PCB power rail diagnosis: A regulated +5 V supply on a board has sagged to about +3.4 V under load. Which fault is the most likely immediate cause?

Introduction / Context:
Power rails that read significantly below their nominal value often indicate excessive loading. In digital systems, unintended shorts or partial shorts can pull a regulated rail down to an intermediate level where the regulator current limits or the source impedance causes droop.

Given Data / Assumptions:

• Nominal +5 V rail measures +3.4 V in-circuit.
• Supply previously operated correctly.
• No evidence of supply being set to +3.3 V intentionally.

Concept / Approach:
Two broad categories cause low rails: source failure (regulator fault) and excessive load (short/overload). A drop from 5.0 V to about 3.4 V strongly suggests the supply is being dragged down by a low-resistance path, often a solder bridge, failed IC, or connector issue. The “half-split method” is a troubleshooting technique, not a fault. Opens usually cause rails to float or rise, not sag under load.

Step-by-Step Solution:
Isolate: Power off, measure resistance from rail to ground.If abnormally low, disconnect sections (pull fuses, unplug cards) to localize the short.Use thermal camera or IPA evaporation to find the hot component/spot.Repair the shorted device or bridge; verify rail returns to +5 V.

Verification / Alternative check:
Bench supply with current limit: bring rail up and observe current draw; a short will pull high current immediately.

Why Other Options Are Wrong:
An open usually removes load and does not pull rails down.

A faulty regulator can cause low voltage, but the most common sudden sag to ~3.4 V is heavy loading.

Half-split is a method, not a failure.

Probe compensation is unrelated to DC rail measurements.

Common Pitfalls:
Confusing an intentional +3.3 V rail with a pulled-down +5 V; always check schematics and labels.

Final Answer:
a circuit short