What is filter circuit? How it works? why filter circuit is necessary in DC regulated Power supply?

What is the filter?

The circuit used for filtering the AC voltage variations from rectifier circuit output voltage is called a filter circuit.

The required output voltage of the rectifier is unidirectional and it has no fluctuations concerning the time. The output voltage of the rectifier is unidirectional but it fluctuates with respect to the time. It has an average or DC value on which AC components of different frequencies are superimposed (mixed).

These undesired AC components mixed in DC are called Ripples. Therefore filter circuits are necessary in DC Regulated Power supply.

How effectively the rectifier converts AC power into DC power is described by the ripple factor. The ripple factor is the ratio of RMS value of the component of output wave to the average or DC value. It is given by

Ripple Factor =RMS value of the component of output wave/average or DC value

For Half wave, it is 1.21 and for full-wave and bridge rectifier, it is 0.482.

Types of Filters circuits

The electronic circuit required constant DC voltage similar to the voltage from the battery for their working. The rectifiers cannot provide ripple-free DC voltage, they provide pulsating DC voltage.

“The circuit used for filtering the AC voltage variations from rectifier output voltage is called a Filter circuit.” Types of filter circuit are

1. Capacitor input filter
2. Inductor filter
3. LC filter
4. RC filter
5. π filter

Capacitor input filter

A large value capacitor is connected in parallel to load resistance R_L therefore it is also called a shunt capacitor filter. The rectified input voltage is applied across the capacitor and the filtered output voltage is taken across load resistor R_L as shown in the following circuit diagram.

How does the Capacitor input filter works?

When the Rectified voltage is applied, the capacitor charges to its peak voltage level. So the voltage across a capacitor becomes equal to the output voltage of the rectifier.

When the rectifier voltage decreases the capacitor starts discharges through load resistance R_L, so the output voltage also decreases. The charging and discharging of the capacitor is shown in the following waveform. The value of load resistance R_L is large so that the capacitor discharges slowly and the voltage across the capacitor falls slightly less than the peak value of rectified voltage.

When there is the next pulse the capacitor charges to a peak value. Thus, the process repeats and the output voltage almost equal to a peak value and smooth DC voltage is maintained across RL.

The discharging time (T) of a capacitor (C) depends on the product (T = C.R). If load resistance R_L is small, the capacitor will discharge quickly before the next half-cycle and the voltage across the load will not remain steady.

 

To avoid this problem, it is essential to keep a large value of load resistance R_L. Hence, this circuit is suitable for low current and high voltage applications only.

The filtering action of the capacitor filter can be also explained by using its reactance. The capacitive reactance of a capacitor is given by X_c =  1/ 2πFC

For DC the frequency is zero therefore the reactance is infinity, it means that the capacitor acts as an open for DC. So the DC components cannot pass through a capacitor it passes only through the load resistance R_L.

As AC having the frequency a capacitor acts as a short and so AC components pass through the capacitor to ground. In this way, a capacitor maintains a smooth DC voltage across it.

Advantages:

1. It is low cost and small size filter.
2. It is not affected by magnetic interference.
3. It is suitable for high voltage applications.

Disadvantages:

1. It is not suitable for heavy load currents.
2. It requires a large value of the capacitor.

you can read details about capacitor filter by other authors here Capacitor filter

Series Inductor Filter

As the capacitor input filter circuit can not work with the heavy load currents so we need to use the inductor filter circuit. In this filter, an inductor is connected in series with a load resistance R_L. It is also called a choke filter.

The inductive reactance is given by X_C = 2πFL. The inductive reactance increases with an increase in the frequency of the current. It means that the inductor opposes any change of current through it. This property of inductor is used in an inductor filter. Whenever the current through inductor changes it produces back emf in it and prevents the change in current.

The inductor opposes the AC component and passes only DC towards the load resistor so the voltage across the load is DC voltage. The working of this filter depends upon the current through it. For higher current, the reactance of the AC component is more and hence its filtering action is better. So it is suitable where high current is required and it is not suitable for the half-wave rectifier.

Advantages: –

1. The Ripples are minimized for the heavy load current.
2. The output of this filter is almost smooth DC voltage.

Disadvantages:-

1. This circuit is not suitable for the low load current.
2. The size of this filter is more than a capacitor filter.
3. The cost of this filter is more than a capacitor filter.