Lesson 1: Power Has a Path

A PC does not simply have power or not have power.

It has several power paths, and they can fail independently.

That is why a machine can light LEDs and spin fans while still refusing to POST. Some circuits are receiving energy. That does not prove the CPU, GPU, or storage has the correct power path.

The PSU changes AC into the DC the computer uses

The wall or UPS provides alternating current, or AC.

The power supply converts that input into regulated direct-current outputs used by the computer.

A simplified path looks like this:

wall / UPS
    ↓
AC input
    ↓
PSU conversion
    ↓
3.3 V / 5 V / 12 V DC rails
    ↓
system connectors
    ↓
components

The PSU is not a battery and it is not just an extension cord. It performs electrical conversion and regulation.

Do not open the PSU enclosure. Stored energy can remain hazardous after external power is removed.

Diagrams open at a readable shape-aware scale. Zoom or expand when you need more detail.

System process animation

Power delivery from wall to silicon

Follow energy conversion and regulation instead of treating every no-power symptom as a failed PSU.

RestartPreviousPlay sequenceNext

1. 1Wall ACThe outlet supplies correct alternating-current voltage.
2. 2PSUThe supply converts AC into regulated DC rails.
3. 3DC railsCables carry power to the board and devices.
4. 4MotherboardBoard planes distribute rails to subsystems.
5. 5VRMRegulators create precise processor voltages.
6. 6LoadsCPU, GPU, memory, storage, and fans consume power.

Technician question: Where does a known-good voltage stop?

Input voltage belongs on the AC side

You may encounter input ranges such as:

• 110–120 VAC;
• 220–240 VAC.

Many current desktop power supplies accept a wide input range automatically. Older or specialized units may use a manual selector.

Do not casually change a voltage selector.

The important reasoning point is that AC input requirements and DC output rails describe opposite sides of the PSU.

The rails feed different parts of the system

PC power supplies provide regulated outputs including 3.3 V, 5 V, and 12 V.

You do not need to memorize wire colors to understand the system model.

You do need to understand that different circuits depend on different supplied voltages and connectors.

That makes this statement weak:

The PSU is fine because the fans spin.

The fans prove that some powered path is active.

They do not prove every required rail is correct, every connector is seated, or the PSU remains stable under load.

The major connectors are separate failure boundaries

20+4-pin motherboard power

The main motherboard power connection is commonly a 20+4 design that forms the 24-pin connector used by modern ATX systems.

4+4 or 8-pin CPU power

The CPU power connector is separate from the main motherboard connector.

A system can have the 24-pin connector installed and still fail to initialize the processor because CPU power is missing.

GPU supplemental power

A discrete graphics card may receive some power through PCIe and still require additional PSU connections.

A card sitting correctly in the slot does not prove its power requirements are satisfied.

SATA power

SATA drives normally require a PSU power path in addition to the SATA data path.

This is another example of one device depending on two independent connections.

Legacy peripheral power

Older systems and accessories may still use legacy peripheral connectors.

Technicians support the systems that exist, not just the connectors they wish had disappeared.

Wattage is capacity, not forced consumption

A 1000 W PSU does not push 1000 watts into the computer at all times.

The rating describes available capacity under defined conditions.

A useful PSU choice needs:

• enough capacity for the actual configuration;
• the correct connectors;
• reasonable headroom;
• appropriate quality and efficiency;
• compatibility with the case and platform.

A huge wattage number cannot compensate for a missing connector or poor-quality power delivery.

Efficiency tells you something different

Power conversion is not perfectly efficient. Some input energy becomes heat.

Higher efficiency can reduce conversion losses under rated conditions.

But efficiency does not prove:

• sufficient wattage;
• correct connectors;
• good voltage regulation;
• healthy operation;
• compatibility with the system.

One specification should not be promoted into a complete health report.

Modular cables are not universal

Modular and semi-modular PSUs let you remove some or all output cables.

That helps cable management.

It also creates a dangerous assumption:

The connector fits the PSU, so the cable must be interchangeable.

No.

Modular PSU pinouts can differ between models and manufacturers. Use only cables specifically approved for that PSU.

A cable can physically connect and still route voltages incorrectly.

Physical fit is weak evidence. You have seen that pattern before.

Redundant power solves a different problem

Some servers and high-availability systems use redundant PSUs.

The design may allow one supply to fail while another continues supporting the system.

That is not the same as a UPS.

A redundant PSU design can still lose power if the upstream electrical source fails. A UPS can help with upstream power interruption but does not make an internal PSU immortal.

Different controls cover different failure domains.

Worked case: absolutely nothing happens

Evidence:

power button pressed
no LEDs
no fans
no display
outlet: known-good

Do not start with a motherboard replacement.

A sensible path begins outside the case:

1. verify the power cable;
2. verify PSU external switch and input state when applicable;
3. disconnect power and inspect required motherboard power connections;
4. inspect the front-panel power-switch path;
5. continue toward PSU or motherboard testing only when the evidence justifies it.

Why start there?

Because the symptom is system-wide and the earliest shared dependency is power delivery.

Worked case: fans spin, but there is no POST

Evidence:

case fans: spinning
motherboard LEDs: on
no firmware display
recent change: motherboard replaced
20+4-pin power: connected
CPU power: not yet verified

A useful next check is obvious once you think in paths.

Verify CPU power.

Do not let the spinning fans convince you that every power requirement has already been proven.

Map the power path

For a desktop with a motherboard, CPU, NVMe system drive, SATA storage drive, and discrete GPU, build a power map showing:

• AC source;
• PSU;
• main motherboard power;
• CPU power;
• GPU supplemental power when required;
• SATA power;
• which components depend on each path.

Then remove one path from the model and predict the narrowest symptom you can defend.

For example, missing SATA power may leave the rest of the computer completely functional while that drive disappears.

That is more useful than writing "computer broken."

Before you move on

Power troubleshooting improves when you separate:

AC input
PSU conversion
DC rail availability
connector delivery
component demand
load behavior

A green LED proves one thing happened.

It does not tell you why the rest of the machine should work.