鎳氫電池充電器原理
1.基準電壓Vref形成
外接電源經插座X、二極管VD1后由電容C1濾波。VD1起保護作用,防止外接電源極性反接時損壞TL431。R3、R4、R5和TL431組成基準電壓Vref,根據圖中參數Vref= 2.5×(100+820)/820=2.80(v),這個數據主要是針對鎳氫充電電池而設計(單節鎳氫充電電池充滿后電壓約
為1.40V)。
2.大電流充電
(1)工作原理
接入電源, 電源指示燈LED(VD2)點亮。裝入電池(參考圖片,實際上是用導線引出到電池盒, 電池裝在電池盒中),當電池電壓低于Vref時,IC1-1輸出低電平,VT1導通,輸出大電流給電池充電。此時,VT1處于放大狀態-這是因為電池電壓和-VD4壓降的和約為3.2V(假設開始充
電(dian)(dian)時(shi)電(dian)(dian)池電(dian)(dian)壓(ya)約為(wei)2.5V),而(er)經(jing)VD1后的電(dian)(dian)壓(ya)大約5.OV,所以(yi)(yi),VT1的發射極(ji)(ji)-集電(dian)(dian)極(ji)(ji)壓(ya)差遠大于(yu)0.2V,當(dang)充電(dian)(dian)電(dian)(dian)流為(wei)300mA時(shi),VT1發熱比(bi)較(jiao)嚴(yan)重,所以(yi)(yi)最好用PT=625mW的S8550,或(huo)者適當(dang)增大基極(ji)(ji)電(dian)(dian)阻以(yi)(yi)減(jian)小充電(dian)(dian)電(dian)(dian)流(注(zhu):由于(yu)LM324低電(dian)(dian)平驅動能(neng)力較(jiao)小,實測IC1-2,IC1-4輸出低電(dian)(dian)平并不(bu)是(shi)0V,而(er)是(shi)約為(wei)0.8V)。
(2)充電的指示
首先看IC1-3的工作情況:其同相端1O腳通過R13接Vref,R14接成正反饋,反相端9腳外接電容,并有一負反饋通路,所以,它實際上構成了滯回比較器。剛開始時C2上端沒有電壓,則IC1-3輸出高電平。這個高電平有兩個放電通路,一個通路是通過R14反饋到10腳,另一通路是經電阻R15對電容C2充電,當充電的電壓高于10腳電壓V+ 時,比較器翻轉輸出低電平;與此同時,由于R14的反饋作用,10腳電壓立即下跳到V-,這時,電容C2通過電阻R15放電,當放電的電壓小于10腳電壓V-時,比較器再次翻轉輸出高電平, 由于R14的反饋作用,10腳電壓立即上跳到V+,此后電路一直重復上述過程,因此,IC1-3的輸出為頻率固定的方波信號。
其次看IC1-4的工作情況:電池電壓經R2、R16分壓,接IC1-4的12腳,因為R2< 顯然它更低于其l3腳電壓 因此,IC1-4輸出穩定的低電平。結合上面的討論,我們可以看出,加在R12和VD 3通路一端為頻率固定的方波電
壓,另一端為穩定的低電平,因此,發光二極管VD3會周期性點亮,給人一閃一閃的感覺。
最后看IC1-1的工作情況:當IC1-2輸出低電平時,顯然IC1-1的3腳為低電平,而其2腳通過R1接Vref所以,IC1-1也輸出低電平。結合上面的討論,我們可以看出,R11和VD5兩端電壓差為零,因此,VD5(飽和指示)不能點亮!
另外,由于IC1-1輸出低電平,無論IC1-3的9腳電壓如何變化(電容充、放電在該腳形成三角波電壓)都不會受IC1-1輸出的影響— 因為IC1-3的9腳電壓(要么高到V+ ,要么低到V-)始終高于IC1-1的輸出,VD6反偏截止!所以,這種狀態下,三只指示燈的工作情況分別為:VD2點亮,指示電源正常;VD3閃爍,指示電池充電正常;VD5不亮。
3.小電流充電
當充電一段時間后,電池電壓慢慢上升到接近Vref時,IC1-2輸出電壓慢慢上升,于是,流過R7的電流慢慢減小,即流經VT1基極的電流慢慢減小,因此VT1輸出的電流也會慢慢減小,但電池電壓還會持續不斷地緩慢上升,當電池電壓幾乎等于Vref時,IC1-2會輸出較高電壓,這時IC1-1的3腳電壓高于2.8OV (反相端2腳的輸入端電壓), 比較器翻轉輸出高電平。該電壓有兩個作用:一方面會使VD5正偏導通被點亮(此時,IC1-4輸出還是低電平),指示充電飽和;另一方面VD6也正偏導通,而R17很小,實際上是強制C2上端為高電平,所以IC1-3的9腳電壓高于10腳電壓,IC1-3被強迫輸出低電平,VD3因無正偏壓而熄滅。
雖然,從外在的表現看充電燈熄滅,飽和燈點亮在某一時刻瞬間轉換完成,但是實際上充電過程卻是逐漸過渡的:當電池電壓遠低于Vref時持續大電流充電,當電池電壓接近于時充電電流慢慢減小,直至逐漸充電趨近零—— 即使飽和燈點亮時,小電流充電仍在繼續!所以這種狀態下,三只指示燈的工作情況分別為:VD2點亮,指示電源正常;VD3不亮;VD5點亮(飽和指示,小電流充電)。
4.IC1-4的用途
從上面2、3內容的分析中可以看出,無論電路是大電流或小電流充電,IC1-4的輸出一直是“低電平”,好像它沒有什么作用似的,還不如直接把VD3、VD5負極接“地”?剛開始設計時,確實沒有考慮用IC1-4,把VD3、VD5的負極直接接地。然而,當制作好后通電工作時發現一個問題:當不裝電池通電時,飽和指示燈VD5點亮—顯然不合適!因為,沒裝電池時VT1處于微導通狀態,IC 1-2的5腳電壓高于 ,IC1—2輸出高電平,于是IC1-2也輸出高電平,VD5點亮。
若在原理圖中接入IC1-4,沒裝電池時VT1處于微導通狀態,IC1-4的1 2腳電壓也會高于 ,因此,IC1-4輸出高電平,這樣VD5就不能點亮。
需要說(shuo)明(ming)一點(dian),外接(jie)輸(shu)入電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)不(bu)(bu)能(neng)太(tai)(tai)高(gao)(gao),也不(bu)(bu)能(neng)太(tai)(tai)低。輸(shu)入電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)太(tai)(tai)高(gao)(gao),大電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)時(shi)(shi)調(diao)整管發熱嚴重(zhong);另(ling)一方面(mian),IC1-2輸(shu)出高(gao)(gao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)平的(de)時(shi)(shi)間(jian)會因為電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)較高(gao)(gao)而(er)提前超過(guo)Vref(設定值),這樣(yang)就會給我們一個錯覺,電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)池很快就充(chong)滿了!實際上并非如此。輸(shu)入電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)太(tai)(tai)低也不(bu)(bu)好(hao),同上面(mian)的(de)分析一樣(yang),IC1-2輸(shu)出高(gao)(gao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)平的(de)時(shi)(shi)間(jian)會因為電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)源(yuan)(yuan)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)較低而(er)遲后,更有甚者,也可能(neng)永遠(yuan)達不(bu)(bu)到(dao)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)指(zhi)(zhi)示燈一直閃爍,但(dan)大電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)流(liu)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)過(guo)程早已結束。所以,外接(jie)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓(ya)太(tai)(tai)高(gao)(gao)或太(tai)(tai)低,充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)(dian)和(he)飽和(he)指(zhi)(zhi)示的(de)狀態是不(bu)(bu)準確的(de).