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Let's go a step further and redraw the filter circuit so that you can see the voltage divider action. Refer to view A of figure 424. Remember, the 165 volts peak 60 hertz provided by the rectifier consists of both an ac and a dc component. This first discussion will be about the ac component. From the figure, you see that the capacitor (C1) offers the least opposition (265 ohms) to the ac component. Therefore, the greater amount of ac will flow through C1. (The heavy line in view B indicates the ac current flow through the capacitor.) Thus the capacitor bypasses, or shunts, most of the ac around the load. Figure 424.  Ac component in an LC chokeinput filter.
By combining the X_{C} of C1 and the resistance of R_{L} into an equivalent circuit (view B), you will have an equivalent impedance of 265 ohms. As a formula;
You now have a voltage divider as illustrated in figure 425. You should see that because of the impedance ratios, a large amount of ripple voltage is dropped across L1, and a substantially smaller amount is dropped across C1 and R_{L}. You can further increase the ripple voltage across L1 by increasing the inductance (X_{L} = 2pfL).
Figure 425.  Equivalent circuit of an LC chokeinput filter.
Now let's discuss the dc component of the applied voltage. Remember, a capacitor offers an infinite impedance to the flow of direct current. The dc component, therefore, must flow through R_{L} and L1. As far as the dc is concerned, the capacitor does not exist. The coil and the load are therefore in series with each other. The dc resistance of a filter choke is very low (50 ohms average). Consequently, most of the dc component is developed across the load and a very small amount of the dc voltage is dropped across the coil, as shown in figure 426. Figure 426.  Dc component in an LC chokeinput filter.
As you may have noticed, both the ac and the dc components flow through L1. Because it is frequency sensitive, the coil provides a large resistance to ac and a small resistance to dc. In other words, the coil opposes any change in current. This property makes the coil a highly desirable filter component. Note that the filtering action of the LC chokeinput filter is improved when the filter is used in conjunction with a fullwave rectifier, as shown in figure 427. This is due to the decrease in the X_{C} of the filter capacitor and the increase in the X_{L }of the choke. Remember, ripple frequency of a fullwave rectifier is twice that of a halfwave rectifier. For 60hertz input, the ripple will be 120 hertz. The X_{C} of C1 and the X_{L} of L1 are calculated as follows:
Figure 427.  Fullwave rectifier with an LC chokeinput filter.
When the X_{C} of a filter capacitor is decreased, it provides less opposition to the flow of ac. The greater the ac flow through the capacitor, the lower the flow through the load. Conversely, the larger the X_{L} of the choke, the greater the amount of ac ripple developed across the choke; consequently, less ripple is developed across the load and better filtering is obtained. Q.21 In an LC chokeinput filter, what prevents the rapid charging of the capacitor? 