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可調電源模塊的選擇

2019/5/24 9:11:52??????點擊:
        LM317作為輸出電壓(ya)(ya)可變的(de)集(ji)成三端穩壓(ya)(ya)塊,是一(yi)種使用方便、應用廣(guang)泛的(de)集(ji)成穩壓(ya)(ya)塊。317系列(lie)穩壓(ya)(ya)塊的(de)型號很多:例(li)如LM317HVH、W317L等。電子(zi)愛好者經常用317穩壓(ya)(ya)塊制作輸出電壓(ya)(ya)可變的(de)穩壓(ya)(ya)電源(yuan)。
穩(wen)壓電源的(de)(de)輸出電壓可(ke)用下式(shi)計(ji)(ji)算(suan),Vo=1.25(1+R2/R1)。僅僅從公式(shi)本身看,R1、R2的(de)(de)電阻(zu)值(zhi)可(ke)以隨意(yi)設(she)定。然而作為穩(wen)壓電源的(de)(de)輸出電壓計(ji)(ji)算(suan)公式(shi),R1和R2的(de)(de)阻(zu)值(zhi)是(shi)不能隨意(yi)設(she)定的(de)(de)。
首先317穩壓(ya)塊的輸(shu)(shu)出(chu)電壓(ya)變(bian)化范圍是Vo=1.25V—37V(高輸(shu)(shu)出(chu)電壓(ya)的317穩壓(ya)塊如LM317HVA、LM317HVK等,其輸(shu)(shu)出(chu)電壓(ya)變(bian)化范圍是Vo=1.25V—45V),所以R2/R1的比(bi)值范圍只能是0—28.6。

其(qi)次是317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)都有(you)(you)一(yi)個最(zui)(zui)小(xiao)穩(wen)(wen)(wen)定(ding)(ding)工(gong)作(zuo)(zuo)(zuo)電(dian)流(liu),有(you)(you)的(de)(de)資料(liao)稱為最(zui)(zui)小(xiao)輸出(chu)電(dian)流(liu),也有(you)(you)的(de)(de)資料(liao)稱為最(zui)(zui)小(xiao)泄放電(dian)流(liu)。最(zui)(zui)小(xiao)穩(wen)(wen)(wen)定(ding)(ding)工(gong)作(zuo)(zuo)(zuo)電(dian)流(liu)的(de)(de)值(zhi)一(yi)般為1.5mA。由于(yu)317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)的(de)(de)生產廠(chang)家(jia)不(bu)同(tong)(tong)、型號不(bu)同(tong)(tong),其(qi)最(zui)(zui)小(xiao)穩(wen)(wen)(wen)定(ding)(ding)工(gong)作(zuo)(zuo)(zuo)電(dian)流(liu)也不(bu)相(xiang)同(tong)(tong),但一(yi)般不(bu)大于(yu)5mA。當317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)的(de)(de)輸出(chu)電(dian)流(liu)小(xiao)于(yu)其(qi)最(zui)(zui)小(xiao)穩(wen)(wen)(wen)定(ding)(ding)工(gong)作(zuo)(zuo)(zuo)電(dian)流(liu)時,317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)就(jiu)不(bu)能正常工(gong)作(zuo)(zuo)(zuo)。當317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)的(de)(de)輸出(chu)電(dian)流(liu)大于(yu)其(qi)最(zui)(zui)小(xiao)穩(wen)(wen)(wen)定(ding)(ding)工(gong)作(zuo)(zuo)(zuo)電(dian)流(liu)時,317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)就(jiu)可以輸出(chu)穩(wen)(wen)(wen)定(ding)(ding)的(de)(de)直(zhi)流(liu)電(dian)壓(ya)(ya)(ya)(ya)。如果用317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)制作(zuo)(zuo)(zuo)穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)電(dian)源時,沒(mei)有(you)(you)注(zhu)意(yi)317穩(wen)(wen)(wen)壓(ya)(ya)(ya)(ya)塊(kuai)的(de)(de)最(zui)(zui)小(xiao)穩(wen)(wen)(wen)定(ding)(ding)工(gong)作(zuo)(zuo)(zuo)電(dian)流(liu),那么你制作(zuo)(zuo)(zuo)的(de)(de)可能會出(chu)現(xian)下述(shu)不正(zheng)常現(xian)象:穩壓電(dian)源(yuan)輸出(chu)的有載(zai)電(dian)壓和空載(zai)電(dian)壓差別較大。

        在應(ying)用(yong)中,為了電(dian)路的(de)穩定工作(zuo),在一般情況下,還需要接二極管作(zuo)為保護電(dian)路,防止電(dian)路中的(de)電(dian)容放電(dian)時的(de)高壓把317燒壞。
1.要解決317穩(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)最(zui)小(xiao)(xiao)穩(wen)定(ding)(ding)工作(zuo)電流(liu)(liu)的問題,可以通(tong)過(guo)設定(ding)(ding)R1和R2阻值的大(da)(da)小(xiao)(xiao),而(er)使317穩(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)空(kong)(kong)(kong)載時(shi)輸出的電流(liu)(liu)大(da)(da)于或(huo)等(deng)于其最(zui)小(xiao)(xiao)穩(wen)定(ding)(ding)工作(zuo)電流(liu)(liu),從而(er)保證317穩(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)在空(kong)(kong)(kong)載時(shi)能夠(gou)穩(wen)定(ding)(ding)地工作(zuo)。此時(shi),只要保證Vo/(R1+R2)≥1.5mA,就可以保證317穩(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)在空(kong)(kong)(kong)載時(shi)能夠(gou)穩(wen)定(ding)(ding)地工作(zuo)。上式中的1.5mA為317穩(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)的最(zui)小(xiao)(xiao)穩(wen)定(ding)(ding)工作(zuo)電流(liu)(liu)。當然,只要能保證317穩(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)在空(kong)(kong)(kong)載時(shi)能夠(gou)穩(wen)定(ding)(ding)地工作(zuo),Vo/(R1+R2)的值也可以設定(ding)(ding)為大(da)(da)于1.5mA的任意值。
2.經計算可知R1的(de)(de)最(zui)大取(qu)值為(wei)R1≈0.83KΩ。又因為(wei)R2/R1的(de)(de)最(zui)大值為(wei)28.6。所以R2的(de)(de)最(zui)大取(qu)值為(wei)R2≈23.74KΩ。在使用317穩壓(ya)塊的(de)(de)輸出電(dian)壓(ya)計算公式計算其輸出電(dian)壓(ya)時(shi),必須保證R1≥0.83KΩ,R2≤23.74KΩ兩個不等式同時(shi)成立,才能保證317穩壓(ya)塊在空載時(shi)能夠穩定(ding)地工(gong)作。

3.當然在317穩壓(ya)(ya)塊的(de)輸(shu)(shu)出(chu)端并聯泄流(liu)電阻R也可(ke)以為317穩壓(ya)(ya)塊提(ti)供(gong)最小穩定(ding)(ding)工作電流(liu)。但是(shi),由于并聯的(de)泄流(liu)電阻不(bu)能隨輸(shu)(shu)出(chu)電壓(ya)(ya)的(de)變化而變化,如果要保證317穩壓(ya)(ya)塊在輸(shu)(shu)出(chu)電壓(ya)(ya)為1.25V時(shi),其輸(shu)(shu)出(chu)電流(liu)大于其最小穩定(ding)(ding)工作電流(liu),則在317穩壓(ya)(ya)塊的(de)輸(shu)(shu)出(chu)電壓(ya)(ya)為37V時(shi),流(liu)過泄流(liu)電阻的(de)電流(liu)就太(tai)大了(le),這樣不(bu)僅浪費了(le)電能,而且(qie)增(zeng)加(jia)了(le)317穩壓(ya)(ya)塊的(de)負(fu)擔,不(bu)是(shi)一種(zhong)妥當的(de)辦法。

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