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    Transformer Power Supply and Filter
    變壓器供電和濾波器
    作者:佚名
    譯者:佚名
    We know that ac power can be changed to dc power, But the dc output of the rectifier circuit was not a pure direct current-it was a pulsating direct current. These pulsations are known as the ac component of the rectifier output. Later we see that a “ripple” seems to “ride “on the dc output of a power supply filter. This is called the ac component. Also electronic circuitry usually requires a pure, unvarying dc supply. To obtain this pure dc supply .To obtain this pure dc supply, filter circuit must be attached to the rectifier. A filter circuit filters out the pulsation (accomponent ,or ripple).There are a number ofdifferent filter circuits, each using a different arrangement of two components which produces filtering of the pulsating dc supply. These two components are the capacitor and inductor. The ability of the capacitor to act as a filter is related to its capacitance and reactance. The larger the capacitor, the better the filtering action attainable. The filtering action of a choke is related to its inductance. As with the capacitor, the larger its reactance and inductance, the better the filtering action.  

    Fig.6-1 (a) full-wave power supply with simple capacitor Filter ;(b)full-wave rectifier output ,unfiltered The simplest possible power supply filter is the single capacitor shown in Fig.6-1

    In Fig.6-1 we realize that ehere is an outout pulse across RL for each half-cycle of input.The unfiltered rectifier output waveform is deawn in Fig.6-1(b).Capacitor C1 is connected across the rectifier output and charges to the peak input pulse voltage. When it is charged as shown in FIG.6-1, the cathodes of D1and D2 are positive and reverse-biased. There is no path for capacitor discharge except through its own leakage or RL. So C1 maintains most of its charge, and D1 and D2 are cut off. As RL uses current from the capacitor, its voltage drops. As this voltage drops,D1and D2 conduct again, allowing C1 to recharge. The recharging replaces the small charge lost by C1 during the time when there was no out put rrom the rectifier, as in Fig.6-1.In this way the filter capacitor C1 stays charged to approximately the peak input voltage. The goal of any filter system is to maintain this voltage so there is no discharge during intervals when the diodes are not conducting. Of course, theirs is impossible for there is always some amount of baritone or ripple, in the output , eventhough it may be on the order of some millivolts. The amount of output voltage variation, or ripple, depends on the size of the capacitor and the size of the load. If RL is small in value, more current can flow from the capacitorduring the diode nonconducting interval. Thus C1 must be large enough to supply operating current to the load without discharging moue than a few millivolts. Likewise , C1 must have a large capacitance value is order to provide operating current for the load when D1 and D2 are not conducting. If C1 is too small, it discharges during the D1and D2 nonconducting in terval, and more output ripples result.
      大家都知道,交流電可以轉換成直流電。但是整流器的輸出并不是純粹的直流,而是脈動直流。其實這些波動成分是整流電路輸出的交流分量。后來我們發現電源濾波器的直流輸出中疊加有紋波,這種紋波也稱為交流分量。通常電子電路中需要穩定不變的直流電源。為了獲得穩定不變的直流電,整流電路中必須附加濾波器。因為濾波電路會濾掉脈動成分(交流分量,或紋波)。目前有許多不同的濾波電路,主要利用兩種不同元件來達到濾波效果。這兩種元件是電容器和電感。電容器的濾波效果與它的電容量和容抗有關。電容器越大,濾波效果越好。電感線圈的濾波效果與它的電感量和感抗有關。與電容一樣,電抗和電感越大,濾波效果就越好。 最簡單可行的濾波電源采用一個電容器,如圖6-1所示。



      在圖6-l中我們發現在輸入回路每半波內 RL兩端均有波形輸出。圖 6-1(b)給出了未加濾波器的整流電路輸出波形。若將電容C1接于整流電路的輸出瑞,它可以充電到輸入電壓峰值、如圖6-1所示,此時C1和C2的陰極為貧,呈現反偏。除了本身的泄漏和RL外,電容器再無效電途徑。所以CI貯存了大量也荷,而D1和D2處于截止狀態,位于電容器向RL放電。它的電壓波形呈下降趨勢。當RL兩端電壓下降時,D1和D2開始導電,C1開始再充電。如圖6-1所示,在整流器設有輸出電流期間,C1損失的少量充行可在再充電時得到補充。這樣濾波電容器C1兩端電壓大體總保持在輸入電壓的峰值左右。任何濾波系統的目的都是保持這種電壓則保證在二極管截止時本會放電。當然這是不可能的,因為濾波器輸出總有一定波動或紋波,此波動或紋波有時僅為幾毫伏。 輸出電壓的波動或波紋的大小與電容器電容量的大小和負荷有關。如果 RL數值小。在二極管不導通期間。電容器的放電電流很大。這樣C1容量必須很大才能保證C1為負荷提供運行電流而放電量不超過幾毫伏。同樣,C1容量必須足夠大,才可以保證在D1和D2不導通期間為負載提供運行電流。如果C1容量太小。它在D1和D2不導通期間C1放電很大,會增大輸電壓的紋波。
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