可調電源模塊的選擇
穩(wen)(wen)壓電(dian)源(yuan)(yuan)的輸出(chu)電(dian)壓可用下式計算，Vo＝1.25（1＋R2/R1）。僅僅從公式本身看，R1、R2的電(dian)阻值(zhi)可以(yi)隨(sui)意設定(ding)。然而作(zuo)為穩(wen)(wen)壓電(dian)源(yuan)(yuan)的輸出(chu)電(dian)壓計算公式，R1和R2的阻值(zhi)是不能(neng)隨(sui)意設定(ding)的。
首先317穩(wen)壓(ya)塊的輸出電(dian)壓(ya)變化(hua)范圍(wei)是Vo＝1.25V—37V（高(gao)輸出電(dian)壓(ya)的317穩(wen)壓(ya)塊如LM317HVA、LM317HVK等，其(qi)輸出電(dian)壓(ya)變化(hua)范圍(wei)是Vo＝1.25V—45V），所以R2/R1的比值(zhi)范圍(wei)只能(neng)是0—28.6。
其(qi)次(ci)是317穩(wen)(wen)(wen)(wen)(wen)(wen)壓(ya)塊(kuai)都(dou)有一(yi)個(ge)最(zui)小(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)(wen)定(ding)(ding)(ding)(ding)工(gong)(gong)作(zuo)(zuo)(zuo)電(dian)(dian)(dian)(dian)流(liu)，有的(de)資(zi)料(liao)稱(cheng)為最(zui)小(xiao)(xiao)輸(shu)出電(dian)(dian)(dian)(dian)流(liu)，也有的(de)資(zi)料(liao)稱(cheng)為最(zui)小(xiao)(xiao)泄放電(dian)(dian)(dian)(dian)流(liu)。最(zui)小(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)(wen)定(ding)(ding)(ding)(ding)工(gong)(gong)作(zuo)(zuo)(zuo)電(dian)(dian)(dian)(dian)流(liu)的(de)值一(yi)般為1.5mA。由于317穩(wen)(wen)(wen)(wen)(wen)(wen)壓(ya)塊(kuai)的(de)生產廠家不同(tong)、型號不同(tong)，其(qi)最(zui)小(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)(wen)定(ding)(ding)(ding)(ding)工(gong)(gong)作(zuo)(zuo)(zuo)電(dian)(dian)(dian)(dian)流(liu)也不相同(tong)，但一(yi)般不大于5mA。當317穩(wen)(wen)(wen)(wen)(wen)(wen)壓(ya)塊(kuai)的(de)輸(shu)出電(dian)(dian)(dian)(dian)流(liu)小(xiao)(xiao)于其(qi)最(zui)小(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)(wen)定(ding)(ding)(ding)(ding)工(gong)(gong)作(zuo)(zuo)(zuo)電(dian)(dian)(dian)(dian)流(liu)時，317穩(wen)(wen)(wen)(wen)(wen)(wen)壓(ya)塊(kuai)就不能(neng)正常(chang)工(gong)(gong)作(zuo)(zuo)(zuo)。當317穩(wen)(wen)(wen)(wen)(wen)(wen)壓(ya)塊(kuai)的(de)輸(shu)出電(dian)(dian)(dian)(dian)流(liu)大于其(qi)最(zui)小(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)(wen)定(ding)(ding)(ding)(ding)工(gong)(gong)作(zuo)(zuo)(zuo)電(dian)(dian)(dian)(dian)流(liu)時，317穩(wen)(wen)(wen)(wen)(wen)(wen)壓(ya)塊(kuai)就可以(yi)輸(shu)出穩(wen)(wen)(wen)(wen)(wen)(wen)定(ding)(ding)(ding)(ding)的(de)直流(liu)電(dian)(dian)(dian)(dian)壓(ya)。如果用(yong)317穩(wen)(wen)(wen)(wen)(wen)(wen)壓(ya)塊(kuai)制作(zuo)(zuo)(zuo)穩(wen)(wen)(wen)(wen)(wen)(wen)壓(ya)電(dian)(dian)(dian)(dian)源時,沒(mei)有注意317穩(wen)(wen)(wen)(wen)(wen)(wen)壓(ya)塊(kuai)的(de)最(zui)小(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)(wen)定(ding)(ding)(ding)(ding)工(gong)(gong)作(zuo)(zuo)(zuo)電(dian)(dian)(dian)(dian)流(liu)，那么你制作(zuo)(zuo)(zuo)的(de)可能會(hui)出(chu)現下述不(bu)正常現象(xiang)：穩壓電(dian)源輸出(chu)的有(you)載電(dian)壓和空載電(dian)壓差別較(jiao)大。
在(zai)應用中(zhong)(zhong)，為(wei)了電(dian)路的穩定工(gong)作(zuo)，在(zai)一般(ban)情況下(xia)，還需要接二極管作(zuo)為(wei)保護電(dian)路，防止電(dian)路中(zhong)(zhong)的電(dian)容放電(dian)時的高壓把317燒壞。1.要解決317穩(wen)(wen)(wen)壓塊最小穩(wen)(wen)(wen)定工作(zuo)(zuo)電流(liu)(liu)的(de)問(wen)題，可以(yi)(yi)通過設定R1和R2阻值的(de)大(da)小，而(er)使317穩(wen)(wen)(wen)壓塊空(kong)載(zai)(zai)時輸出的(de)電流(liu)(liu)大(da)于(yu)或等(deng)于(yu)其最小穩(wen)(wen)(wen)定工作(zuo)(zuo)電流(liu)(liu)，從(cong)而(er)保(bao)證(zheng)317穩(wen)(wen)(wen)壓塊在空(kong)載(zai)(zai)時能夠(gou)穩(wen)(wen)(wen)定地(di)工作(zuo)(zuo)。此(ci)時，只(zhi)要保(bao)證(zheng)Vo/（R1＋R2）≥1.5mA，就可以(yi)(yi)保(bao)證(zheng)317穩(wen)(wen)(wen)壓塊在空(kong)載(zai)(zai)時能夠(gou)穩(wen)(wen)(wen)定地(di)工作(zuo)(zuo)。上式中的(de)1.5mA為(wei)317穩(wen)(wen)(wen)壓塊的(de)最小穩(wen)(wen)(wen)定工作(zuo)(zuo)電流(liu)(liu)。當然，只(zhi)要能保(bao)證(zheng)317穩(wen)(wen)(wen)壓塊在空(kong)載(zai)(zai)時能夠(gou)穩(wen)(wen)(wen)定地(di)工作(zuo)(zuo)，Vo/（R1＋R2）的(de)值也可以(yi)(yi)設定為(wei)大(da)于(yu)1.5mA的(de)任(ren)意值。
2.經計算可知R1的(de)(de)最大取值(zhi)為(wei)R1≈0.83KΩ。又因為(wei)R2/R1的(de)(de)最大值(zhi)為(wei)28.6。所以R2的(de)(de)最大取值(zhi)為(wei)R2≈23.74KΩ。在(zai)使(shi)用317穩壓(ya)塊的(de)(de)輸(shu)出電(dian)壓(ya)計算公式(shi)計算其輸(shu)出電(dian)壓(ya)時(shi)，必須保證R1≥0.83KΩ，R2≤23.74KΩ兩個不等式(shi)同(tong)時(shi)成立，才(cai)能保證317穩壓(ya)塊在(zai)空(kong)載(zai)時(shi)能夠(gou)穩定地(di)工作。
3.當然在(zai)317穩壓(ya)(ya)塊(kuai)(kuai)(kuai)的(de)輸(shu)(shu)出端(duan)并聯泄流(liu)電(dian)阻(zu)(zu)R也可以為317穩壓(ya)(ya)塊(kuai)(kuai)(kuai)提供最小(xiao)穩定(ding)工作電(dian)流(liu)。但是，由于并聯的(de)泄流(liu)電(dian)阻(zu)(zu)不能隨輸(shu)(shu)出電(dian)壓(ya)(ya)的(de)變(bian)化(hua)而變(bian)化(hua)，如果(guo)要保證(zheng)317穩壓(ya)(ya)塊(kuai)(kuai)(kuai)在(zai)輸(shu)(shu)出電(dian)壓(ya)(ya)為1.25V時(shi)，其輸(shu)(shu)出電(dian)流(liu)大于其最小(xiao)穩定(ding)工作電(dian)流(liu)，則在(zai)317穩壓(ya)(ya)塊(kuai)(kuai)(kuai)的(de)輸(shu)(shu)出電(dian)壓(ya)(ya)為37V時(shi)，流(liu)過泄流(liu)電(dian)阻(zu)(zu)的(de)電(dian)流(liu)就(jiu)太大了，這樣(yang)不僅(jin)浪費了電(dian)能，而且增加了317穩壓(ya)(ya)塊(kuai)(kuai)(kuai)的(de)負擔，不是一種妥當的(de)辦法(fa)。
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