可調電源模塊的選擇
穩壓(ya)電(dian)源(yuan)的輸出電(dian)壓(ya)可(ke)用下(xia)式(shi)計(ji)算(suan)，Vo＝1.25（1＋R2/R1）。僅僅從(cong)公式(shi)本身(shen)看，R1、R2的電(dian)阻值可(ke)以隨意設定(ding)。然而作為穩壓(ya)電(dian)源(yuan)的輸出電(dian)壓(ya)計(ji)算(suan)公式(shi)，R1和R2的阻值是(shi)不能隨意設定(ding)的。
首先317穩壓(ya)塊的(de)(de)輸(shu)出(chu)電壓(ya)變化范(fan)圍(wei)是Vo＝1.25V—37V（高輸(shu)出(chu)電壓(ya)的(de)(de)317穩壓(ya)塊如LM317HVA、LM317HVK等，其輸(shu)出(chu)電壓(ya)變化范(fan)圍(wei)是Vo＝1.25V—45V），所以R2/R1的(de)(de)比(bi)值范(fan)圍(wei)只能是0—28.6。
其(qi)(qi)次(ci)是317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)(kuai)都有一個最(zui)小(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)定(ding)工(gong)(gong)作(zuo)(zuo)電(dian)(dian)(dian)流(liu)(liu)(liu)(liu)(liu)，有的(de)資料稱為(wei)最(zui)小(xiao)(xiao)輸(shu)出電(dian)(dian)(dian)流(liu)(liu)(liu)(liu)(liu)，也有的(de)資料稱為(wei)最(zui)小(xiao)(xiao)泄放電(dian)(dian)(dian)流(liu)(liu)(liu)(liu)(liu)。最(zui)小(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)定(ding)工(gong)(gong)作(zuo)(zuo)電(dian)(dian)(dian)流(liu)(liu)(liu)(liu)(liu)的(de)值一般(ban)為(wei)1.5mA。由于(yu)317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)(kuai)的(de)生(sheng)產廠家不(bu)同、型(xing)號不(bu)同，其(qi)(qi)最(zui)小(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)定(ding)工(gong)(gong)作(zuo)(zuo)電(dian)(dian)(dian)流(liu)(liu)(liu)(liu)(liu)也不(bu)相(xiang)同，但一般(ban)不(bu)大(da)于(yu)5mA。當317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)(kuai)的(de)輸(shu)出電(dian)(dian)(dian)流(liu)(liu)(liu)(liu)(liu)小(xiao)(xiao)于(yu)其(qi)(qi)最(zui)小(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)定(ding)工(gong)(gong)作(zuo)(zuo)電(dian)(dian)(dian)流(liu)(liu)(liu)(liu)(liu)時(shi)，317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)(kuai)就不(bu)能正常工(gong)(gong)作(zuo)(zuo)。當317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)(kuai)的(de)輸(shu)出電(dian)(dian)(dian)流(liu)(liu)(liu)(liu)(liu)大(da)于(yu)其(qi)(qi)最(zui)小(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)定(ding)工(gong)(gong)作(zuo)(zuo)電(dian)(dian)(dian)流(liu)(liu)(liu)(liu)(liu)時(shi)，317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)(kuai)就可(ke)以(yi)輸(shu)出穩(wen)(wen)(wen)(wen)(wen)定(ding)的(de)直流(liu)(liu)(liu)(liu)(liu)電(dian)(dian)(dian)壓(ya)(ya)。如果(guo)用317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)(kuai)制作(zuo)(zuo)穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)電(dian)(dian)(dian)源(yuan)時(shi),沒有注意317穩(wen)(wen)(wen)(wen)(wen)壓(ya)(ya)塊(kuai)(kuai)(kuai)的(de)最(zui)小(xiao)(xiao)穩(wen)(wen)(wen)(wen)(wen)定(ding)工(gong)(gong)作(zuo)(zuo)電(dian)(dian)(dian)流(liu)(liu)(liu)(liu)(liu)，那么你制作(zuo)(zuo)的(de)可能會出(chu)(chu)現(xian)下(xia)述不正(zheng)常現(xian)象(xiang)：穩壓(ya)電(dian)源輸出(chu)(chu)的有載(zai)電(dian)壓(ya)和(he)空載(zai)電(dian)壓(ya)差別較大(da)。
在應用中，為(wei)了電(dian)路的(de)穩定工作(zuo)，在一般情況(kuang)下，還需要接(jie)二(er)極管作(zuo)為(wei)保護電(dian)路，防止電(dian)路中的(de)電(dian)容(rong)放電(dian)時的(de)高(gao)壓把317燒(shao)壞。1.要解(jie)決317穩(wen)壓(ya)塊最(zui)(zui)小穩(wen)定(ding)工作(zuo)電(dian)(dian)(dian)流(liu)(liu)的(de)(de)問題(ti)，可(ke)以(yi)通過設定(ding)R1和R2阻值的(de)(de)大小，而(er)使317穩(wen)壓(ya)塊空載時(shi)輸出(chu)的(de)(de)電(dian)(dian)(dian)流(liu)(liu)大于或等于其(qi)最(zui)(zui)小穩(wen)定(ding)工作(zuo)電(dian)(dian)(dian)流(liu)(liu)，從而(er)保(bao)(bao)證(zheng)(zheng)(zheng)317穩(wen)壓(ya)塊在空載時(shi)能(neng)(neng)夠(gou)穩(wen)定(ding)地工作(zuo)。此時(shi)，只(zhi)(zhi)要保(bao)(bao)證(zheng)(zheng)(zheng)Vo/（R1＋R2）≥1.5mA，就可(ke)以(yi)保(bao)(bao)證(zheng)(zheng)(zheng)317穩(wen)壓(ya)塊在空載時(shi)能(neng)(neng)夠(gou)穩(wen)定(ding)地工作(zuo)。上式中(zhong)的(de)(de)1.5mA為317穩(wen)壓(ya)塊的(de)(de)最(zui)(zui)小穩(wen)定(ding)工作(zuo)電(dian)(dian)(dian)流(liu)(liu)。當(dang)然，只(zhi)(zhi)要能(neng)(neng)保(bao)(bao)證(zheng)(zheng)(zheng)317穩(wen)壓(ya)塊在空載時(shi)能(neng)(neng)夠(gou)穩(wen)定(ding)地工作(zuo)，Vo/（R1＋R2）的(de)(de)值也可(ke)以(yi)設定(ding)為大于1.5mA的(de)(de)任(ren)意值。
2.經計(ji)算可知R1的(de)(de)最大(da)取值(zhi)(zhi)為R1≈0.83KΩ。又因為R2/R1的(de)(de)最大(da)值(zhi)(zhi)為28.6。所以R2的(de)(de)最大(da)取值(zhi)(zhi)為R2≈23.74KΩ。在使用317穩(wen)壓(ya)塊的(de)(de)輸(shu)出電壓(ya)計(ji)算公式計(ji)算其輸(shu)出電壓(ya)時，必須保證R1≥0.83KΩ，R2≤23.74KΩ兩個不等式同(tong)時成立，才能保證317穩(wen)壓(ya)塊在空載時能夠(gou)穩(wen)定地(di)工作。
3.當然在317穩(wen)壓(ya)(ya)塊的(de)(de)輸出(chu)(chu)端并(bing)聯泄(xie)流(liu)電(dian)(dian)(dian)(dian)(dian)阻(zu)R也可(ke)以為(wei)317穩(wen)壓(ya)(ya)塊提供(gong)最小穩(wen)定工(gong)作電(dian)(dian)(dian)(dian)(dian)流(liu)。但是(shi)，由于并(bing)聯的(de)(de)泄(xie)流(liu)電(dian)(dian)(dian)(dian)(dian)阻(zu)不能(neng)隨(sui)輸出(chu)(chu)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)的(de)(de)變(bian)化而(er)變(bian)化，如果要保證317穩(wen)壓(ya)(ya)塊在輸出(chu)(chu)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)為(wei)1.25V時，其(qi)輸出(chu)(chu)電(dian)(dian)(dian)(dian)(dian)流(liu)大于其(qi)最小穩(wen)定工(gong)作電(dian)(dian)(dian)(dian)(dian)流(liu)，則在317穩(wen)壓(ya)(ya)塊的(de)(de)輸出(chu)(chu)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)為(wei)37V時，流(liu)過泄(xie)流(liu)電(dian)(dian)(dian)(dian)(dian)阻(zu)的(de)(de)電(dian)(dian)(dian)(dian)(dian)流(liu)就太大了(le)(le)，這樣不僅浪費了(le)(le)電(dian)(dian)(dian)(dian)(dian)能(neng)，而(er)且增加了(le)(le)317穩(wen)壓(ya)(ya)塊的(de)(de)負擔，不是(shi)一種妥當的(de)(de)辦法。
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