可調電源模塊的選擇
穩(wen)壓(ya)(ya)電(dian)源的輸出(chu)(chu)電(dian)壓(ya)(ya)可用下式計算，Vo＝1.25（1＋R2/R1）。僅僅從公式本(ben)身看，R1、R2的電(dian)阻值(zhi)可以隨(sui)意設(she)定(ding)。然而作為(wei)穩(wen)壓(ya)(ya)電(dian)源的輸出(chu)(chu)電(dian)壓(ya)(ya)計算公式，R1和R2的阻值(zhi)是不能隨(sui)意設(she)定(ding)的。
首先317穩(wen)壓(ya)塊(kuai)的輸(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317穩(wen)壓(ya)塊(kuai)如LM317HVA、LM317HVK等(deng)，其輸(shu)出(chu)電壓(ya)變化范(fan)圍(wei)是Vo＝1.25V—45V），所以(yi)R2/R1的比值(zhi)范(fan)圍(wei)只能是0—28.6。
其次是317穩(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)都(dou)有一個最(zui)(zui)(zui)小穩(wen)定(ding)工作(zuo)(zuo)(zuo)(zuo)電(dian)流(liu)(liu)(liu)，有的(de)(de)(de)資料稱為(wei)最(zui)(zui)(zui)小輸出電(dian)流(liu)(liu)(liu)，也有的(de)(de)(de)資料稱為(wei)最(zui)(zui)(zui)小泄放電(dian)流(liu)(liu)(liu)。最(zui)(zui)(zui)小穩(wen)定(ding)工作(zuo)(zuo)(zuo)(zuo)電(dian)流(liu)(liu)(liu)的(de)(de)(de)值一般為(wei)1.5mA。由于(yu)317穩(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)的(de)(de)(de)生產廠家不同、型號(hao)不同，其最(zui)(zui)(zui)小穩(wen)定(ding)工作(zuo)(zuo)(zuo)(zuo)電(dian)流(liu)(liu)(liu)也不相同，但一般不大于(yu)5mA。當(dang)317穩(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)的(de)(de)(de)輸出電(dian)流(liu)(liu)(liu)小于(yu)其最(zui)(zui)(zui)小穩(wen)定(ding)工作(zuo)(zuo)(zuo)(zuo)電(dian)流(liu)(liu)(liu)時，317穩(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)就不能正(zheng)常(chang)工作(zuo)(zuo)(zuo)(zuo)。當(dang)317穩(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)的(de)(de)(de)輸出電(dian)流(liu)(liu)(liu)大于(yu)其最(zui)(zui)(zui)小穩(wen)定(ding)工作(zuo)(zuo)(zuo)(zuo)電(dian)流(liu)(liu)(liu)時，317穩(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)就可以輸出穩(wen)定(ding)的(de)(de)(de)直(zhi)流(liu)(liu)(liu)電(dian)壓(ya)(ya)(ya)(ya)。如(ru)果用317穩(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)制(zhi)作(zuo)(zuo)(zuo)(zuo)穩(wen)壓(ya)(ya)(ya)(ya)電(dian)源時,沒有注意317穩(wen)壓(ya)(ya)(ya)(ya)塊(kuai)(kuai)的(de)(de)(de)最(zui)(zui)(zui)小穩(wen)定(ding)工作(zuo)(zuo)(zuo)(zuo)電(dian)流(liu)(liu)(liu)，那么你制(zhi)作(zuo)(zuo)(zuo)(zuo)的(de)(de)(de)可(ke)能會出現(xian)(xian)下(xia)述不正常現(xian)(xian)象：穩壓電源輸出的(de)有載電壓和空載電壓差別(bie)較大。
在(zai)應用中(zhong)，為(wei)了電(dian)路(lu)的穩定工作，在(zai)一(yi)般(ban)情況(kuang)下，還需(xu)要接二極管作為(wei)保護(hu)電(dian)路(lu)，防止電(dian)路(lu)中(zhong)的電(dian)容放電(dian)時的高壓(ya)把317燒壞。1.要(yao)(yao)解(jie)決(jue)317穩(wen)(wen)壓塊最(zui)小穩(wen)(wen)定(ding)(ding)(ding)工(gong)(gong)作電流的(de)問題(ti)，可以通過設定(ding)(ding)(ding)R1和(he)R2阻值(zhi)(zhi)的(de)大(da)小，而(er)(er)使317穩(wen)(wen)壓塊空(kong)載時(shi)輸出的(de)電流大(da)于(yu)(yu)或等于(yu)(yu)其最(zui)小穩(wen)(wen)定(ding)(ding)(ding)工(gong)(gong)作電流，從(cong)而(er)(er)保(bao)證317穩(wen)(wen)壓塊在(zai)空(kong)載時(shi)能夠(gou)穩(wen)(wen)定(ding)(ding)(ding)地工(gong)(gong)作。此(ci)時(shi)，只要(yao)(yao)保(bao)證Vo/（R1＋R2）≥1.5mA，就可以保(bao)證317穩(wen)(wen)壓塊在(zai)空(kong)載時(shi)能夠(gou)穩(wen)(wen)定(ding)(ding)(ding)地工(gong)(gong)作。上(shang)式中的(de)1.5mA為317穩(wen)(wen)壓塊的(de)最(zui)小穩(wen)(wen)定(ding)(ding)(ding)工(gong)(gong)作電流。當然，只要(yao)(yao)能保(bao)證317穩(wen)(wen)壓塊在(zai)空(kong)載時(shi)能夠(gou)穩(wen)(wen)定(ding)(ding)(ding)地工(gong)(gong)作，Vo/（R1＋R2）的(de)值(zhi)(zhi)也(ye)可以設定(ding)(ding)(ding)為大(da)于(yu)(yu)1.5mA的(de)任意值(zhi)(zhi)。
2.經計(ji)算可知R1的(de)(de)最(zui)大取(qu)值(zhi)為(wei)R1≈0.83KΩ。又(you)因為(wei)R2/R1的(de)(de)最(zui)大值(zhi)為(wei)28.6。所(suo)以R2的(de)(de)最(zui)大取(qu)值(zhi)為(wei)R2≈23.74KΩ。在使用317穩壓塊的(de)(de)輸(shu)出電壓計(ji)算公式計(ji)算其(qi)輸(shu)出電壓時(shi)(shi)，必(bi)須保證(zheng)R1≥0.83KΩ，R2≤23.74KΩ兩個不等式同時(shi)(shi)成立，才(cai)能保證(zheng)317穩壓塊在空載(zai)時(shi)(shi)能夠穩定(ding)地(di)工作。
3.當(dang)然在(zai)317穩(wen)(wen)壓(ya)(ya)塊(kuai)的(de)(de)輸出端并聯泄流(liu)電(dian)(dian)(dian)阻R也可以為317穩(wen)(wen)壓(ya)(ya)塊(kuai)提供最小穩(wen)(wen)定(ding)工(gong)作電(dian)(dian)(dian)流(liu)。但是，由于并聯的(de)(de)泄流(liu)電(dian)(dian)(dian)阻不(bu)能隨輸出電(dian)(dian)(dian)壓(ya)(ya)的(de)(de)變(bian)化(hua)而變(bian)化(hua)，如(ru)果要保證317穩(wen)(wen)壓(ya)(ya)塊(kuai)在(zai)輸出電(dian)(dian)(dian)壓(ya)(ya)為1.25V時，其輸出電(dian)(dian)(dian)流(liu)大于其最小穩(wen)(wen)定(ding)工(gong)作電(dian)(dian)(dian)流(liu)，則在(zai)317穩(wen)(wen)壓(ya)(ya)塊(kuai)的(de)(de)輸出電(dian)(dian)(dian)壓(ya)(ya)為37V時，流(liu)過泄流(liu)電(dian)(dian)(dian)阻的(de)(de)電(dian)(dian)(dian)流(liu)就(jiu)太大了，這樣(yang)不(bu)僅(jin)浪費了電(dian)(dian)(dian)能，而且增加了317穩(wen)(wen)壓(ya)(ya)塊(kuai)的(de)(de)負(fu)擔，不(bu)是一種妥當(dang)的(de)(de)辦法。
- 上一篇：可調直流穩壓電源有哪些分類？使用場合 2019/5/24
- 下一篇：大功率直流穩壓電源的設計一般包括那些環節 2019/5/23