What is Filter Circuits ?
Generally, a rectifier is required to produce pure d.c. supply for using at various places in the electronic circuits. However, the output of a rectifier has pulsating *character i.e. it contains a.c. and d.c. components. The a.c. component is undesirable and must be kept away from the load. To do so, a filter circuit is used which removes (or filters out) the a.c. component and allows only the d.c. component to reach the load.
A filter circuit is a device which removes the a.c. component of rectifier output but allows the
d.c. component to reach the load. Obviously, a filter circuit should be installed between the rectifier and the load as shown in Fig. 6.40. A filter circuit is generally a combination of inductors (L) and capacitors (C). The filtering action of L and C depends upon the basic electrical principles. A capacitor passes a.c. readily but does not **pass d.c. at all. On the other hand, an inductor †opposes a.c. but allows d.c. to pass through it. It then becomes clear that suitable network of L and C can effectively remove the a.c. component, allowing the d.c. component to reach the load.
Types of Filter Circuits
The most commonly used filter circuits are capacitor filter, choke input filter and capacitor input
filter or π-filter. We shall discuss these filters in turn.
Capacitor filter circuit
Fig. 6.41 (ii) shows a typical capacitor filter circuit. It consists of a capacitor C placed across the rectifier output in parallel with load RL. The pulsating direct voltage of the rectifier is applied across the capacitor. As the rectifier voltage increases, it charges the capacitor and also supplies current to the load. At the end of quarter cycle [Point A in Fig. 6.41 (iii)], the capacitor is charged to the peak value Vm of the rectifier voltage. Now, the rectifier voltage starts to decrease. As this occurs, the capacitor discharges through the load and voltage across it (i.e. across parallel combination of R-C) decreases as shown by the line AB in Fig. 6.41 (iii). The voltage across load will decrease only slightly because immediately the next voltage peak comes and recharges the capacitor. This process is repeated again and again and the output voltage waveform becomes ABCDEFG. It may be seen that very little ripple is left in the output. Moreover, output voltage is higher as it remains substantially near the peak value of rectifier output voltage.
The capacitor filter circuit is extremely popular because of its low cost, small size, little weight and good characteristics. For small load currents (say upto 50 mA), this type of filter is preferred. It is commonly used in transistor radio battery eliminators.
Choke input filter circuit
Fig. 6.42 shows a typical choke input filter circuit. It consists of a *choke L connected in series with the rectifier output and a filter capacitor C across the load. Only single filter section is shown, but several identical sections are often used to reduce the pulsations as effectively as possible. The pulsating output of the rectifier is applied across terminals 1 and 2 of the filter circuit. As discussed before, the pulsating output of rectifier contains a.c. and d.c. components. The choke offers high opposition to the passage of a.c. component but negligible opposition to the d.c. component. The result is that most of the a.c. component appears across the choke while whole of d.c. component passes through the choke on its way to load. This results in the reduced pulsations at terminal 3.
At terminal 3, the rectifier output contains d.c. component and the remaining part of a.c. component which has managed to pass through the choke. Now, the low reactance of filter capacitor bypasses the a.c. component but prevents the d.c. component to flow through it. Therefore, only d.c. component reaches the load. In this way, the filter circuit has filtered out the a.c. component from the rectifier output, allowing d.c. component to reach the load.
Capacitor input filter or π-filter circuit
Fig. 6.43 shows a typical capacitor input filter or **πfilter. It consists of a filter capacitor C1
connected across the rectifier output, a choke L in series and
another filter capacitor C2 connected across the load. Only one filter section is shown but several identical sections
are often used to improve the smoothing action.
The pulsating output from the
rectifier is applied across the input terminals (i.e. terminals 1 and 2) of the filter. The filtering action of the three components viz C1, L and C2 of this filter is described below :
(a) The filter capacitor C1 offers low reactance to a.c. component of rectifier output while it offers infinite reactance to the d.c. component. Therefore, capacitor C1 bypasses an appreciable amount of a.c. component while the d.c. component continues its journey to the choke L.
(b) The choke L offers high reactance to the a.c. component but it offers almost zero reactance to the d.c. component. Therefore, it allows the d.c. component to flow through it, while the *unbypassed a.c. component is blocked
(c) The filter capacitor C2 bypasses the a.c. component which the choke has failed to block.
Therefore, only d.c. component appears across the load and that is what we desire.
Example 6.23. For the circuit shown in Fig. 6.44, find the output d.c. voltage.
Solution. It can be proved that output d.c. voltage is given by :
Example 6.24. The choke of Fig. 6.45 has a d.c. resistance of 25 Ω. What is the d.c. voltage if the
full-wave signal into the choke has a peak value of 25.7 V ?
Solution. The output of a full-wave rectifier has a d.c. component and an a.c. component. Due to
the presence of a.c. component, the rectifier output has a pulsating character as shown in Fig. 6.46.
The maximum value of the pulsating output is Vm and d.c. component is V′dc = 2 Vm/π.
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Reference: Principles Of Electronics By V K Mehta And Rohit Mehta