BySquadron Leader Zahid Mir

CS&IT Department , Superior University

PHY-BE -09 Rectifier Filters

Rectifier Filter Circuits

• The purpose of the power supply is to reduce the fluctuations in the output voltage of a half-wave or full-wave rectifier and produce a nearly constant-level dc voltage.

• Filtering is done using capacitors, inductors or combination of both.

Capacitor Filter

Fig a

Fig b

• During the positive first quarter-cycle of the input, the diode is forward-biased, allowing the capacitor to charge to within a diode drop of the input peak (Fig a).

• When the input begins to decrease below its peak, the capacitor retains its charge and the diode becomes reverse-biased.

• During the remaining part of the cycle, the capacitor discharge only through the load resistance at a rate determined by the RLC time constant. (Fig b)

• Larger the time constant, the less the capacitor will discharge.

Capacitor Filter

• During the first quarter of the next cycle, the diode will again become forward-biased when the input voltage exceeds the capacitor voltage by approximately a diode drop (Fig c).

Fig c

Results • Large ripples means less effective

filtering .

• Smaller ripple means more effective filtering.

• For a given input frequency, the output frequency of a half-wave rectifier is twice that of a half-wave rectifier.

• When filtered, the full-wave rectified voltage has less ripple than does a half-wave rectified voltage.– This is because the capacitor

discharges less during the shorter interval between full-wave pulses.

Ripple Voltage

The variation in the output voltage due to charging and discharging of the capacitor is called ripple voltage.

Ripple Factor

• Ripple factor is the indication of the effectiveness of the filter and is defined as;

r = Vr / Vdc

Vr = rms ripple voltage.

Vdc = dc (average) value of the filter’s output voltage.

• The lower the ripple factor, the better the filter.• Ripple factor can be lowered by increasing the value of the filter capacitor.

References

• “Electronic Devices”by

Thomas L. Floyd (7th Edition)