An ATX power supply is the most common power electronics board most people will ever open, and it is also the one most often misdiagnosed. From wall plug to 12 V rail, the board walks through a defined chain — input bridge, PFC, rectification, down-conversion — and each step has a diode with a job and a failure signature. This article walks that chain, names the diode at each stage, and then reads the failure signatures of a loaded PC so a builder or technician can tell whether the diode, the capacitor, or the fan is the story.
From Wall Plug to 12 V Rail: The Power Path
The ATX power path starts at the wall plug and ends at the 12 V rail, and the chain in between is the route the power takes through the board. The path is worth walking once because every later diagnosis refers to a point on it.
The input stage is the first stop: the AC line enters the PSU, passes through the input filter and the bridge rectifier, and becomes the DC bus that everything downstream uses. The PFC stage sits just after the input, shaping the input current to keep the power factor near unity. The main conversion stage — the DC-DC or phase-shifted converter — steps the bus to the 12 V rail that feeds the CPU, GPU, and the board. From the 12 V rail, the down-conversion stages create the 5 V and 3.3 V rails for the peripherals and the memory. Each stage has a rectifier or diode with a defined job, and each is the subject of a later section. The data center power article and the server power supply article document the same chain at the higher power levels, and this article reads the consumer ATX version.
The power path is the map of the board. A technician with the map can trace a symptom back to its stage — a 12 V rail that sags points upstream of the rail, a 3.3 V rail that fails points at the down-conversion. The failure signatures at the end depend on knowing which stage each part serves.
The Input Bridge and PFC Diode: First to See the Surge
The input bridge and the PFC diode are the first parts to see everything the wall and the grid throw at the supply, and their failure signatures are the ones that look like “the PSU died” rather than “the rail is weak.”
The input bridge rectifies the AC line into DC, and its diodes carry the full input current and see the line’s surges first. The PFC diode works at the switching frequency of the PFC stage, carrying the fast pulses that shape the power factor. Both live at the front of the board, and both are exposed to the surge events from the grid and the fast switching of the PFC algorithm. The rectifier selection for SMPS article names the three roles and their enemies, and the surge and ESD testing guide covers the surge waves the input must survive.
The failure signature of the input stage is the supply that is dead at the input: nothing powers on, the fan does not spin, and the failure is in the first stage that the wall reaches. The signature of the PFC is more subtle — a supply that runs but loses efficiency or trips under load, because the PFC diode’s recovery loss has grown or the stage is running over its rating. The input stage is where surge damage concentrates, and the surge damage article shows how to read the evidence.
12 V Rail and the Rectifier That Feeds It
The 12 V rail is the heart of the ATX supply — the rail that feeds the CPU, GPU, and most of the board — and the rectifier that feeds it is the part most PC builders will meet.
The main conversion stage produces the 12 V rail through its own rectifier (or synchronous-rectifier stage), and that rectifier carries the largest continuous current on the board under a gaming or rendering load. Its rating must cover the load’s average and peak, its drop sets the rail’s efficiency, and its thermal path carries the heat of the loaded PC. Under a heavy GPU load the rail’s rectifier runs at its warmest, and a part chosen without margin fails exactly there. The server power supply article documents the thermal-first method at the server scale, and the synchronous rectifier article shows the low-loss upgrade for the high-current rail.
The failure signature of the 12 V rail is the loaded crash: the PC idles fine, then shuts down or restarts under load, because the rail sags or the rectifier trips thermally when the GPU ramps. The diagnosis separates the rail rectifier from the fan — a hot, over-current rail is a rectifier or capacitor story, not a fan story — and the ripple and noise guide covers the rail measurements that confirm it.
5 V and 3.3 V: Where the Down-Conversion Diode Lives
Below the 12 V rail, the down-conversion stages create the 5 V and 3.3 V rails, and these stages carry the quiet diodes that fail quietly.
The 5 V and 3.3 V stages step the 12 V rail down for the peripherals and the memory, and their output rectifiers — or synchronous replacements — carry the smaller currents of those rails. The parts are smaller and run cooler than the 12 V rectifier, and their failures are the confusing ones: a rail that is weak or noisy without a dramatic hot part. The down-conversion stage’s rectifier drop is a larger share of the low output voltage, so the low-VF part matters more at 3.3 V than at 12 V, and a degraded or wrong part shows as regulation loss or ripple on the rail. The low-VF Schottky article and the 250 V loss worksheet develop the low-voltage drop logic that the 3.3 V stage lives on.
The failure signature of the down-conversion stage is the stable-but-wrong rail: the PC runs, but devices on the 3.3 V rail drop out, the memory becomes unstable, or the rail’s ripple upsets a sensitive peripheral. Because no loud part is hot, the diagnosis is the scope on the rail — the ripple or sag names the stage, and the ripple diagnosis guide shows the read.
The signature map is compact enough for the table that technicians actually use:
| Symptom in a loaded PC | Stage | Likely part | Confirming measurement |
|---|---|---|---|
| Dead at power-on | Input bridge / fuse | Input rectifier or surge | Nothing powers; check input stage |
| Crash or restart under GPU load | 12 V rail | Rail rectifier or capacitor | Rail scope, stage temperature |
| Device dropouts, unstable memory | Down-conversion | 3.3 V / 5 V rectifier or cap | Scope on the low rail |
| Runs hot, fan loud, no sag | Enclosure / thermal | Fan, airflow, thermal path | Case and stage temperatures |
| Efficiency loss, trips under load | PFC stage | PFC diode | Power analyzer, stage heat |
The table is the diagnosis in one view: each symptom names the stage, the part, and the measurement that confirms it. The rectifier failure modes guide and the power supply repair article provide the deeper test sequences, and the ripple diagnosis guide covers the rail measurements that confirm the capacitor and down-conversion rows.
Failure Signatures in Loaded PCs: Diode, Cap, or Fan?
The payoff of the anatomy is the failure diagnosis, and the loaded PC provides three distinct signatures that separate the diode, the capacitor, and the fan.
The diode signature is heat and sag: the supply runs hot at a specific stage, the rail sags under load, and the part at that stage is past its derated limit. The capacitor signature is ripple and instability: the PC runs, but the rail is noisy, devices drop out, or the supply cannot hold the rail under a load step — the classic aging-capacitor pattern. The fan signature is temperature: the supply overheats for reasons of airflow or enclosure, and the failure is thermal management, not a failed part. Reading the three requires the measurements, not just the smell of warm solder: the case temperature of the stage, the scope on the rail, and the load profile. The rectifier failure modes guide and the power supply repair article give the test order, and the field reliability checklist turns the diagnosis into a pre-power audit.
The three signatures close the anatomy. The ATX supply is a chain — input bridge, PFC, 12 V rectifier, down-conversion — and each link has a diode with a job and a failure mode. The supply that is dead at the input is a first-stage story; the supply that crashes under load is a 12 V or capacitor story; the supply that runs wrong is a down-conversion story; and the supply that just runs hot is the fan story. Walk the board from wall to rail, name the diode at each stage, and the loaded PC stops being a mystery. The general rectifier category supplies the parts the anatomy maps, and the server power supply article is the high-power companion to the consumer read.