Power Supply Diagnostics: Measuring AC Ripple on a PC PSU To check for excessive ripple on a DC output rail of a PC power supply with a multimeter, which measurement mode should be used?

Introduction / Context:
Switch-mode PC power supplies output regulated DC with some residual AC ripple. Excessive ripple can cause instability, crashes, or component stress. A quick field check can be done with a multimeter; deeper analysis uses an oscilloscope. This question asks for the correct meter setting for ripple measurement.

Given Data / Assumptions:

• Measurement taken across a DC rail (e.g., +12 V, +5 V) and ground.
• Using a multimeter capable of AC RMS measurement on a DC offset (AC-coupled measurement).
• Ripple is small compared to the DC level.

Concept / Approach:

Ripple is the AC component superimposed on a DC output. To measure it with a DMM, select AC voltage mode so the meter filters out (or ignores) the DC component and reports the AC magnitude only. For precise ripple amplitude and frequency content, an oscilloscope is preferred.

Step-by-Step Solution:

Verification / Alternative check:

Use an oscilloscope with AC coupling to view ripple waveform and peak-to-peak amplitude, confirming the DMM reading and revealing switching artifacts.

Why Other Options Are Wrong:

DC scale measures total including the large DC offset and does not isolate ripple. OHM and Farad scales are not for live-voltage ripple measurement. “None of the above” is incorrect because AC voltage mode is appropriate.

Common Pitfalls:

Using long ground leads introducing noise, or measuring under no-load conditions that misrepresent in-service ripple. Always measure under realistic load when possible.

Final Answer:

AC voltage