Adding a capacitor filter to a rectifier output: When a suitably sized capacitor is connected as a filter across the load of a rectifier, what is the principal effect on the output waveform?

Introduction / Context:
Rectifiers deliver a pulsating output. Adding a filter capacitor after the rectifier smooths the waveform, moving the output closer to a steady DC level. Understanding this effect is fundamental for designing linear power supplies and for estimating ripple under load.

Given Data / Assumptions:

• Pulsating rectified input (half- or full-wave) feeding a resistive load.
• A capacitor connected in parallel with the load (across the output).
• Idealized diodes for conceptual clarity; real ESR and diode drops are secondary effects.

Concept / Approach:
The filter capacitor charges up to near the peak of the rectified voltage and discharges slowly through the load between peaks. This “peak hold” and gradual discharge reduce the depth of the valleys between peaks, thus reducing ripple. With sufficient capacitance and light load, the output approaches a nearly-constant DC level close to the rectified peak (minus diode drops). Ripple magnitude Vr(pp) ≈ I_load / (f_ripple * C) for simple estimates, where f_ripple is mains frequency for half-wave and twice mains for full-wave.

Step-by-Step Solution:

Verification / Alternative check:
Scope the output before and after adding the capacitor. You will see the valleys filled in, producing a flatter waveform. Increasing C or using a full-wave rectifier reduces ripple further, consistent with Vr(pp) ≈ I/(f*C).

Why Other Options Are Wrong:

• Increase ripple voltage: The capacitor filter reduces ripple for a given load and frequency.
• Lower the surge current at turn-on only: In fact, a large capacitor can increase inrush at turn-on.
• Reduce the output voltage under all conditions: With light loads, the DC level typically increases toward the peak.
• Invert polarity: Filtering does not change polarity; rectification determines polarity.

Common Pitfalls:
Ignoring load dependence of ripple; assuming infinite smoothing with any capacitor; overlooking inrush current and diode/transformer stress at power-up.

Final Answer:
produce a nearly-constant dc voltage