PC power-supply diagnostics: To check AC ripple on a computer's DC power rails with a multimeter, which meter setting should you select?

Introduction / Context:
Switch-mode PC power supplies output DC voltages, but excessive AC ripple indicates failing capacitors or poor regulation. Measuring ripple correctly is vital for troubleshooting random resets and data errors.

Given Data / Assumptions:

• You need to measure residual AC ripple superimposed on DC outputs (for example, +12 V, +5 V, +3.3 V).
• A standard handheld multimeter is available.
• Safety and correct measurement technique are required.

Concept / Approach:

Ripple is the AC component riding on a DC rail. To see it with a multimeter, set the meter to AC voltage and measure across the rail and ground. The DC setting measures average DC and largely ignores high-frequency AC; resistance and capacitance scales are irrelevant to in-circuit ripple measurement.

Step-by-Step Solution:

Verification / Alternative check:

An oscilloscope provides a far clearer picture of ripple frequency and amplitude; however, a DMM on AC volts is a quick screening tool to flag abnormal ripple.

Why Other Options Are Wrong:

• DC voltage scale: reads the DC level, not the AC ripple content.
• OHM scale: used on de-energized circuits; not for ripple on live rails.
• Farad scale: for capacitance, not ripple voltage.
• None of the above: incorrect because AC voltage is correct.

Common Pitfalls:

Measuring ripple with the system idle (load-dependent); placing probes unsafely near primary side; drawing conclusions from noisy DMM readings without cross-checking with a scope.

Final Answer:

AC voltage