To remove the AC components or filter them out in a rectifier circuit, a filter circuit is used. A filter circuit is a device to remove the A.C components of the rectified output, but allows the D.C components to reach the load. A filter circuit is in general a combination of inductor (L) and Capacitor (C) called LC filter circuit. A capacitor allows A.C only and inductor allows D.C only to pass. So a suitable L and C network can effectively filter out the A.C component from rectified wave.
A filter circuit consists of passive circuit elements i.e, inductors, capacitors, resistors and their combination. The filter action depends upon the electrical properties of passive circuit elements. For example, an inductor allows the D.C to pass through it. But it blocks A.C. On the other hand, a capacitor allows the A.C to pass through it. But it blocks the D.C. Some of the important filters are given below.
This type of filter is also called choke filter. It consists of an inductor L which is inserted between the rectifier and the load resistance RL. The rectifier contains A.C components as well as D.C components. When the output passes through the inductor, it offers a high resistance to the A.C component and no resistance to D.C components. Therefore, A.C components of the rectified output is blocked and only D.C components reached at the load.
In this filter a capacitor is connected across the load during the rise of voltage cycle it gets charge and this charge is supply to the load during the fall in the voltage cycle. This process is repeated for each cycle and thus the repel is reduced across the load. It is shown in the above Figure. It is popular, because of its low cost, small size, less weight and good characteristics. Useful for load up to 50mA as in transistor radio battery eliminators.
In inductor filter, the ripple factor is directly proportional to the load resistance. On the other hand in a capacitor filter, it is varying inversely with the load resistance. Hence if we combine the inductor filter with the capacitor the ripple factor will become almost independent of the load filter. It is also known as inductor input filter, choke input filter, L input or LC-section.
In this circuit a choke is connected in series with the load. It offers high resistances to the AC components and allows DC component to flow through the load. The capacitor across the load is connected in parallel which filter out any AC component flowing through the choke. In this way the reppls are rectified and a smooth DC is provided through the load.
It consists of one inductor and two capacitor connected across its each end. The three components are arranged in shape of Greek letter Pi. It is also called capacitor input Pi filter. The input capacitor C1 is selected to offer very low reactance to the repel frequency hence major parts of filtering is done by C1. Most of the remaining repels are removed by the combining action of L and C2. This circuit gives much better filter then LC filter. However C1 is still directly connected across the supply and would need high pulse of current if load current is large. This filter is used for the low current equipment’s.