Power Supplies Explained: Modular, Efficiency and Sizing for A+

The power supply converts mains AC into the DC rails a computer needs. The A+ exam covers the connector types, efficiency ratings, sizing and — most usefully — how to recognise a failing one.

Modular, semi-modular, non-modular

Type	Cables	Suits
Non-modular	All permanently attached	Budget builds; cheapest
Semi-modular	Essential cables fixed, rest detachable	Most builds; good balance
Fully modular	Every cable detachable	Small cases, custom builds, servicing

The advantage of modular is cable management: you connect only what you need, so there is less clutter, better airflow, and easier building in a small case. The disadvantages are slightly higher cost and one additional connection point per cable.

The critical warning, and a genuine exam item: modular cables are not interchangeable between manufacturers or even between models from the same manufacturer. The connector on the PSU side may be physically identical while the pinout differs, and connecting the wrong cable can put 12 V where a device expects ground and destroy hardware. Always use the cables that came with that specific unit.

80 PLUS efficiency

Efficiency is the proportion of drawn power delivered as DC rather than lost as heat. The tiers, measured at 50% load, are 80 PLUS, Bronze, Silver, Gold, Platinum and Titanium, running from about 80% to over 94%.

Two consequences beyond the electricity bill. A less efficient supply produces more heat inside the case, raising temperatures for everything else. And efficiency varies with load, peaking around 50% — which is a practical argument for sizing so that typical load sits near half the rated output rather than buying the largest unit available.

Connectors

24-pin ATX — main motherboard power, usually split 20+4 for compatibility.
4/8-pin EPS (CPU) — supplies the processor, plugged in at the top of the board. This is the one most often forgotten, and the symptom is a system that does not POST despite fans spinning.
6/8-pin PCIe — graphics cards. An 8-pin supplies 150 W, a 6-pin 75 W.
12VHPWR / 12V-2x6 — the newer high-power connector for modern GPUs, up to 600 W. Must be fully seated; partial insertion has caused melted connectors.
SATA power — 15-pin, for drives.
Molex — legacy 4-pin, fans and older peripherals.

A note on adapters: a PCIe connector is not the same as an EPS connector despite both being 8-pin, and forcing one into the other damages hardware. They are keyed differently for that reason.

Sizing

Add up the components, with the graphics card and processor dominating, then leave headroom — 20 to 30% above expected peak is the usual guidance. That covers transient spikes, which modern GPUs produce in bursts far above their rated draw, and it places typical load near the efficiency sweet spot.

Undersizing produces the most confusing failure mode in computing: random reboots and shutdowns under load with no error, because the supply cannot meet demand during a transient. It looks like a software fault, a memory fault, or overheating, and people replace several other components before suspecting the PSU.

Also check the +12 V rail capacity rather than only total wattage, since that rail feeds the CPU and GPU and is what actually constrains a modern system.

Diagnosing a failing supply

The symptoms, roughly in order of how often they are misdiagnosed.

Random reboots or shutdowns under load — classic insufficient or degrading supply.
No power at all — no fans, no lights. Check the mains lead, the rear switch and the wall socket before condemning the unit.
Fans spin but no POST — frequently the EPS CPU connector, not the PSU itself.
Burning smell or visible bulging capacitors — replace immediately and do not power on again.
Coil whine — usually harmless, occasionally an early sign.
Intermittent device dropouts — a rail sagging under load.

Testing: a PSU tester confirms the rails are present but loads them only lightly, so it can pass a supply that fails under real demand. A multimeter measures the rails under actual load, which is the better test. Swapping in a known-good supply of adequate capacity remains the definitive one.

The tolerances worth knowing: rails should be within 5% of nominal, so 12 V should read between 11.4 and 12.6.

Safety and redundancy

Never open a power supply. Capacitors retain a lethal charge long after disconnection, and there is nothing user-serviceable inside — this is the one component the A+ explicitly tells you not to repair.

In servers, redundant hot-swappable supplies allow one to fail or be replaced without downtime, and should be fed from separate circuits so a single breaker does not take both.

What to remember

Modular cables are specific to the unit and must never be swapped between models. 80 PLUS tiers run Bronze to Titanium, with efficiency peaking around 50% load. The EPS CPU connector is the most commonly forgotten cable. Size 20-30% above peak and check the +12 V rail. Random shutdowns under load point at the supply. Never open one.

Related reading: CPU sockets · SODIMM memory · MTBF and MTTF