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 os 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.filter circuits 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.1. 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 -filter. We shall discuss these filters in turn.

(i) Capacitor filter. Fig. 2 shows a typical capacitor filter circuit. It
consists of a capacitor C placed across the rectifier output in parallel with
load R L . 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. 2], the

…………………………………………………………………………………………………………………………..

• If such a d.c. is applied in an electronic circuit, it will produce a hum.
  ** Capacitor offers infinite reactance to d.c. For d.c., =0.
  X c = = =∞

Hence, a capacitor does not allow d.c. to pass through it.
† We know X L = For d.c., = 0 and, therefore, X L = 0. Hence inductor passes k.c.
quite readily. For a.c., it offers opposition and drops a part of it.
…………………………………………………………………………………………………………………………..
the capacitor is charged to the peak value V m rectifier voltage. Now, the rectifier
voltage starts to decrease. As these occurs, the capacitor discharges through the load
and voltage across it (i.e. across parallel combination of R-C) decreases as shows by
the line AB in Fig.1.

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 ABCEFG. 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 prefer.

It is commonly used in transistor radio battery eliminators.

(ii) Choke input filter. Fig. 2 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 a single filter section is show, but
several identical sections are often use to reduce the pulsations as
effectively as possible.
The pulsating output of the rectifier is apply 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 wholw 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 by passes 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.
(iii) Capacitor input filter or - filter. shows a typical capacitor input
filter or **-filter.

It consists of a filter capacitor C 1 connected across the
rectifier output, a choke L in series and

…………………………………………………………………………………………………………………………..

 The shorthand name of inductor coil is choke.
 The shape of the circuit diagram of this filter circuit appears like Greek letter (pi
and hence the name -filter.
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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 terminal (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 by passes an appreciable amount of a.c. component while the
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 *un by passed a.c. component is blocked.
(c) The filter capacitor C₂ by passes 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.

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