可調電源模塊的選擇
穩壓(ya)電(dian)源的輸出電(dian)壓(ya)可(ke)用下式計算(suan)，Vo＝1.25（1＋R2/R1）。僅(jin)僅(jin)從公式本身(shen)看，R1、R2的電(dian)阻值(zhi)可(ke)以隨意(yi)設定。然而作(zuo)為穩壓(ya)電(dian)源的輸出電(dian)壓(ya)計算(suan)公式，R1和R2的阻值(zhi)是(shi)不能隨意(yi)設定的。
首先317穩壓(ya)(ya)塊的輸(shu)出電(dian)壓(ya)(ya)變(bian)化(hua)(hua)范(fan)圍是Vo＝1.25V—37V（高輸(shu)出電(dian)壓(ya)(ya)的317穩壓(ya)(ya)塊如LM317HVA、LM317HVK等，其(qi)輸(shu)出電(dian)壓(ya)(ya)變(bian)化(hua)(hua)范(fan)圍是Vo＝1.25V—45V），所以R2/R1的比值范(fan)圍只能是0—28.6。
其(qi)次是317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)都有一(yi)(yi)個最(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)定工(gong)(gong)作(zuo)電(dian)(dian)(dian)流(liu)(liu)(liu)，有的(de)資料稱為(wei)最(zui)(zui)小(xiao)(xiao)(xiao)輸(shu)出電(dian)(dian)(dian)流(liu)(liu)(liu)，也(ye)有的(de)資料稱為(wei)最(zui)(zui)小(xiao)(xiao)(xiao)泄放電(dian)(dian)(dian)流(liu)(liu)(liu)。最(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)定工(gong)(gong)作(zuo)電(dian)(dian)(dian)流(liu)(liu)(liu)的(de)值(zhi)一(yi)(yi)般為(wei)1.5mA。由于317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)的(de)生產廠家不(bu)同(tong)(tong)、型(xing)號不(bu)同(tong)(tong)，其(qi)最(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)定工(gong)(gong)作(zuo)電(dian)(dian)(dian)流(liu)(liu)(liu)也(ye)不(bu)相同(tong)(tong)，但(dan)一(yi)(yi)般不(bu)大(da)于5mA。當317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)的(de)輸(shu)出電(dian)(dian)(dian)流(liu)(liu)(liu)小(xiao)(xiao)(xiao)于其(qi)最(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)定工(gong)(gong)作(zuo)電(dian)(dian)(dian)流(liu)(liu)(liu)時(shi)，317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)就(jiu)不(bu)能(neng)正常工(gong)(gong)作(zuo)。當317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)的(de)輸(shu)出電(dian)(dian)(dian)流(liu)(liu)(liu)大(da)于其(qi)最(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)定工(gong)(gong)作(zuo)電(dian)(dian)(dian)流(liu)(liu)(liu)時(shi)，317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)就(jiu)可以(yi)輸(shu)出穩(wen)(wen)(wen)(wen)(wen)定的(de)直流(liu)(liu)(liu)電(dian)(dian)(dian)壓(ya)(ya)(ya)。如果用317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)制作(zuo)穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)電(dian)(dian)(dian)源(yuan)時(shi),沒有注意317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)的(de)最(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)定工(gong)(gong)作(zuo)電(dian)(dian)(dian)流(liu)(liu)(liu)，那么你制作(zuo)的(de)可能(neng)會(hui)出現下述(shu)不正常現象：穩壓(ya)電(dian)(dian)源輸出的有載電(dian)(dian)壓(ya)和空載電(dian)(dian)壓(ya)差別較大。
在(zai)應用(yong)中，為了電路(lu)的(de)穩定工作，在(zai)一般(ban)情況下，還需(xu)要(yao)接(jie)二極(ji)管作為保護電路(lu)，防(fang)止電路(lu)中的(de)電容放電時的(de)高壓把317燒壞。1.要解決317穩(wen)(wen)(wen)壓塊(kuai)(kuai)最(zui)(zui)小(xiao)穩(wen)(wen)(wen)定(ding)工(gong)(gong)作(zuo)電(dian)流的問(wen)題(ti)，可以(yi)通(tong)過設(she)定(ding)R1和R2阻(zu)值的大(da)小(xiao)，而(er)使317穩(wen)(wen)(wen)壓塊(kuai)(kuai)空(kong)載時(shi)(shi)(shi)輸出的電(dian)流大(da)于或等(deng)于其最(zui)(zui)小(xiao)穩(wen)(wen)(wen)定(ding)工(gong)(gong)作(zuo)電(dian)流，從而(er)保(bao)(bao)證(zheng)317穩(wen)(wen)(wen)壓塊(kuai)(kuai)在(zai)空(kong)載時(shi)(shi)(shi)能夠(gou)穩(wen)(wen)(wen)定(ding)地工(gong)(gong)作(zuo)。此(ci)時(shi)(shi)(shi)，只要保(bao)(bao)證(zheng)Vo/（R1＋R2）≥1.5mA，就可以(yi)保(bao)(bao)證(zheng)317穩(wen)(wen)(wen)壓塊(kuai)(kuai)在(zai)空(kong)載時(shi)(shi)(shi)能夠(gou)穩(wen)(wen)(wen)定(ding)地工(gong)(gong)作(zuo)。上式中的1.5mA為317穩(wen)(wen)(wen)壓塊(kuai)(kuai)的最(zui)(zui)小(xiao)穩(wen)(wen)(wen)定(ding)工(gong)(gong)作(zuo)電(dian)流。當然，只要能保(bao)(bao)證(zheng)317穩(wen)(wen)(wen)壓塊(kuai)(kuai)在(zai)空(kong)載時(shi)(shi)(shi)能夠(gou)穩(wen)(wen)(wen)定(ding)地工(gong)(gong)作(zuo)，Vo/（R1＋R2）的值也可以(yi)設(she)定(ding)為大(da)于1.5mA的任意值。
2.經計算可知R1的(de)(de)(de)最(zui)大取值為(wei)R1≈0.83KΩ。又因為(wei)R2/R1的(de)(de)(de)最(zui)大值為(wei)28.6。所以R2的(de)(de)(de)最(zui)大取值為(wei)R2≈23.74KΩ。在使(shi)用317穩(wen)壓塊(kuai)的(de)(de)(de)輸出電(dian)壓計算公(gong)式計算其輸出電(dian)壓時(shi)，必須保(bao)證R1≥0.83KΩ，R2≤23.74KΩ兩個不等式同時(shi)成立，才能(neng)保(bao)證317穩(wen)壓塊(kuai)在空載時(shi)能(neng)夠(gou)穩(wen)定(ding)地工作(zuo)。
3.當然在317穩(wen)(wen)(wen)(wen)壓(ya)塊(kuai)的(de)(de)輸(shu)出(chu)端(duan)并聯泄(xie)流(liu)(liu)電(dian)阻R也可以(yi)為(wei)317穩(wen)(wen)(wen)(wen)壓(ya)塊(kuai)提供最小(xiao)(xiao)穩(wen)(wen)(wen)(wen)定(ding)工作電(dian)流(liu)(liu)。但是，由于并聯的(de)(de)泄(xie)流(liu)(liu)電(dian)阻不(bu)能隨輸(shu)出(chu)電(dian)壓(ya)的(de)(de)變(bian)化而變(bian)化，如果(guo)要保(bao)證(zheng)317穩(wen)(wen)(wen)(wen)壓(ya)塊(kuai)在輸(shu)出(chu)電(dian)壓(ya)為(wei)1.25V時(shi)，其輸(shu)出(chu)電(dian)流(liu)(liu)大于其最小(xiao)(xiao)穩(wen)(wen)(wen)(wen)定(ding)工作電(dian)流(liu)(liu)，則(ze)在317穩(wen)(wen)(wen)(wen)壓(ya)塊(kuai)的(de)(de)輸(shu)出(chu)電(dian)壓(ya)為(wei)37V時(shi)，流(liu)(liu)過泄(xie)流(liu)(liu)電(dian)阻的(de)(de)電(dian)流(liu)(liu)就太大了，這樣不(bu)僅浪費(fei)了電(dian)能，而且增加了317穩(wen)(wen)(wen)(wen)壓(ya)塊(kuai)的(de)(de)負擔，不(bu)是一種(zhong)妥當的(de)(de)辦法(fa)。
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