一、電阻是什么
電(dian)阻器是限制電(dian)流(liu)的(de)元件,通常簡(jian)稱(cheng)為電(dian)阻,是一種最常用(yong)的(de)電(dian)子(zi)元件,普遍(bian)應用(yong)在各類電(dian)子(zi)電(dian)路中。不但金屬導體(ti)(ti)有電(dian)阻,其他物(wu)體(ti)(ti)也有電(dian)阻。
簡單來說,電(dian)(dian)(dian)阻(zu)(zu)就(jiu)是(shi)指電(dian)(dian)(dian)流(liu)(liu)在電(dian)(dian)(dian)路中所(suo)遇到的阻(zu)(zu)力,或者說是(shi)指物(wu)體對電(dian)(dian)(dian)流(liu)(liu)的阻(zu)(zu)礙(ai)才能。電(dian)(dian)(dian)阻(zu)(zu)越(yue)(yue)(yue)(yue)大,電(dian)(dian)(dian)流(liu)(liu)所(suo)遭(zao)到的阻(zu)(zu)力就(jiu)越(yue)(yue)(yue)(yue)大,因而電(dian)(dian)(dian)流(liu)(liu)就(jiu)越(yue)(yue)(yue)(yue)小。反之,電(dian)(dian)(dian)阻(zu)(zu)越(yue)(yue)(yue)(yue)小,電(dian)(dian)(dian)流(liu)(liu)所(suo)遭(zao)到的阻(zu)(zu)力就(jiu)越(yue)(yue)(yue)(yue)小,因而電(dian)(dian)(dian)流(liu)(liu)就(jiu)越(yue)(yue)(yue)(yue)大。
電阻(zu)的(de)符號是(shi)“R”。電阻(zu)的(de)單位為歐(ou)(ou)姆(mu),簡(jian)稱(cheng)歐(ou)(ou),用字母“Ω”表(biao)示。常用的(de)電阻(zu)單位還(huan)有千(qian)歐(ou)(ou)姆(mu)(KΩ),兆歐(ou)(ou)姆(mu)(MΩ)。
二、電阻在電路中的作用
1、限流作用
為使通(tong)過用(yong)電(dian)(dian)(dian)器(qi)的(de)(de)(de)(de)電(dian)(dian)(dian)流(liu)不超過額定(ding)值(zhi)(zhi)或實際工(gong)作(zuo)需要(yao)的(de)(de)(de)(de)規定(ding)值(zhi)(zhi),以(yi)保證(zheng)用(yong)電(dian)(dian)(dian)器(qi)的(de)(de)(de)(de)正常(chang)工(gong)作(zuo),通(tong)常(chang)可(ke)在電(dian)(dian)(dian)路中串聯一(yi)個(ge)可(ke)變電(dian)(dian)(dian)阻(zu)(zu)。當改變這個(ge)電(dian)(dian)(dian)阻(zu)(zu)的(de)(de)(de)(de)大(da)小時(shi),電(dian)(dian)(dian)流(liu)的(de)(de)(de)(de)大(da)小也隨(sui)之(zhi)改變。我們把這種可(ke)以(yi)限制電(dian)(dian)(dian)流(liu)大(da)小的(de)(de)(de)(de)電(dian)(dian)(dian)阻(zu)(zu)叫(jiao)做限流(liu)電(dian)(dian)(dian)阻(zu)(zu)。
2、分流作用
當在電路的干路上需同時接入幾個額定電流不同的用電器時,可以在額定電流較小的用電器兩端并聯接入一個品牌電阻,這個(ge)電阻的作用是“分(fen)流”。
3、分壓作用
一般用(yong)(yong)(yong)電(dian)(dian)(dian)器(qi)(qi)上都(dou)標有(you)額(e)定(ding)電(dian)(dian)(dian)壓(ya)值(zhi),若電(dian)(dian)(dian)源比用(yong)(yong)(yong)電(dian)(dian)(dian)器(qi)(qi)的(de)額(e)定(ding)電(dian)(dian)(dian)壓(ya)高,則(ze)不可把用(yong)(yong)(yong)電(dian)(dian)(dian)器(qi)(qi)直接接在(zai)電(dian)(dian)(dian)源上。在(zai)這(zhe)種情況(kuang)下,可給用(yong)(yong)(yong)電(dian)(dian)(dian)器(qi)(qi)串接一個合適阻(zu)值(zhi)的(de)電(dian)(dian)(dian)阻(zu),讓(rang)它分擔一部分電(dian)(dian)(dian)壓(ya),用(yong)(yong)(yong)電(dian)(dian)(dian)器(qi)(qi)便能在(zai)額(e)定(ding)電(dian)(dian)(dian)壓(ya)下工作。我(wo)們稱這(zhe)樣的(de)電(dian)(dian)(dian)阻(zu)為分壓(ya)電(dian)(dian)(dian)阻(zu)。
三、影響電阻的因素
1、長度(du):當(dang)材料和橫(heng)截(jie)面(mian)積相同時,導體的長度(du)越長,電阻越大。
2、橫截面(mian)積:當(dang)材(cai)料和(he)長度相同時(shi),導體的橫截面(mian)積越小,電阻越大。
3、材料:當長度和橫(heng)截(jie)面積(ji)相同時(shi),不同材料的(de)導體電阻(zu)不同。
4、溫度(du)(du):對大多(duo)數(shu)導體來說,溫度(du)(du)越(yue)高,電阻越(yue)大,如金屬等;對少(shao)數(shu)導體來說,溫度(du)(du)越(yue)高,電阻越(yue)小,如碳。
四、電阻壞了有什么現象
電(dian)(dian)(dian)阻是(shi)電(dian)(dian)(dian)器設備中數量(liang)最(zui)(zui)多的元件(jian),但(dan)不是(shi)損壞率最(zui)(zui)高的元件(jian)。電(dian)(dian)(dian)阻損壞以開路(lu)最(zui)(zui)常(chang)(chang)見(jian)(jian)(jian),阻值(zhi)變大較少見(jian)(jian)(jian),阻值(zhi)變小十分少見(jian)(jian)(jian)。常(chang)(chang)見(jian)(jian)(jian)的有(you)碳膜(mo)電(dian)(dian)(dian)阻、金(jin)屬(shu)膜(mo)電(dian)(dian)(dian)阻、線繞電(dian)(dian)(dian)阻和合金(jin)電(dian)(dian)(dian)阻等幾種,在這(zhe)些電(dian)(dian)(dian)阻中如何(he)判(pan)斷它們在電(dian)(dian)(dian)路(lu)中是(shi)否損壞,它們損壞后又(you)有(you)哪些特征呢(ni)?
1、線繞電阻損壞的特征
線(xian)繞電(dian)阻一(yi)般用作(zuo)大電(dian)流(liu)限流(liu),阻值不大。
圓柱形線繞(rao)電阻燒(shao)壞(huai)時有(you)的會發(fa)黑(hei)或表面(mian)爆(bao)皮(pi)、裂紋,有(you)的沒有(you)痕跡。
水泥電阻(zu)是線繞電阻(zu)的一(yi)種,燒(shao)壞時(shi)可能會斷裂(lie),否則也(ye)沒有可見痕跡。
保險電阻燒(shao)壞時有的表面會炸掉一塊(kuai)皮,有的也沒有什么痕跡,但絕不(bu)會燒(shao)焦發黑。
根據以上(shang)特(te)點,在檢查電(dian)(dian)阻(zu)時可有所(suo)側重,快速找出損壞的電(dian)(dian)阻(zu)。
2、金屬膜電阻和碳膜電阻損壞特征
金屬膜電(dian)(dian)阻(zu)(zu)和碳(tan)膜電(dian)(dian)阻(zu)(zu)應用(yong)是最(zui)廣,其損(sun)壞(huai)的(de)特點:一是低阻(zu)(zu)值(zhi)(100Ω以(yi)(yi)下)和高(gao)阻(zu)(zu)值(zhi)(100kΩ以(yi)(yi)上)的(de)損(sun)壞(huai)率較(jiao)高(gao),中間(jian)阻(zu)(zu)值(zhi)(如幾百歐到幾十千歐)的(de)極少損(sun)壞(huai);二是低阻(zu)(zu)值(zhi)電(dian)(dian)阻(zu)(zu)損(sun)壞(huai)時往往是燒焦發黑,很(hen)容易發現(xian),而高(gao)阻(zu)(zu)值(zhi)電(dian)(dian)阻(zu)(zu)損(sun)壞(huai)時很(hen)少有痕(hen)跡。
五、怎么測量電阻好壞
1、外觀檢查
對于固定電(dian)(dian)阻(zu)首先查看標志清晰,保護漆完好,無(wu)(wu)燒焦,無(wu)(wu)傷痕(hen),無(wu)(wu)裂(lie)痕(hen),無(wu)(wu)腐蝕,電(dian)(dian)阻(zu)體與(yu)引腳(jiao)緊(jin)密接觸等。對于電(dian)(dian)位器還(huan)應(ying)檢查轉軸(zhou)靈活(huo),松緊(jin)適(shi)當,手感(gan)舒適(shi)。有開關的(de)要檢查開關動作是否正常。
2、萬用表檢測
①固定電阻的檢測
用萬用表(biao)(biao)的(de)(de)電(dian)阻(zu)(zu)(zu)擋對電(dian)阻(zu)(zu)(zu)進行測量(liang),對于測量(liang)不同阻(zu)(zu)(zu)值(zhi)(zhi)的(de)(de)電(dian)阻(zu)(zu)(zu)選擇萬用表(biao)(biao)的(de)(de)不同倍(bei)乘擋。對于指(zhi)(zhi)針式(shi)萬用表(biao)(biao),由(you)于電(dian)阻(zu)(zu)(zu)擋的(de)(de)示(shi)(shi)數(shu)(shu)是非(fei)線性(xing)的(de)(de),阻(zu)(zu)(zu)值(zhi)(zhi)越大(da),示(shi)(shi)數(shu)(shu)越密,所以選擇合適的(de)(de)量(liang)程(cheng)(cheng),應使表(biao)(biao)針偏轉角大(da)些,指(zhi)(zhi)示(shi)(shi)于1/3~2/3滿量(liang)程(cheng)(cheng),讀數(shu)(shu)更為(wei)(wei)準確(que)。若測得阻(zu)(zu)(zu)值(zhi)(zhi)超過該(gai)(gai)電(dian)阻(zu)(zu)(zu)的(de)(de)誤(wu)差范圍、阻(zu)(zu)(zu)值(zhi)(zhi)無限大(da)、阻(zu)(zu)(zu)值(zhi)(zhi)為(wei)(wei)0或阻(zu)(zu)(zu)值(zhi)(zhi)不穩(wen),說明該(gai)(gai)電(dian)阻(zu)(zu)(zu)器(qi)已(yi)壞。
在測量(liang)中注(zhu)意拿電(dian)(dian)阻的手不要與電(dian)(dian)阻器的兩個引腳相接觸,這樣會使手所(suo)呈現的電(dian)(dian)阻與被(bei)測電(dian)(dian)阻并聯,影(ying)響測量(liang)準(zhun)確。另外,不能帶電(dian)(dian)情況下用萬用表電(dian)(dian)阻擋檢測電(dian)(dian)路中電(dian)(dian)阻器的阻值。在線檢測應首(shou)先(xian)斷(duan)電(dian)(dian),再將(jiang)電(dian)(dian)阻從(cong)電(dian)(dian)路中斷(duan)開出(chu)來,然后進(jin)行測量(liang)。
②保險絲電阻和敏感電阻的檢測
保險(xian)絲電(dian)阻一般阻值(zhi)只有幾到幾十歐,若測(ce)(ce)得阻值(zhi)為無限大,則已熔斷。也可在線檢測(ce)(ce)保險(xian)絲電(dian)阻的(de)好壞,分別測(ce)(ce)量(liang)其兩端對(dui)地電(dian)壓,若一端為電(dian)源電(dian)壓,一端電(dian)壓為0伏(fu),保險(xian)絲電(dian)阻已熔斷。
敏(min)感電(dian)(dian)阻(zu)種類較多,以熱(re)敏(min)電(dian)(dian)阻(zu)為例,又分正溫(wen)(wen)度(du)系(xi)數和負(fu)溫(wen)(wen)度(du)系(xi)數熱(re)敏(min)電(dian)(dian)阻(zu)。對于正溫(wen)(wen)度(du)系(xi)(PTC)熱(re)敏(min)電(dian)(dian)阻(zu),在常溫(wen)(wen)下一般阻(zu)值(zhi)不大(da),在測量中用燒熱(re)的電(dian)(dian)烙(luo)鐵靠近電(dian)(dian)阻(zu),這(zhe)時阻(zu)值(zhi)應明顯增大(da),說明該電(dian)(dian)阻(zu)正常,若無(wu)變化(hua)說明元件損壞,負(fu)溫(wen)(wen)度(du)系(xi)熱(re)敏(min)電(dian)(dian)阻(zu)則(ze)相反。
光敏電阻(zu)在(zai)無光照(zhao)(用手(shou)或(huo)物遮住光)的情況下萬用表測得阻(zu)值大,有(you)(you)光照(zhao)表針指示電阻(zu)值有(you)(you)明顯減小。若(ruo)無變(bian)化,則元件損(sun)壞。
③可變電阻和電位器的檢測
首先測量兩固定端(duan)之(zhi)間電阻(zu)(zu)值(zhi)(zhi)是否正常,若為(wei)無(wu)(wu)限大或(huo)(huo)為(wei)零(ling)歐,或(huo)(huo)與標稱相差(cha)較大,超過(guo)誤差(cha)允許范圍,都說(shuo)明已損(sun)壞(huai);電阻(zu)(zu)體阻(zu)(zu)值(zhi)(zhi)正常,再(zai)將萬用表一只表筆接電位(wei)器(qi)滑(hua)動端(duan),另一只表筆接電位(wei)器(qi)(可調電阻(zu)(zu))的任一固定端(duan),緩(huan)慢(man)旋(xuan)動軸柄,觀察表針是否平(ping)穩變化(hua),當(dang)從(cong)一端(duan)旋(xuan)向另一端(duan)時,阻(zu)(zu)值(zhi)(zhi)從(cong)零(ling)歐變化(hua)到標稱值(zhi)(zhi)(或(huo)(huo)相反),并且無(wu)(wu)跳變或(huo)(huo)抖動等現(xian)象,則說(shuo)明電位(wei)器(qi)正常,若在旋(xuan)轉(zhuan)的過(guo)程中有跳變或(huo)(huo)抖動現(xian)象,說(shuo)明滑(hua)動點現(xian)電阻(zu)(zu)體接觸不良(liang)。
3、用電橋測量電阻
如(ru)果要(yao)求精確測量(liang)電(dian)阻(zu)(zu)(zu)器(qi)的阻(zu)(zu)(zu)值,可通過電(dian)橋(qiao)(數字式(shi))進行測試(shi)。將電(dian)阻(zu)(zu)(zu)插入電(dian)橋(qiao)元件測量(liang)端,選擇合適的量(liang)程,即(ji)可從顯(xian)示器(qi)上(shang)讀出(chu)電(dian)阻(zu)(zu)(zu)器(qi)的阻(zu)(zu)(zu)值。例如(ru),用電(dian)阻(zu)(zu)(zu)絲自(zi)制電(dian)阻(zu)(zu)(zu)或對固(gu)定電(dian)阻(zu)(zu)(zu)器(qi)進行處理(li)來獲得(de)某(mou)一較為(wei)精確的電(dian)阻(zu)(zu)(zu)值時(shi),就必須用電(dian)橋(qiao)測量(liang)自(zi)制電(dian)阻(zu)(zu)(zu)的阻(zu)(zu)(zu)值。