Filters-LC and Pi.pdf - Shunt Capacitor Filter Figure 1 1 1...

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Unformatted text preview: Shunt Capacitor Filter Figure 1: 1 1 Use this gure while wrtting notes on shunt capacitor lter. Notes already shared does not cotain gure 1 Series Inductor Filter Figure 2: 2 2 Use this gure while wrtting notes on Series inductor lter lter. Notes already shared does not cotain gure 2 Choke-Input LC Filter Figure shows a choke-input lter using an inductor L in series and capacitor C in shunt with load. We have seen that a series inductor lter has the feature of decreasing the ripples when the load current is increased. Reverse is the case with a shunt capacitor lter. In this case, as the load current is increased, the ripples also increase. An LC lter combines the features of both the series inductor lter and shunt capacitor lter. There fore, the ripples remain fairly the same even when the load current changes. The choke (iron-core inductor) allows the dc component to pass through casily because its dc resistance R is very small. For dc, the capacitor appears as open circuit and all the dc current passes through the load resistance RL . Therefore, the circuit acts like a voltage divider, and the output dc voltage is given by Eq. Vdc = RL 0 R + RL Vdc The fundamental frequency of the ac component in the output of the rectier is 100 Hz (twice the line frequency). For this ac, the reactance XL (= 2πf L) is high. The ac current has diculty in passing through the inductor. Even if some ac current manages to pass through the choke, it fows through the low reactance XC (=1/2πf C ) rather than through load resistance RL . The ripples are reduced very eectively because XL is much greater than Xc , and Xc is much smaller than R. The cirecuit works like the ac voltage divider of Fig. 0 If Vr is the rms value of the ripple voltage from the full-wave rectier, then the rms value of the output ripple is given as Vdc = Xc 0 V XL r 3 Figure 3: In the capacitor-input lter, the current ows through the transformer in a series of pulses. But in the choke-input lter, the current ows continuously. This means, the transformer is utilized more efciently. A further advantage of the choke-input lter is that the ripple content at the output is not only low but is also less dependent on the load current. π -Filter Very often, in addition to the LC lter, we use an additional capacitor C for providing smoother output voltage. This lter is called π lter (its shape is like the Greek letter π ). Such a lter is shown in Fig. 4 Figure 4: The rectier now feeds directly into the capacitor C. Therefore, the lter is also called capacitor- input lter. Since the rectier feeds into the capacitorC1 , this type of lter can be used together with a half-wave rectier. (The chock-input lter cannot be used with a half-wave rectier.) The disadvantages of the capacitor-input LC lter are the cost, weight, size and external eld produced by the series inductor. 5 ...
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