Filter Circuits

A rectifier's raw output -- whether half wave or full wave -- is unidirectional but still far from steady: it keeps fluctuating because of the ripple component present in it. A FILTER CIRCUIT is the block placed right after the rectifier specifically to remove this AC ripple component (or as much of it as possible) and pass through mainly the DC component, so that the load sees a much steadier voltage than the raw rectified output. The simplest and most widely used filter is the CAPACITOR filter: a capacitor C is connected in PARALLEL with the load resistor $R_L$, directly across the rectifier's pulsating output. As the rectified voltage rises during each pulse, the capacitor charges up, reaching the peak rectified voltage $V_p$ at the end of a quarter cycle. Once the rectified voltage begins to fall again (as the diode's conduction pulse ends), the capacitor cannot discharge instantly -- it discharges only gradually through $R_L$, so the voltage across the load drops only slightly, down to a point B, before the NEXT rectified pulse arrives and recharges the capacitor straight back up to $V_p$. Repeating every cycle, this charge-discharge action smooths the sharply pulsating rectified waveform into a much gentler, nearly-flat output with only a small residual ripple (Fig. 16.6c) -- still technically an UNREGULATED DC supply (its output level still shifts somewhat if the load current changes), but a large practical improvement over the raw rectifier output. The capacitor filter's low cost, small size, and light weight make it especially popular for small-load-current applications such as battery eliminators. A power supply whose output voltage changes noticeably w …