What Is A Filter Circuit

What Is A Filter Circuit

A rectifier is actually required to produce pure d.c. supply for using  at various places in the electronics circuits. However, the output of a rectifier is  pulsating. That means it contains both  a.c.component and d.c. component. If such a pulsating d.c. is applied in an electronics circuit, it will produce a hum. So the a.c. component in the pulsating rectifier output is undesirable and must be kept away  from the load. To do so, a filter circuit is used which removes the a.c. component  and allows only the d.c. component .

A filter circuit is a device which removes the a.c. component of rectifier output  and allows only d.c. component  to reach the load.

A filter circuit is installed between the rectifier and the load  as shown in fig.1(i)

Fig.1(i)

Rectifier Output                                                          Pure D.C. Output

Fig.1(ii)

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 C passes a.c. readily but does not pass d.c. at all. On the other hand, an inductor L  opposes a.c. but allows d.c. to pass  through it.  Hence a suitable network of  L and C can effectively  remove the a.c. component  and allows only  the d.c. component to reach the load.

Types of Filter Circuits

The three most commonly used filter circuits are:

1. Capacitor Filter
2. Choke input Filter
3. Capacitor input filter or π-Filter

Capacitor Filter

The circuit diagram of a typical capacitor filter is shown in Fig.2(i).Its input and output waveform are shown  in fig.2(ii) and Fig.2(iii) respectively.

Fig.2(i)

Fig.2(ii)

Fig.2(iii)

The capacitor filter circuit consists of a capacitor C placed across the rectifier output in parallel with load resistance R_L. The pulsating d.c. output of the rectifier is applied across the capacitor.

As the rectifier output voltage increases, it charges the capacitor and also supplies current to the load. At the end of quarter cycle i.e. at point A in fig.2(iii), the capacitor is charged to the peak value of the rectifier voltage i.e. V_m.

As the rectifier voltage now starts to decrease, the capacitor discharges through the load and voltage across it decreases. So voltage across R_L also decreases. This is shown by the line AB in fig.2(iii).

The voltage across the 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 as shown in the fig.2(iii).

We can see that very little ripple is left in the output.

The output voltage is also higher as it remains substantially near the peak value of rectifier output voltage.

Advantages of Capacitor Filter

1. Capacitor filter circuits are extremely popular because of its low cost.
2. These filters are of very small size.
3. It has a little weight.
4. It has good characteristics.

For small load currents up to 50mA this type of filter is preferred. It is commonly used in transistor radio battery eliminators.

Choke Input Filter

Fig.3(i) shows a typical choke input filter circuit.The rectifier output which is applied as input to the choke input filter is shown in fig.3(ii) and the output of this filter circuit is shown in Fig.3(iii).

Fig.3(i)

Fig.3(ii)

Fig.3(iii)

The circuit of a choke input filter consists of a choke L connected in series with the rectifier output and a filter capacitor C, which is connected across the load resistance R_L. Here in this fig.3(i) only a single filter section is shown. But normally several identical sections are used to reduce the pulsations as effectively as possible.

The pulsating output of the rectifier is applied across the terminal 1 and 2 of the filter circuit.

This pulsating output contains both a.c. and d.c. component.

As we know, the choke L offers a high resistance to the passage of a.c. component and passes the d.c. component readily.

So most of the a.c. component appears across the choke L , while all the d.c. component passes through the choke L on its way to the load.

This results in the reduced pulsation at terminal 3 as most of the a.c. component are blocked by the choke L now.

At terminal 3, the rectifier output contains d.c. component and the remaining part of a.c. component which are managed to pass through the choke L.

Now, the filter capacitor by passes the a.c. component but opposes the d.c. component to flow through it.

Therefore, only the d.c. component reaches the load R_L.

Capacitor Input Filter or π-Filter              

Fig.4(i) shows the circuit diagram of a typical capacitor input filter or π–filter. Fig.4(ii) shows the rectifier output, which is applied as the filter input and the filter output wave forms.

Fig.4(i)

Fig.4(ii)

As we can see in Fig.4(i) the shape of the circuit diagram of this filter circuit looks like π, hence it is also known as π-filter.

In this circuit a filter capacitor C₁ is connected across the rectifier output.

A choke L is connected in series and another filter capacitor C₂ is connected across the load.

Here only one filter section is shown, but most often several identical sections are used to improve the smoothing action.

The pulsating output from the rectifier is applied across the input terminals 1 and 2 of the filter.

The filtering action of the three components i.e. C₁,L  and C₂ of the filter is described as below.

The filter capacitor C₁ offers low reactance to the a.c. component of the 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.

The choke L offers a high reactance to the a.c. component while 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.

The filter capacitor C₂ bypasses the a.c. component which the chock L has failed to block.

Hence, only the d.c. component appears across the load R_L.