The rectifiers and filters are the circuits which are used for generation DC power supply. A rectifier is a circuit which converts the AC voltage to DC voltage. The output of the of rectifier contains two components namely AC component and DC component. This AC component is undesirable and causes pulsation in the rectifier output. Such issue is solved by the filter circuits. Let's explore more about it.
Filters :
In above figure the full wave rectified output voltage is applied at as input. The output of a filter is not exactly a constant DC level but still contains small amount of AC component. Thus filter is a circuit which removes or minimizes the unwanted AC component of rectifier circuit and allows the DC component to reach the load. Filter circuit consists of passive circuit components such as inductors, capacitors, resistors and their combinations.
The filtering action depends upon the basic electrical properties of the passive circuit elements. For example , an inductor allows the DC current to pass through it and blocks AC current. However a capacitor allows the AC current to pass through it and block DC current.
Inductor Filter :
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| Fig.2 Inductor Filter |
Ripple factor of an inductor filter ( for 50Hz Supply ) is given as
ץ = RL
3√2 .⍵L
= RL
1330L
where RL is in ohms and L is Henry. In equation it is clear that the ripple will decrease as the value of RL is decreased and L is increased.
The inductor filter is most effective for heavy load current i.e. when the load resistance RL is low. The AC component present at a output of filter, can be reduced if we use an inductor of a large value. But it may be noted that an inductor of a large value will also have a higher DC resistance, which will result in a lower DC output voltage.
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| Fig.3 Capacitor Filter |
As discussed earlier, a inductor filter is suitable only for heavy loads i.e. loads with low resistance.
For light loads i.e. load with high resistance, capacitor filter is used. The operation of a capacitor filter depends upon the property of a capacitor to oppose any change in voltage , when it is connected across a pulsating DC voltage. The action of a capacitor filter is to be smoothed out the voltage ripples.
Working : During the positive half-cycle of AC input voltage, the diode is forward biased. It allows the capacitor to charge quickly to its maximum value of the voltage i.e. point a in above fig.3. Since there is no resistance in the charging path, except the diode forward resistance which is almost null, therefore the charging time is almost zero. Therefore the capacitor follows the charging voltage.
When AC input voltage begin to decrease below its maximum value, the capacitor returns its charge.
During the negative half-cycle of the input, the capacitor tries to discharge
through load resistance ( curve ab in above fig.3 ). As diode is reverse biased during negative cycle of input voltage, capacitor cannot discharge through it.
The discharging time is determine by the capacitance of capacitor ( C ) and value of load resistance
( RL ) i.e.
τ = C.RL
It is interesting to know that larger the time-constant, lesser will be the capacitor discharge. In actual practice, the discharging time constant is kept 100 times the larger than that of the charging time.
Thus capacitor does not have enough time to discharge appreciably. As a result of this, the capacitor maintains a large voltage across the load resistance RL.
During the next half positive cycle, when the input voltage exceeds the previous voltage level at capacitor at point b in above fig.3. The diode is again become forward biased as the anode side is more positive than cathode side. The capacitor again charges to its maximum voltage level i.e. point c.
Due to such charging and discharging of a capacitor, there is a ripple ( AC component ) present in the output voltage of a capacitor filter.
The ripple factor of a capacitor filter ( for 50Hz ) is given by ץ,
ripple factor = 1 / 4√3 .f.C.RL
where f is frequency , C is capacitance in µF and RL in ohms.