Sunday, May 20, 2012

Filter Circuits

       We have seen that output a half wave or full wave rectifier circuit is not pure d.c.; but it contains fluctuation or ripple, which are undesired. To minimize the ripple content in the output, filter circuits are used. These circuits are connected between the rectifier and load, as shown in the Fig. 1.
Fig. 1  Power supply using rectifier and filter

       An a.c. input is applied to the rectifier. At the output of the rectifier, there will be d.c. and ripple voltage present, which is the input to the filter. Ideally the output of the filter should be pure d.c. Practically, the filter circuit will try to minimize the ripple at the output, as far as possible.
       Basically the ripple is a.c., i.e. varying with time, while d.c. is a constant with respect to time. Hence in order to separate d.c. from ripple, the filter circuit should use components which have widely different impedance for a.c. and d.c. Two such components are inductance and capacitance. Ideally, the inductance acts as a short circuit for d.c., but it has a large impedance for a.c. Similarly, the capacitor acts as open for d.c. and almost short for a.c. if the value of capacitance is sufficiently large enough.
       Since ideally, inductance acts as short circuit for d.c., it can not be placed in shunt arm across the load, otherwise the d.c. will be shorted.
Note : Hence, in a filter circuit, the inductance is always connected in series with the load.
       The inductance used in filter circuits is also called ''choke''.
        Similarly, since the capacitance is open for d.c., i.e. it blocks dc.; hence it can not be connected in series with the load.
Note : It is always connected in shunt arm, parallel to the load.
       Thus filter is an electronic circuit composed of capacitor, inductor or combination of both and connected between the rectifier and the load so as to convert pulsating d.c. to pure d.c.
      There are basically two types of filter circuits,
- Capacitor input filter
- Choke input filter
       Looking from the rectifier side, if the first element, in the filter circuit is capacitor then the filter circuit is called capacitor input filter. While if the first element is an inductor, it is called choke input filter. The choke input filter is not in use nowadays as inductors are bulky, expensive and consume more power.

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