可調電源模塊的選擇
穩壓(ya)(ya)電(dian)(dian)(dian)源的(de)輸(shu)(shu)出電(dian)(dian)(dian)壓(ya)(ya)可用下式(shi)計算，Vo＝1.25（1＋R2/R1）。僅僅從(cong)公式(shi)本身看(kan)，R1、R2的(de)電(dian)(dian)(dian)阻值(zhi)可以隨(sui)意設定(ding)。然而作為穩壓(ya)(ya)電(dian)(dian)(dian)源的(de)輸(shu)(shu)出電(dian)(dian)(dian)壓(ya)(ya)計算公式(shi)，R1和R2的(de)阻值(zhi)是不能隨(sui)意設定(ding)的(de)。
首(shou)先317穩壓(ya)(ya)塊的(de)輸出(chu)電(dian)壓(ya)(ya)變化(hua)范圍(wei)是(shi)Vo＝1.25V—37V（高輸出(chu)電(dian)壓(ya)(ya)的(de)317穩壓(ya)(ya)塊如LM317HVA、LM317HVK等(deng)，其(qi)輸出(chu)電(dian)壓(ya)(ya)變化(hua)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(qi)(qi)次是317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)都(dou)有一個最(zui)小(xiao)(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)(gong)(gong)(gong)作(zuo)(zuo)電流(liu)(liu)，有的(de)(de)(de)資(zi)料(liao)稱為最(zui)小(xiao)(xiao)(xiao)(xiao)輸(shu)(shu)出(chu)電流(liu)(liu)，也有的(de)(de)(de)資(zi)料(liao)稱為最(zui)小(xiao)(xiao)(xiao)(xiao)泄放電流(liu)(liu)。最(zui)小(xiao)(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)(gong)(gong)(gong)作(zuo)(zuo)電流(liu)(liu)的(de)(de)(de)值一般為1.5mA。由于317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)的(de)(de)(de)生產廠(chang)家不(bu)同、型號不(bu)同，其(qi)(qi)最(zui)小(xiao)(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)(gong)(gong)(gong)作(zuo)(zuo)電流(liu)(liu)也不(bu)相(xiang)同，但一般不(bu)大于5mA。當(dang)317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)的(de)(de)(de)輸(shu)(shu)出(chu)電流(liu)(liu)小(xiao)(xiao)(xiao)(xiao)于其(qi)(qi)最(zui)小(xiao)(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)(gong)(gong)(gong)作(zuo)(zuo)電流(liu)(liu)時，317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)就不(bu)能正(zheng)常工(gong)(gong)(gong)(gong)作(zuo)(zuo)。當(dang)317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)的(de)(de)(de)輸(shu)(shu)出(chu)電流(liu)(liu)大于其(qi)(qi)最(zui)小(xiao)(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)(gong)(gong)(gong)作(zuo)(zuo)電流(liu)(liu)時，317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)就可以輸(shu)(shu)出(chu)穩(wen)(wen)定(ding)(ding)的(de)(de)(de)直流(liu)(liu)電壓(ya)(ya)(ya)。如(ru)果用(yong)317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)制(zhi)作(zuo)(zuo)穩(wen)(wen)壓(ya)(ya)(ya)電源時,沒有注意(yi)317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)的(de)(de)(de)最(zui)小(xiao)(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)工(gong)(gong)(gong)(gong)作(zuo)(zuo)電流(liu)(liu)，那(nei)么你制(zhi)作(zuo)(zuo)的(de)(de)(de)可能會(hui)出(chu)現下述不正常現象：穩壓電源輸出(chu)的(de)有(you)載電壓和空載電壓差別(bie)較大。
在應用(yong)中，為(wei)了(le)電路的穩(wen)定工(gong)作，在一般(ban)情況下，還需要接(jie)二極管作為(wei)保護電路，防止(zhi)電路中的電容放電時的高壓把317燒(shao)壞(huai)。1.要解(jie)決317穩(wen)(wen)壓(ya)塊(kuai)(kuai)(kuai)最(zui)小穩(wen)(wen)定(ding)工作電流的(de)(de)(de)(de)問(wen)題，可(ke)以通過設定(ding)R1和R2阻值的(de)(de)(de)(de)大小，而使317穩(wen)(wen)壓(ya)塊(kuai)(kuai)(kuai)空(kong)載(zai)時(shi)(shi)輸出的(de)(de)(de)(de)電流大于(yu)或等于(yu)其最(zui)小穩(wen)(wen)定(ding)工作電流，從(cong)而保證(zheng)(zheng)317穩(wen)(wen)壓(ya)塊(kuai)(kuai)(kuai)在(zai)空(kong)載(zai)時(shi)(shi)能(neng)(neng)夠穩(wen)(wen)定(ding)地(di)工作。此時(shi)(shi)，只(zhi)要保證(zheng)(zheng)Vo/（R1＋R2）≥1.5mA，就可(ke)以保證(zheng)(zheng)317穩(wen)(wen)壓(ya)塊(kuai)(kuai)(kuai)在(zai)空(kong)載(zai)時(shi)(shi)能(neng)(neng)夠穩(wen)(wen)定(ding)地(di)工作。上式中(zhong)的(de)(de)(de)(de)1.5mA為317穩(wen)(wen)壓(ya)塊(kuai)(kuai)(kuai)的(de)(de)(de)(de)最(zui)小穩(wen)(wen)定(ding)工作電流。當然，只(zhi)要能(neng)(neng)保證(zheng)(zheng)317穩(wen)(wen)壓(ya)塊(kuai)(kuai)(kuai)在(zai)空(kong)載(zai)時(shi)(shi)能(neng)(neng)夠穩(wen)(wen)定(ding)地(di)工作，Vo/（R1＋R2）的(de)(de)(de)(de)值也可(ke)以設定(ding)為大于(yu)1.5mA的(de)(de)(de)(de)任意值。
2.經計算可知R1的(de)最(zui)(zui)大取(qu)值為R1≈0.83KΩ。又因為R2/R1的(de)最(zui)(zui)大值為28.6。所以R2的(de)最(zui)(zui)大取(qu)值為R2≈23.74KΩ。在(zai)使用(yong)317穩壓(ya)塊的(de)輸(shu)(shu)出電(dian)壓(ya)計算公式計算其輸(shu)(shu)出電(dian)壓(ya)時(shi)，必須保證(zheng)R1≥0.83KΩ，R2≤23.74KΩ兩(liang)個不等式同時(shi)成立，才(cai)能保證(zheng)317穩壓(ya)塊在(zai)空載時(shi)能夠穩定地工(gong)作。
3.當然在(zai)317穩(wen)壓塊(kuai)的(de)輸(shu)(shu)出(chu)(chu)端并聯(lian)泄流電(dian)(dian)阻(zu)R也可(ke)以為(wei)317穩(wen)壓塊(kuai)提供最小(xiao)穩(wen)定工(gong)作電(dian)(dian)流。但是(shi)，由于并聯(lian)的(de)泄流電(dian)(dian)阻(zu)不(bu)能隨(sui)輸(shu)(shu)出(chu)(chu)電(dian)(dian)壓的(de)變化(hua)而變化(hua)，如(ru)果要保(bao)證317穩(wen)壓塊(kuai)在(zai)輸(shu)(shu)出(chu)(chu)電(dian)(dian)壓為(wei)1.25V時(shi)，其(qi)輸(shu)(shu)出(chu)(chu)電(dian)(dian)流大于其(qi)最小(xiao)穩(wen)定工(gong)作電(dian)(dian)流，則在(zai)317穩(wen)壓塊(kuai)的(de)輸(shu)(shu)出(chu)(chu)電(dian)(dian)壓為(wei)37V時(shi)，流過泄流電(dian)(dian)阻(zu)的(de)電(dian)(dian)流就(jiu)太(tai)大了，這(zhe)樣不(bu)僅浪費(fei)了電(dian)(dian)能，而且增加了317穩(wen)壓塊(kuai)的(de)負(fu)擔，不(bu)是(shi)一種妥(tuo)當的(de)辦法。
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