可調電源模塊的選擇
穩(wen)壓電(dian)(dian)源的輸出(chu)電(dian)(dian)壓可(ke)用下(xia)式計(ji)算(suan)，Vo＝1.25（1＋R2/R1）。僅僅從公式本身(shen)看，R1、R2的電(dian)(dian)阻值(zhi)可(ke)以隨意(yi)設(she)(she)定。然(ran)而(er)作為穩(wen)壓電(dian)(dian)源的輸出(chu)電(dian)(dian)壓計(ji)算(suan)公式，R1和R2的阻值(zhi)是不(bu)能隨意(yi)設(she)(she)定的。
首先317穩壓(ya)塊(kuai)的(de)輸(shu)出(chu)電壓(ya)變化范圍(wei)是Vo＝1.25V—37V（高輸(shu)出(chu)電壓(ya)的(de)317穩壓(ya)塊(kuai)如LM317HVA、LM317HVK等，其輸(shu)出(chu)電壓(ya)變化范圍(wei)是Vo＝1.25V—45V），所以R2/R1的(de)比(bi)值(zhi)范圍(wei)只(zhi)能(neng)是0—28.6。
其次是317穩(wen)(wen)壓(ya)(ya)(ya)(ya)(ya)塊(kuai)都有一(yi)個最(zui)(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)作(zuo)電(dian)(dian)(dian)流(liu)(liu)，有的(de)(de)(de)資(zi)料(liao)稱(cheng)為最(zui)(zui)(zui)小(xiao)(xiao)(xiao)輸(shu)(shu)出電(dian)(dian)(dian)流(liu)(liu)，也(ye)有的(de)(de)(de)資(zi)料(liao)稱(cheng)為最(zui)(zui)(zui)小(xiao)(xiao)(xiao)泄放(fang)電(dian)(dian)(dian)流(liu)(liu)。最(zui)(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)作(zuo)電(dian)(dian)(dian)流(liu)(liu)的(de)(de)(de)值一(yi)般(ban)為1.5mA。由于317穩(wen)(wen)壓(ya)(ya)(ya)(ya)(ya)塊(kuai)的(de)(de)(de)生產廠家不(bu)同、型號(hao)不(bu)同，其最(zui)(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)作(zuo)電(dian)(dian)(dian)流(liu)(liu)也(ye)不(bu)相同，但一(yi)般(ban)不(bu)大(da)于5mA。當(dang)317穩(wen)(wen)壓(ya)(ya)(ya)(ya)(ya)塊(kuai)的(de)(de)(de)輸(shu)(shu)出電(dian)(dian)(dian)流(liu)(liu)小(xiao)(xiao)(xiao)于其最(zui)(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)作(zuo)電(dian)(dian)(dian)流(liu)(liu)時，317穩(wen)(wen)壓(ya)(ya)(ya)(ya)(ya)塊(kuai)就不(bu)能正常工(gong)作(zuo)。當(dang)317穩(wen)(wen)壓(ya)(ya)(ya)(ya)(ya)塊(kuai)的(de)(de)(de)輸(shu)(shu)出電(dian)(dian)(dian)流(liu)(liu)大(da)于其最(zui)(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)作(zuo)電(dian)(dian)(dian)流(liu)(liu)時，317穩(wen)(wen)壓(ya)(ya)(ya)(ya)(ya)塊(kuai)就可以(yi)輸(shu)(shu)出穩(wen)(wen)定(ding)(ding)的(de)(de)(de)直流(liu)(liu)電(dian)(dian)(dian)壓(ya)(ya)(ya)(ya)(ya)。如果用317穩(wen)(wen)壓(ya)(ya)(ya)(ya)(ya)塊(kuai)制作(zuo)穩(wen)(wen)壓(ya)(ya)(ya)(ya)(ya)電(dian)(dian)(dian)源時,沒有注意(yi)317穩(wen)(wen)壓(ya)(ya)(ya)(ya)(ya)塊(kuai)的(de)(de)(de)最(zui)(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)作(zuo)電(dian)(dian)(dian)流(liu)(liu)，那么你制作(zuo)的(de)(de)(de)可(ke)能會出現(xian)下述不正常現(xian)象(xiang)：穩壓(ya)電源輸(shu)出的有載電壓(ya)和空載電壓(ya)差別較大。
在(zai)應用中，為了電路(lu)(lu)的(de)穩定工作，在(zai)一般情況下，還需(xu)要接(jie)二極管作為保護電路(lu)(lu)，防止(zhi)電路(lu)(lu)中的(de)電容放電時的(de)高壓把317燒壞(huai)。1.要(yao)(yao)解決317穩(wen)(wen)壓塊最(zui)小穩(wen)(wen)定(ding)(ding)(ding)工(gong)(gong)作電(dian)流(liu)的(de)(de)問題，可(ke)以(yi)通過設(she)定(ding)(ding)(ding)R1和R2阻(zu)值(zhi)的(de)(de)大小，而(er)使317穩(wen)(wen)壓塊空(kong)載(zai)時輸出的(de)(de)電(dian)流(liu)大于(yu)或(huo)等(deng)于(yu)其最(zui)小穩(wen)(wen)定(ding)(ding)(ding)工(gong)(gong)作電(dian)流(liu)，從而(er)保證(zheng)(zheng)317穩(wen)(wen)壓塊在(zai)空(kong)載(zai)時能夠穩(wen)(wen)定(ding)(ding)(ding)地工(gong)(gong)作。此時，只(zhi)(zhi)要(yao)(yao)保證(zheng)(zheng)Vo/（R1＋R2）≥1.5mA，就可(ke)以(yi)保證(zheng)(zheng)317穩(wen)(wen)壓塊在(zai)空(kong)載(zai)時能夠穩(wen)(wen)定(ding)(ding)(ding)地工(gong)(gong)作。上式中的(de)(de)1.5mA為(wei)(wei)317穩(wen)(wen)壓塊的(de)(de)最(zui)小穩(wen)(wen)定(ding)(ding)(ding)工(gong)(gong)作電(dian)流(liu)。當然，只(zhi)(zhi)要(yao)(yao)能保證(zheng)(zheng)317穩(wen)(wen)壓塊在(zai)空(kong)載(zai)時能夠穩(wen)(wen)定(ding)(ding)(ding)地工(gong)(gong)作，Vo/（R1＋R2）的(de)(de)值(zhi)也可(ke)以(yi)設(she)定(ding)(ding)(ding)為(wei)(wei)大于(yu)1.5mA的(de)(de)任意(yi)值(zhi)。
2.經計算可知R1的最(zui)(zui)大取(qu)值(zhi)(zhi)為(wei)(wei)R1≈0.83KΩ。又(you)因為(wei)(wei)R2/R1的最(zui)(zui)大值(zhi)(zhi)為(wei)(wei)28.6。所以R2的最(zui)(zui)大取(qu)值(zhi)(zhi)為(wei)(wei)R2≈23.74KΩ。在使用317穩壓塊(kuai)的輸(shu)出(chu)電(dian)壓計算公式計算其(qi)輸(shu)出(chu)電(dian)壓時，必(bi)須保證R1≥0.83KΩ，R2≤23.74KΩ兩個不(bu)等式同時成立(li)，才能保證317穩壓塊(kuai)在空載時能夠(gou)穩定(ding)地工(gong)作。
3.當然在317穩(wen)壓(ya)塊的(de)(de)輸(shu)(shu)(shu)出端并(bing)聯(lian)泄(xie)流(liu)(liu)(liu)電(dian)(dian)阻R也可以為317穩(wen)壓(ya)塊提供最小穩(wen)定工作(zuo)電(dian)(dian)流(liu)(liu)(liu)。但(dan)是(shi)，由于并(bing)聯(lian)的(de)(de)泄(xie)流(liu)(liu)(liu)電(dian)(dian)阻不(bu)能隨輸(shu)(shu)(shu)出電(dian)(dian)壓(ya)的(de)(de)變(bian)化而(er)變(bian)化，如(ru)果要保證(zheng)317穩(wen)壓(ya)塊在輸(shu)(shu)(shu)出電(dian)(dian)壓(ya)為1.25V時，其輸(shu)(shu)(shu)出電(dian)(dian)流(liu)(liu)(liu)大于其最小穩(wen)定工作(zuo)電(dian)(dian)流(liu)(liu)(liu)，則在317穩(wen)壓(ya)塊的(de)(de)輸(shu)(shu)(shu)出電(dian)(dian)壓(ya)為37V時，流(liu)(liu)(liu)過(guo)泄(xie)流(liu)(liu)(liu)電(dian)(dian)阻的(de)(de)電(dian)(dian)流(liu)(liu)(liu)就太大了，這樣(yang)不(bu)僅浪(lang)費了電(dian)(dian)能，而(er)且(qie)增加了317穩(wen)壓(ya)塊的(de)(de)負擔，不(bu)是(shi)一種妥當的(de)(de)辦法。
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