In power electronics and rectifier theory, which description best characterizes a pulsating DC voltage waveform?

Introduction / Context:
Pulsating DC appears at the output of rectifier circuits before filtering. It is not pure DC (constant level) nor pure AC (alternating polarity). Recognizing the waveform’s essential properties matters when specifying filters, ripple voltage tolerances, and load regulation. This question contrasts pulsating DC with steady DC and true AC waveforms.

Given Data / Assumptions:

• Consider typical rectifier outputs (half-wave or full-wave) without smoothing capacitors.
• Polarity relative to a reference (ground) is maintained for pulsating DC.
• Periodicity is due to the rectified mains or source frequency.

Concept / Approach:
Pulsating DC is characterized by unidirectional polarity with time-varying magnitude. A half-wave rectifier yields pulses separated by zero intervals; a full-wave rectifier yields more frequent pulses at twice the AC input frequency. Only after adding filters (capacitors, inductors, or regulators) does the waveform approach steady DC. If the polarity reverses, the signal is AC, not pulsating DC.

Step-by-Step Solution:
Identify the polarity behavior: remains one-sided (positive or negative with respect to ground).Identify magnitude behavior: varies over time, peaking and dropping periodically.Match to the option that explicitly states “one polarity but changes value periodically.”Select option B.

Verification / Alternative check:
Examine oscilloscope traces of a bridge-rectified sine wave before the filter: all peaks are above zero, but the level rises and falls each half-cycle, confirming the definition.

Why Other Options Are Wrong:
A describes regulated DC, not pulsating. C is physically contradictory (current cannot flow in two opposite directions simultaneously through a simple two-terminal element). D describes AC, which alternates polarity. “None” is incorrect because option B is precise.

Common Pitfalls:
Equating “pulsating” with “alternating”; forgetting that “DC” in this context means unidirectional, not necessarily constant magnitude.

Final Answer:
The voltage remains at one polarity but changes value periodically