，輸(shu)入0-3.3v，輸(shu)出4mA-20mA，可采用運放LM358，供(gong)電+12v。

  咱們系(xi)統(tong)地來瞧看模擬仿(fang)真量生產設(she)備為甚(shen)呢都寵愛用4~20mA傳輸信號~

     4-20mA. DC(1-5V.DC)訊號制是全國安裝電工理事會會( IEC )具體步驟控制體統用于的虛擬訊號文件文件文件傳輸原則的。我們國家也用于這種全國原則的訊號制，儀表板文件文件文件傳輸訊號用于4-20mA.DC，發收訊號用于1-5V.DC，即用于瞬時電流文件文件文件傳輸、額定電壓發收的訊號體統。

尋常(chang)機器設備電(dian)(dian)子儀(yi)表的(de)電(dian)(dian)磁(ci)波電(dian)(dian)流(liu)(liu)值都為4-20mA，指最(zui)小電(dian)(dian)流(liu)(liu)為4mA,最(zui)大(da)電(dian)(dian)流(liu)(liu)為20mA 。傳輸(shu)信號(hao)時候，因為導線上也有電(dian)(dian)阻，如果用電(dian)(dian)壓(ya)傳輸(shu)則(ze)會(hui)在導線內產(chan)生一定(ding)的(de)壓(ya)降，那(nei)接(jie)收端的(de)信號(hao)就會(hui)產(chan)生一定(ding)的(de)誤差了(le)，所以一般(ban)使用電(dian)(dian)流(liu)(liu)信號(hao)作為變送(song)器的(de)標準傳輸(shu)。

些什么才是4~20mA.DC（1~5V.DC）信號制？

        4~20mA.DC（1~5V.DC）無(wu)線(xian)(xian)(xian)衛(wei)星(xing)衛(wei)星(xing)訊(xun)(xun)(xun)號(hao)(hao)制(zhi)(zhi)是國(guo)外電(dian)焊工編委會會（IEC）：流(liu)程管理機系統(tong)用(yong)模擬(ni)系統(tong)無(wu)線(xian)(xian)(xian)衛(wei)星(xing)衛(wei)星(xing)訊(xun)(xun)(xun)號(hao)(hao)基準(zhun)。我國(guo)從DDZ-Ⅲ型電(dian)動伸縮儀表(biao)盤板進行用(yong)到(dao)(dao)一項國(guo)外基準(zhun)無(wu)線(xian)(xian)(xian)衛(wei)星(xing)衛(wei)星(xing)訊(xun)(xun)(xun)號(hao)(hao)制(zhi)(zhi)，儀表(biao)盤板網絡輸送無(wu)線(xian)(xian)(xian)衛(wei)星(xing)衛(wei)星(xing)訊(xun)(xun)(xun)號(hao)(hao)用(yong)到(dao)(dao)4~20mA.DC，頻繁聯系無(wu)線(xian)(xian)(xian)衛(wei)星(xing)衛(wei)星(xing)訊(xun)(xun)(xun)號(hao)(hao)用(yong)到(dao)(dao)1~5V.DC，即用(yong)到(dao)(dao)電(dian)流(liu)量網絡輸送、相電(dian)壓吸收的無(wu)線(xian)(xian)(xian)衛(wei)星(xing)衛(wei)星(xing)訊(xun)(xun)(xun)號(hao)(hao)機系統(tong)。

         4~20mA感應電流環運行原里：

        在工(gong)業(ye)企業(ye)工(gong)地，用1個儀表(biao)盤(pan)盤(pan)調(diao)小器來完工(gong)數(shu)據(ju)數(shu)據(ju)信(xin)息(xi)的(de)調(diao)里齊頭并進行(xing)長期發(fa)送，會制造以上原因：第(di)(di)一名，因為發(fa)送的(de)數(shu)據(ju)數(shu)據(ju)信(xin)息(xi)是電阻(zu)(zu)(zu)值(zhi)降數(shu)據(ju)數(shu)據(ju)信(xin)息(xi)，發(fa)送線會會受到噪(zao)音的(de)打擾(rao)；第(di)(di)五，發(fa)送線的(de)分布(bu)圖制作(zuo)電阻(zu)(zu)(zu)功(gong)率會制造電阻(zu)(zu)(zu)值(zhi)降降；再次，在工(gong)地怎么才能(neng)具(ju)備儀表(biao)盤(pan)盤(pan)調(diao)小器的(de)操作(zuo)電阻(zu)(zu)(zu)值(zhi)降也是個原因。

    只為徹(che)底(di)解決所述毛病和(he)躲開有關系背景(jing)噪音污染的導(dao)致，我門電費流(liu)來傳(chuan)導(dao)訊(xun)號(hao)，因此瞬時電流(liu)對背景(jing)噪音污染并不皮膚敏感。4～20mA的電流(liu)環便是用4mA表示(shi)零信(xin)號(hao)，用20mA表示(shi)信(xin)號(hao)的滿(man)刻度，而低于(yu)4mA高于(yu)20mA的信(xin)號(hao)用于(yu)各種故障的報警。

     二、4~20mA.DC（1~5V.DC）信號(hao)制(zhi)的優點？

     直播儀器(qi)多(duo)功(gong)能儀表(biao)(biao)可改變(bian)2線(xian)(xian)制，其實2線(xian)(xian)制即電(dian)、負荷并聯(lian)在來，下(xia)有(you)公(gong)共服務(wu)點(dian)，而直播壓力變(bian)送器(qi)與保持室(shi)儀器(qi)多(duo)功(gong)能儀表(biao)(biao)之后的數字信息交(jiao)流信息及變(bian)電(dian)僅(jin)用多(duo)根家裝電(dian)線(xian)(xian)。是由(you)于數字信息最低為瞬時(shi)電(dian)流為4mA.DC，為變(bian)送器(qi)提供了靜態(tai)工作電(dian)流，同時(shi)儀表(biao)(biao)電(dian)氣零(ling)點(dian)為4mA.DC，不與機械零(ling)點(dian)重(zhong)合，這種(zhong)“活零(ling)點(dian)”有(you)利(li)于識別斷(duan)電(dian)和(he)斷(duan)線(xian)(xian)等故障。而且(qie)兩(liang)線(xian)(xian)制還便于使用安(an)全柵，利(li)于安(an)全防(fang)爆。

調整室電(dian)(dian)(dian)子(zi)(zi)儀(yi)(yi)表(biao)板(ban)(ban)盤板(ban)(ban)主要包括(kuo)電(dian)(dian)(dian)流電(dian)(dian)(dian)壓(ya)串聯衛星信(xin)號高(gao)速傳輸，不同個調整體統歸(gui)屬的(de)電(dian)(dian)(dian)子(zi)(zi)儀(yi)(yi)表(biao)板(ban)(ban)盤板(ban)(ban)范圍內有(you)公共服務端，為了便(bian)捷檢查電(dian)(dian)(dian)子(zi)(zi)儀(yi)(yi)表(biao)板(ban)(ban)盤板(ban)(ban)、調試電(dian)(dian)(dian)子(zi)(zi)儀(yi)(yi)表(biao)板(ban)(ban)盤板(ban)(ban)、計算出(chu)機、報警提(ti)升(sheng)裝置提(ti)升(sheng)裝置通用，并便(bian)捷接法。

  活動現場儀器設(she)(she)備與管(guan)控(kong)室儀器設(she)(she)備之間的(de)(de)(de)溝通網絡信(xin)號(hao)選擇4~20mA.DC的(de)(de)(de)理由是：因為現場與控(kong)制(zhi)室之間的(de)(de)(de)距離較遠，連接電(dian)線的(de)(de)(de)電(dian)阻較大，如果用電(dian)壓信(xin)號(hao)遠傳(chuan)，優于電(dian)線電(dian)阻與接收儀表輸入電(dian)阻的(de)(de)(de)分(fen)壓，將產生(sheng)較大的(de)(de)(de)誤差，而(er)(er)用恒(heng)流(liu)源信(xin)號(hao)作為遠傳(chuan)，只要(yao)傳(chuan)送回路不(bu)出現分(fen)支，回路中的(de)(de)(de)電(dian)流(liu)就不(bu)會隨電(dian)線長短而(er)(er)改變，從而(er)(er)保(bao)證(zheng)了傳(chuan)送的(de)(de)(de)精(jing)度(du)。

 掌(zhang)控室儀容儀表內(nei)的(de)(de)接洽網絡信(xin)(xin)號選用(yong)1~5V.DC理由是：為了便(bian)于多(duo)臺儀表共同(tong)接收(shou)同(tong)一個信(xin)(xin)號，并有利(li)于接線和構成各(ge)種復雜的(de)(de)控制系統。如果(guo)用(yong)電(dian)流源作聯絡信(xin)(xin)號，當多(duo)臺儀表共同(tong)接收(shou)同(tong)一個信(xin)(xin)號時(shi)，它們的(de)(de)輸入電(dian)阻必須串(chuan)聯起來，這會(hui)使(shi)最(zui)大負載電(dian)阻超過變送儀表的(de)(de)負載能力(li)，而(er)且各(ge)接收(shou)儀表的(de)(de)信(xin)(xin)號負端電(dian)位(wei)各(ge)不(bu)(bu)相同(tong)，會(hui)引入干擾，而(er)且不(bu)(bu)能做到(dao)單一集中供(gong)電(dian)。

采取功(gong)(gong)率(lv)量(liang)(liang)(liang)源信(xin)息聯席會，與場地智能(neng)儀表的(de)聯席會用的(de)功(gong)(gong)率(lv)量(liang)(liang)(liang)信(xin)息可(ke)以換為(wei)為(wei)功(gong)(gong)率(lv)量(liang)(liang)(liang)信(xin)息，最容易的(de)方案是：在功(gong)(gong)率(lv)量(liang)(liang)(liang)拷貝(bei)電(dian)路中(zhong)關聯一(yi)250Ω的(de)標準電(dian)阻，把4~20mA.DC轉(zhuan)換為(wei)1~5V.DC，通常(chang)由配電(dian)器來完成這一(yi)任務。

     三、為什么會這樣樣的液位變送器(qi)選(xuan)取4~20mA.DC作傳送信號(hao)？

       1、首選都在直播 應用的安全保障確定

       應(ying)急重中之重是(shi)以防火應(ying)急電(dian)火花(hua)型(xing)電(dian)子儀器儀器來(lai)要(yao)考慮的(de)(de)，并且(qie)以控制電(dian)子儀器儀器能量轉(zhuan)換(huan)為首先，把確保電(dian)子儀器儀器常見工作任務的(de)(de)冗余和各式各樣(yang)功率降(jiang)到低程度。輸出(chu)電(dian)壓(ya)4~20mA.DC標(biao)準信號的(de)(de)變(bian)送(song)器，其電(dian)源電(dian)壓(ya)通常采(cai)用(yong)24V.DC，采(cai)用(yong)直流電(dian)壓(ya)的(de)(de)主(zhu)要(yao)原因是(shi)可以不(bu)用(yong)大容量的(de)(de)電(dian)容器及電(dian)感(gan)(gan)器，就只需考慮變(bian)送(song)器與控制室儀表連接導線的(de)(de)分布電(dian)容及電(dian)感(gan)(gan)，如2mm2 的(de)(de)導線其分布電(dian)容為0.05μ/km左右；對于單線的(de)(de)電(dian)感(gan)(gan)為0.4mH/km左右；大大低于引爆(bao)氫氣的(de)(de)數值(zhi)，顯然(ran)這對防爆(bao)是(shi)非常有利的(de)(de)。

     2、傳送數據的信號使用電流源遠遠高于電流源

畢竟工地與(yu)設定室(shi)范圍內的(de)(de)空距(ju)太遠，連結家裝線(xian)纜(lan)(lan)電(dian)(dian)路的(de)(de)電(dian)(dian)容(rong)很大時，一(yi)旦配電(dian)(dian)壓源(yuan)網(wang)絡網(wang)絡信(xin)(xin)號遠傳(chuan)，可(ke)能家裝線(xian)纜(lan)(lan)電(dian)(dian)路電(dian)(dian)容(rong)與(yu)接(jie)受到(dao)義表導入電(dian)(dian)容(rong)的(de)(de)分(fen)壓，將生產很大的(de)(de)測(ce)量誤差，一(yi)旦配感(gan)應電(dian)(dian)流(liu)源(yuan)網(wang)絡網(wang)絡信(xin)(xin)號是(shi)遠傳(chuan)，一(yi)旦傳(chuan)雙(shuang)雙(shuang)回路不冒出支(zhi)系，雙(shuang)雙(shuang)回路中的(de)(de)感(gan)應電(dian)(dian)流(liu)就會隨(sui)家裝線(xian)纜(lan)(lan)電(dian)(dian)路多(duo)少(shao)而變換，為(wei)了(le)要確(que)保(bao)了(le)傳(chuan)的(de)(de)gps精度。

      3、信號燈大電壓電流挑選20mA的因為

 ; 上限工作電(dian)流(liu)20mA的(de)選擇是基(ji)于安(an)全、實用(yong)、功耗、成(cheng)本的(de)考慮(lv)。安(an)全火花(hua)儀表只能采用(yong)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(an)全的(de)，對(dui)于24V.DC氫氣的(de)引爆電(dian)流(liu)為200mA，遠在(zai)20mA以上，此外還要綜合考慮(lv)生(sheng)產現場儀表之間的(de)連接距離，所帶(dai)負載等因(yin)素(su)；還有功耗及成(cheng)本問題，對(dui)電(dian)子元件(jian)的(de)要求，供電(dian)功率的(de)要求等因(yin)素(su)。

    4、網絡信號起步直流電壓決定4mA的原因分析

  打出(chu)為(wei)4~20mA的(de)(de)變(bian)送(song)器(qi)(qi)以兩(liang)線(xian)制的(de)(de)居多，兩(liang)線(xian)制即電(dian)源、負載串聯在一(yi)起(qi)，有一(yi)公共點(dian)，而現場(chang)變(bian)送(song)器(qi)(qi)與控(kong)制室(shi)儀(yi)表之間(jian)的(de)(de)信(xin)號(hao)聯絡及(ji)供(gong)電(dian)僅用(yong)兩(liang)根(gen)電(dian)線(xian)。為(wei)什(shen)么起(qi)點(dian)信(xin)號(hao)不是(shi)0mA？這是(shi)基(ji)于兩(liang)點(dian)：一(yi)是(shi)變(bian)送(song)器(qi)(qi)電(dian)路沒有靜態工作(zuo)電(dian)流將無法工作(zuo)，信(xin)號(hao)起(qi)點(dian)電(dian)流4mA.DC，不與機械零點(dian)重(zhong)合，這種“活零點(dian)”有利于識(shi)別斷電(dian)和斷線(xian)等故障。

     四、4~20mA傳(chuan)感(gan)器的由(you)來(lai)？

    按照交流(liu)電(dian)(dian)(dian)(dian)網絡信(xin)號(hao)的問題都是不(bu)非(fei)常容易受(shou)干擾(rao)信(xin)號(hao)、同(tong)時(shi)交流(liu)電(dian)(dian)(dian)(dian)源電(dian)(dian)(dian)(dian)阻(zu)值(zhi)無窮大，輸(shu)(shu)電(dian)(dian)(dian)(dian)線電(dian)(dian)(dian)(dian)阻(zu)值(zhi)并聯(lian)電(dian)(dian)(dian)(dian)路圖(tu)在漏電(dian)(dian)(dian)(dian)開關中(zhong)不(bu)會(hui)影響gps精度，在尋常雙絕(jue)緣線上會(hui)高(gao)速傳輸(shu)(shu)千余米。 

  通過電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)數(shu)據信(xin)息(xi)的(de)(de)(de)緣由是(shi)不(bu)能(neng)有可能(neng)受(shou)侵擾(rao)，畢竟工業廠房(fang)的(de)(de)(de)躁(zao)聲(sheng)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)的(de)(de)(de)力度有可能(neng)滿足(zu)數(shu)V，但是(shi)噪聲(sheng)的(de)(de)(de)功(gong)率很弱(ruo)(ruo)，所以噪聲(sheng)電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)通常(chang)小(xiao)于nA級(ji)(ji)別，因此給4－20mA傳輸(shu)帶來的(de)(de)(de)誤差非(fei)常(chang)小(xiao)；電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)源內(nei)(nei)阻趨于無窮大(da)，導線電(dian)(dian)(dian)(dian)阻串聯在回路中不(bu)影響精度，因此在普通雙絞線上可以傳輸(shu)數(shu)百米(mi)；由于電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)源的(de)(de)(de)大(da)內(nei)(nei)阻和恒流(liu)(liu)(liu)輸(shu)出，在接(jie)收端我們只需放(fang)置一(yi)個250歐姆到(dao)地的(de)(de)(de)電(dian)(dian)(dian)(dian)阻就可以獲得0－5V的(de)(de)(de)電(dian)(dian)(dian)(dian)壓(ya)，低輸(shu)入阻抗的(de)(de)(de)接(jie)收器(qi)的(de)(de)(de)好處是(shi)nA級(ji)(ji)的(de)(de)(de)輸(shu)入電(dian)(dian)(dian)(dian)流(liu)(liu)(liu)噪聲(sheng)只產生非(fei)常(chang)微(wei)弱(ruo)(ruo)的(de)(de)(de)電(dian)(dian)(dian)(dian)壓(ya)噪聲(sheng)。

 最大值(zhi)取(qu)20mA是因(yin)(yin)(yin)為(wei)防爆的(de)要(yao)求(qiu)：20mA的(de)電(dian)(dian)(dian)(dian)(dian)流(liu)通斷引起的(de)火花能量(liang)不足以(yi)引燃(ran)瓦斯。下限沒(mei)有取(qu)0mA的(de)原因(yin)(yin)(yin)是為(wei)了能檢(jian)測斷線(xian)(xian)：正常工(gong)作(zuo)時(shi)不會低于4mA，當傳輸(shu)線(xian)(xian)因(yin)(yin)(yin)故障斷路(lu)，環路(lu)電(dian)(dian)(dian)(dian)(dian)流(liu)降為(wei)0。常取(qu)2mA作(zuo)為(wei)斷線(xian)(xian)報警值(zhi)。電(dian)(dian)(dian)(dian)(dian)流(liu)型變送(song)(song)器(qi)(qi)(qi)(qi)將物理量(liang)轉(zhuan)換成4~20mA電(dian)(dian)(dian)(dian)(dian)流(liu)輸(shu)出(chu)(chu)，必然(ran)要(yao)有外電(dian)(dian)(dian)(dian)(dian)源為(wei)其供電(dian)(dian)(dian)(dian)(dian)。最典型的(de)是變送(song)(song)器(qi)(qi)(qi)(qi)需(xu)要(yao)兩(liang)根(gen)(gen)電(dian)(dian)(dian)(dian)(dian)源線(xian)(xian)，加上(shang)兩(liang)根(gen)(gen)電(dian)(dian)(dian)(dian)(dian)流(liu)輸(shu)出(chu)(chu)線(xian)(xian)，總(zong)共(gong)要(yao)接4根(gen)(gen)線(xian)(xian)，稱之(zhi)(zhi)(zhi)為(wei)四線(xian)(xian)制變送(song)(song)器(qi)(qi)(qi)(qi)。當然(ran)，電(dian)(dian)(dian)(dian)(dian)流(liu)輸(shu)出(chu)(chu)可以(yi)與電(dian)(dian)(dian)(dian)(dian)源公用一根(gen)(gen)線(xian)(xian)公用VCC或者(zhe)GND，可節省一根(gen)(gen)線(xian)(xian)，稱之(zhi)(zhi)(zhi)為(wei)三線(xian)(xian)制變送(song)(song)器(qi)(qi)(qi)(qi)。其實大家可能注意到，4-20mA電(dian)(dian)(dian)(dian)(dian)流(liu)本(ben)身就可以(yi)為(wei)變送(song)(song)器(qi)(qi)(qi)(qi)供電(dian)(dian)(dian)(dian)(dian)。變送(song)(song)器(qi)(qi)(qi)(qi)在電(dian)(dian)(dian)(dian)(dian)路(lu)中相當于一個特殊的(de)負載，特殊之(zhi)(zhi)(zhi)處在于變送(song)(song)器(qi)(qi)(qi)(qi)的(de)耗電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)在4~20mA之(zhi)(zhi)(zhi)間根(gen)(gen)據傳感(gan)器(qi)(qi)(qi)(qi)輸(shu)出(chu)(chu)而變化。顯示儀表(biao)只需(xu)要(yao)串在電(dian)(dian)(dian)(dian)(dian)路(lu)中即可。這種(zhong)變送(song)(song)器(qi)(qi)(qi)(qi)只需(xu)外接2根(gen)(gen)線(xian)(xian)，因(yin)(yin)(yin)而被稱為(wei)兩(liang)線(xian)(xian)制變送(song)(song)器(qi)(qi)(qi)(qi)。工(gong)業電(dian)(dian)(dian)(dian)(dian)流(liu)環標準下限為(wei)4mA，因(yin)(yin)(yin)此(ci)只要(yao)在量(liang)程范圍內，變送(song)(song)器(qi)(qi)(qi)(qi)至少有4mA供電(dian)(dian)(dian)(dian)(dian)。

 所以、4-20mA的信號輸(shu)出一般不容易受干(gan)擾(rao)而且安全(quan)可靠、所以工業上普(pu)遍使用的都是二(er)線制(zhi)4-20mA的電源(yuan)輸(shu)出信號。但為了能更好的處理傳(chuan)感器的信號、目前還(huan)有更多(duo)其它形式的輸(shu)出信號：3.33MV/V；2MV/V；0-5V; 0-10V等。

另附一種4到(dao)20mA轉電壓信號(hao)的簡單電路圖：

      這張圖(tu)操作(zuo)同一(yi)個(ge)250歐(ou)姆(mu)的電阻將4到(dao)20mA的電流(liu)信(xin)號轉換成1到(dao)5V的電壓信(xin)號，然后使用一(yi)個(ge)RC濾波(bo)加(jia)一(yi)個(ge)二極(ji)管（原諒我(wo)模擬電路不好(hao)，并不知(zhi)道是(shi)什么(me)意(yi)思）接到(dao)單(dan)片機的AD轉換引腳。