可調電源模塊的選擇
穩壓電源(yuan)的(de)輸出電壓可用下(xia)式計算(suan)，Vo＝1.25（1＋R2/R1）。僅僅從公式本身看，R1、R2的(de)電阻(zu)值可以隨意(yi)設定(ding)。然而作為(wei)穩壓電源(yuan)的(de)輸出電壓計算(suan)公式，R1和R2的(de)阻(zu)值是不能(neng)隨意(yi)設定(ding)的(de)。
首先317穩壓(ya)塊的(de)輸(shu)出電(dian)(dian)壓(ya)變化范圍(wei)是(shi)Vo＝1.25V—37V（高(gao)輸(shu)出電(dian)(dian)壓(ya)的(de)317穩壓(ya)塊如LM317HVA、LM317HVK等，其輸(shu)出電(dian)(dian)壓(ya)變化范圍(wei)是(shi)Vo＝1.25V—45V），所以R2/R1的(de)比值范圍(wei)只能是(shi)0—28.6。
其次是317穩(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)都有一(yi)個最小(xiao)(xiao)穩(wen)(wen)(wen)(wen)定(ding)(ding)工(gong)作(zuo)(zuo)電(dian)流(liu)(liu)(liu)，有的(de)(de)(de)資(zi)料(liao)稱(cheng)為(wei)最小(xiao)(xiao)輸(shu)出(chu)電(dian)流(liu)(liu)(liu)，也有的(de)(de)(de)資(zi)料(liao)稱(cheng)為(wei)最小(xiao)(xiao)泄(xie)放電(dian)流(liu)(liu)(liu)。最小(xiao)(xiao)穩(wen)(wen)(wen)(wen)定(ding)(ding)工(gong)作(zuo)(zuo)電(dian)流(liu)(liu)(liu)的(de)(de)(de)值一(yi)般為(wei)1.5mA。由于(yu)(yu)317穩(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)的(de)(de)(de)生產廠家(jia)不同(tong)、型號不同(tong)，其最小(xiao)(xiao)穩(wen)(wen)(wen)(wen)定(ding)(ding)工(gong)作(zuo)(zuo)電(dian)流(liu)(liu)(liu)也不相(xiang)同(tong)，但一(yi)般不大于(yu)(yu)5mA。當317穩(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)的(de)(de)(de)輸(shu)出(chu)電(dian)流(liu)(liu)(liu)小(xiao)(xiao)于(yu)(yu)其最小(xiao)(xiao)穩(wen)(wen)(wen)(wen)定(ding)(ding)工(gong)作(zuo)(zuo)電(dian)流(liu)(liu)(liu)時，317穩(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)就不能(neng)正常(chang)工(gong)作(zuo)(zuo)。當317穩(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)的(de)(de)(de)輸(shu)出(chu)電(dian)流(liu)(liu)(liu)大于(yu)(yu)其最小(xiao)(xiao)穩(wen)(wen)(wen)(wen)定(ding)(ding)工(gong)作(zuo)(zuo)電(dian)流(liu)(liu)(liu)時，317穩(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)就可以輸(shu)出(chu)穩(wen)(wen)(wen)(wen)定(ding)(ding)的(de)(de)(de)直流(liu)(liu)(liu)電(dian)壓(ya)(ya)(ya)。如果用317穩(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)制(zhi)作(zuo)(zuo)穩(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)電(dian)源時,沒有注意317穩(wen)(wen)(wen)(wen)壓(ya)(ya)(ya)塊(kuai)的(de)(de)(de)最小(xiao)(xiao)穩(wen)(wen)(wen)(wen)定(ding)(ding)工(gong)作(zuo)(zuo)電(dian)流(liu)(liu)(liu)，那么你制(zhi)作(zuo)(zuo)的(de)(de)(de)可能會出現(xian)下述不正常現(xian)象：穩壓電源輸出的有載(zai)電壓和空載(zai)電壓差別較大。
在(zai)應用中，為(wei)了電(dian)(dian)路的(de)(de)穩定工作(zuo)，在(zai)一般情(qing)況(kuang)下，還需要接二極管作(zuo)為(wei)保護電(dian)(dian)路，防(fang)止電(dian)(dian)路中的(de)(de)電(dian)(dian)容放電(dian)(dian)時的(de)(de)高壓把317燒壞。1.要(yao)解決317穩(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)最小穩(wen)(wen)(wen)定(ding)(ding)工(gong)作電流的(de)(de)問題，可(ke)(ke)(ke)以(yi)(yi)通過設定(ding)(ding)R1和R2阻(zu)值的(de)(de)大(da)小，而(er)使317穩(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)空(kong)載時(shi)(shi)(shi)(shi)輸出(chu)的(de)(de)電流大(da)于或等(deng)于其最小穩(wen)(wen)(wen)定(ding)(ding)工(gong)作電流，從(cong)而(er)保證(zheng)(zheng)317穩(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)在空(kong)載時(shi)(shi)(shi)(shi)能(neng)(neng)夠穩(wen)(wen)(wen)定(ding)(ding)地(di)(di)工(gong)作。此時(shi)(shi)(shi)(shi)，只要(yao)保證(zheng)(zheng)Vo/（R1＋R2）≥1.5mA，就可(ke)(ke)(ke)以(yi)(yi)保證(zheng)(zheng)317穩(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)在空(kong)載時(shi)(shi)(shi)(shi)能(neng)(neng)夠穩(wen)(wen)(wen)定(ding)(ding)地(di)(di)工(gong)作。上式(shi)中的(de)(de)1.5mA為317穩(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)的(de)(de)最小穩(wen)(wen)(wen)定(ding)(ding)工(gong)作電流。當然，只要(yao)能(neng)(neng)保證(zheng)(zheng)317穩(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)在空(kong)載時(shi)(shi)(shi)(shi)能(neng)(neng)夠穩(wen)(wen)(wen)定(ding)(ding)地(di)(di)工(gong)作，Vo/（R1＋R2）的(de)(de)值也可(ke)(ke)(ke)以(yi)(yi)設定(ding)(ding)為大(da)于1.5mA的(de)(de)任意值。
2.經計算可知(zhi)R1的最大(da)取(qu)值為R1≈0.83KΩ。又因為R2/R1的最大(da)值為28.6。所(suo)以R2的最大(da)取(qu)值為R2≈23.74KΩ。在使用(yong)317穩(wen)壓塊的輸出電壓計算公式(shi)計算其輸出電壓時(shi)，必須保證R1≥0.83KΩ，R2≤23.74KΩ兩個不等式(shi)同(tong)時(shi)成(cheng)立，才(cai)能保證317穩(wen)壓塊在空載時(shi)能夠穩(wen)定地工作。
3.當然在317穩(wen)壓(ya)(ya)塊的(de)(de)輸(shu)(shu)出(chu)端并聯泄流(liu)電(dian)(dian)阻(zu)R也(ye)可以(yi)為(wei)317穩(wen)壓(ya)(ya)塊提(ti)供最小穩(wen)定工作(zuo)電(dian)(dian)流(liu)。但是，由于并聯的(de)(de)泄流(liu)電(dian)(dian)阻(zu)不能(neng)(neng)隨輸(shu)(shu)出(chu)電(dian)(dian)壓(ya)(ya)的(de)(de)變化(hua)而(er)(er)變化(hua)，如果要保證317穩(wen)壓(ya)(ya)塊在輸(shu)(shu)出(chu)電(dian)(dian)壓(ya)(ya)為(wei)1.25V時，其(qi)輸(shu)(shu)出(chu)電(dian)(dian)流(liu)大(da)于其(qi)最小穩(wen)定工作(zuo)電(dian)(dian)流(liu)，則在317穩(wen)壓(ya)(ya)塊的(de)(de)輸(shu)(shu)出(chu)電(dian)(dian)壓(ya)(ya)為(wei)37V時，流(liu)過(guo)泄流(liu)電(dian)(dian)阻(zu)的(de)(de)電(dian)(dian)流(liu)就太大(da)了(le)，這(zhe)樣不僅浪(lang)費(fei)了(le)電(dian)(dian)能(neng)(neng)，而(er)(er)且(qie)增(zeng)加了(le)317穩(wen)壓(ya)(ya)塊的(de)(de)負擔，不是一種妥當的(de)(de)辦法。
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