可調電源模塊的選擇
穩壓(ya)電(dian)源的輸(shu)出電(dian)壓(ya)可用下(xia)式(shi)計算，Vo＝1.25（1＋R2/R1）。僅僅從公(gong)式(shi)本身看(kan)，R1、R2的電(dian)阻值(zhi)可以隨意設定(ding)(ding)。然而作為穩壓(ya)電(dian)源的輸(shu)出電(dian)壓(ya)計算公(gong)式(shi)，R1和R2的阻值(zhi)是不能隨意設定(ding)(ding)的。
首先317穩壓(ya)(ya)塊的(de)輸(shu)出電壓(ya)(ya)變化(hua)范(fan)圍是Vo＝1.25V—37V（高輸(shu)出電壓(ya)(ya)的(de)317穩壓(ya)(ya)塊如(ru)LM317HVA、LM317HVK等，其輸(shu)出電壓(ya)(ya)變化(hua)范(fan)圍是Vo＝1.25V—45V），所以R2/R1的(de)比(bi)值范(fan)圍只能是0—28.6。
其(qi)(qi)次(ci)是(shi)317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)(kuai)都有(you)(you)(you)一個最(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)(ding)工(gong)(gong)作電(dian)(dian)流(liu)(liu)，有(you)(you)(you)的(de)(de)(de)(de)資料稱為最(zui)(zui)小(xiao)(xiao)(xiao)輸出(chu)(chu)電(dian)(dian)流(liu)(liu)，也(ye)有(you)(you)(you)的(de)(de)(de)(de)資料稱為最(zui)(zui)小(xiao)(xiao)(xiao)泄放電(dian)(dian)流(liu)(liu)。最(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)(ding)工(gong)(gong)作電(dian)(dian)流(liu)(liu)的(de)(de)(de)(de)值一般(ban)為1.5mA。由于(yu)317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)(kuai)的(de)(de)(de)(de)生產廠家不(bu)同(tong)、型號不(bu)同(tong)，其(qi)(qi)最(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)(ding)工(gong)(gong)作電(dian)(dian)流(liu)(liu)也(ye)不(bu)相同(tong)，但(dan)一般(ban)不(bu)大(da)于(yu)5mA。當317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)(kuai)的(de)(de)(de)(de)輸出(chu)(chu)電(dian)(dian)流(liu)(liu)小(xiao)(xiao)(xiao)于(yu)其(qi)(qi)最(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)(ding)工(gong)(gong)作電(dian)(dian)流(liu)(liu)時，317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)(kuai)就不(bu)能正常工(gong)(gong)作。當317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)(kuai)的(de)(de)(de)(de)輸出(chu)(chu)電(dian)(dian)流(liu)(liu)大(da)于(yu)其(qi)(qi)最(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)(ding)工(gong)(gong)作電(dian)(dian)流(liu)(liu)時，317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)(kuai)就可(ke)以輸出(chu)(chu)穩(wen)(wen)定(ding)(ding)(ding)的(de)(de)(de)(de)直流(liu)(liu)電(dian)(dian)壓(ya)(ya)(ya)。如(ru)果(guo)用317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)(kuai)制作穩(wen)(wen)壓(ya)(ya)(ya)電(dian)(dian)源時,沒有(you)(you)(you)注意(yi)317穩(wen)(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)(kuai)的(de)(de)(de)(de)最(zui)(zui)小(xiao)(xiao)(xiao)穩(wen)(wen)定(ding)(ding)(ding)工(gong)(gong)作電(dian)(dian)流(liu)(liu)，那(nei)么你(ni)制作的(de)(de)(de)(de)可能(neng)會(hui)出現下述不正(zheng)常現象：穩壓(ya)電(dian)(dian)源(yuan)輸出的有(you)載電(dian)(dian)壓(ya)和空載電(dian)(dian)壓(ya)差別較大(da)。
在(zai)應用中，為了電(dian)路的穩定工作(zuo)，在(zai)一般情況(kuang)下，還需要接二極管作(zuo)為保(bao)護電(dian)路，防(fang)止電(dian)路中的電(dian)容放電(dian)時的高(gao)壓把317燒壞。1.要(yao)解決317穩(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)最小(xiao)(xiao)穩(wen)定(ding)工(gong)(gong)作(zuo)(zuo)(zuo)電(dian)流的(de)(de)問題，可(ke)(ke)以通過設定(ding)R1和R2阻值(zhi)的(de)(de)大(da)小(xiao)(xiao)，而(er)使317穩(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)空載(zai)時輸出(chu)的(de)(de)電(dian)流大(da)于或(huo)等于其最小(xiao)(xiao)穩(wen)定(ding)工(gong)(gong)作(zuo)(zuo)(zuo)電(dian)流，從而(er)保證317穩(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)在空載(zai)時能(neng)夠穩(wen)定(ding)地工(gong)(gong)作(zuo)(zuo)(zuo)。此時，只要(yao)保證Vo/（R1＋R2）≥1.5mA，就可(ke)(ke)以保證317穩(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)在空載(zai)時能(neng)夠穩(wen)定(ding)地工(gong)(gong)作(zuo)(zuo)(zuo)。上式中的(de)(de)1.5mA為(wei)317穩(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)的(de)(de)最小(xiao)(xiao)穩(wen)定(ding)工(gong)(gong)作(zuo)(zuo)(zuo)電(dian)流。當然，只要(yao)能(neng)保證317穩(wen)壓(ya)(ya)(ya)塊(kuai)(kuai)在空載(zai)時能(neng)夠穩(wen)定(ding)地工(gong)(gong)作(zuo)(zuo)(zuo)，Vo/（R1＋R2）的(de)(de)值(zhi)也可(ke)(ke)以設定(ding)為(wei)大(da)于1.5mA的(de)(de)任意值(zhi)。
2.經計算(suan)可知R1的(de)最大(da)取值(zhi)(zhi)為R1≈0.83KΩ。又因為R2/R1的(de)最大(da)值(zhi)(zhi)為28.6。所以R2的(de)最大(da)取值(zhi)(zhi)為R2≈23.74KΩ。在(zai)使用317穩壓塊的(de)輸(shu)出(chu)(chu)電(dian)壓計算(suan)公式計算(suan)其輸(shu)出(chu)(chu)電(dian)壓時，必須保(bao)(bao)證(zheng)R1≥0.83KΩ，R2≤23.74KΩ兩個不(bu)等式同(tong)時成立，才能保(bao)(bao)證(zheng)317穩壓塊在(zai)空載時能夠穩定地工作。
3.當然在317穩(wen)壓(ya)(ya)塊(kuai)(kuai)(kuai)的(de)輸(shu)出(chu)(chu)端(duan)并(bing)(bing)聯泄流(liu)(liu)電(dian)(dian)阻(zu)R也(ye)可以為317穩(wen)壓(ya)(ya)塊(kuai)(kuai)(kuai)提供最(zui)小穩(wen)定工(gong)(gong)作電(dian)(dian)流(liu)(liu)。但(dan)是(shi)，由于并(bing)(bing)聯的(de)泄流(liu)(liu)電(dian)(dian)阻(zu)不能隨輸(shu)出(chu)(chu)電(dian)(dian)壓(ya)(ya)的(de)變化而變化，如果(guo)要保(bao)證(zheng)317穩(wen)壓(ya)(ya)塊(kuai)(kuai)(kuai)在輸(shu)出(chu)(chu)電(dian)(dian)壓(ya)(ya)為1.25V時(shi)，其(qi)輸(shu)出(chu)(chu)電(dian)(dian)流(liu)(liu)大于其(qi)最(zui)小穩(wen)定工(gong)(gong)作電(dian)(dian)流(liu)(liu)，則在317穩(wen)壓(ya)(ya)塊(kuai)(kuai)(kuai)的(de)輸(shu)出(chu)(chu)電(dian)(dian)壓(ya)(ya)為37V時(shi)，流(liu)(liu)過泄流(liu)(liu)電(dian)(dian)阻(zu)的(de)電(dian)(dian)流(liu)(liu)就太(tai)大了(le)(le)，這樣不僅(jin)浪費(fei)了(le)(le)電(dian)(dian)能，而且增加了(le)(le)317穩(wen)壓(ya)(ya)塊(kuai)(kuai)(kuai)的(de)負(fu)擔，不是(shi)一種妥當的(de)辦法。
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