By Squadron 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 R L C 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 = V r / V dc V r = rms ripple voltage. V dc = 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(7 th Edition)