We know that ac power can be changed to dc power, But the dc output of the rectifier circuit was not a pure direct current-it was a pulsating direct current. These pulsations are known as the ac component of the rectifier output. Later we see that a “ripple” seems to “ride “on the dc output of a power supply filter. This is called the ac component. Also electronic circuitry usually requires a pure, unvarying dc supply. To obtain this pure dc supply .To obtain this pure dc supply, filter circuit must be attached to the rectifier. A filter circuit filters out the pulsation (accomponent ,or ripple).There are a number ofdifferent filter circuits, each using a different arrangement of two components which produces filtering of the pulsating dc supply. These two components are the capacitor and inductor. The ability of the capacitor to act as a filter is related to its capacitance and reactance. The larger the capacitor, the better the filtering action attainable. The filtering action of a choke is related to its inductance. As with the capacitor, the larger its reactance and inductance, the better the filtering action.
Fig.6-1 (a) full-wave power supply with simple capacitor Filter ;(b)full-wave rectifier output ,unfiltered The simplest possible power supply filter is the single capacitor shown in Fig.6-1
In Fig.6-1 we realize that ehere is an outout pulse across RL for each half-cycle of input.The unfiltered rectifier output waveform is deawn in Fig.6-1(b).Capacitor C1 is connected across the rectifier output and charges to the peak input pulse voltage. When it is charged as shown in FIG.6-1, the cathodes of D1and D2 are positive and reverse-biased. There is no path for capacitor discharge except through its own leakage or RL. So C1 maintains most of its charge, and D1 and D2 are cut off. As RL uses current from the capacitor, its voltage drops. As this voltage drops,D1and D2 conduct again, allowing C1 to recharge. The recharging replaces the small charge lost by C1 during the time when there was no out put rrom the rectifier, as in Fig.6-1.In this way the filter capacitor C1 stays charged to approximately the peak input voltage. The goal of any filter system is to maintain this voltage so there is no discharge during intervals when the diodes are not conducting. Of course, theirs is impossible for there is always some amount of baritone or ripple, in the output , eventhough it may be on the order of some millivolts. The amount of output voltage variation, or ripple, depends on the size of the capacitor and the size of the load. If RL is small in value, more current can flow from the capacitorduring the diode nonconducting interval. Thus C1 must be large enough to supply operating current to the load without discharging moue than a few millivolts. Likewise , C1 must have a large capacitance value is order to provide operating current for the load when D1 and D2 are not conducting. If C1 is too small, it discharges during the D1and D2 nonconducting in terval, and more output ripples result.