IT FundamentalsA+

Power Supplies for CompTIA A+ 220-1101

The power supply unit (PSU) converts AC power from the wall outlet to DC power required by PC components. CompTIA A+ 220-1101 tests PSU wattage, connectors, efficiency ratings, and failure symptoms. A faulty or undersized PSU can cause instability, random shutdowns, failure to boot, and in rare cases damage other components — correct PSU selection is critical for system reliability.

7 min

2 sections · 7 exam key points

1 practice questions

PSU Wattage and Efficiency

Wattage: total power the PSU can deliver. Calculate system power requirements by adding component TDPs: CPU (65–125W), GPU (75–250W+), RAM (15W total), drives (5–15W each), fans (2–5W each), motherboard (30–80W). Add 20–30% headroom for peaks and future upgrades. An undersized PSU causes instability or system shutdowns under load. Oversized PSU wastes money and slightly reduces efficiency at low load.

80 PLUS efficiency ratings: measure how much AC power is converted to usable DC power (vs wasted as heat). 80 PLUS: 80% efficient at 100% load. 80 PLUS Bronze: 82% at 100% load. 80 PLUS Gold: 87% at 100% load. 80 PLUS Platinum: 90%+. 80 PLUS Titanium: 94%+. Higher efficiency = less electricity wasted as heat = lower operating cost. Gold is the sweet spot for value vs efficiency for most builds.

Form factors: ATX: standard desktop PSU (150×86×140mm). SFX: smaller PSU for mini-ITX cases. TFX: thin form factor for slim desktop cases. Laptop PSUs: external AC adapters (brick), connect via DC barrel jack or USB-C Power Delivery. Always verify PSU form factor matches the case before purchasing.

PSU Connectors

24-pin ATX main connector: primary power to the motherboard — 24 pins (20+4 adapter fits some older boards). This is the largest connector on the PSU. Always required.

8-pin (EPS/CPU) connector: powers the CPU voltage regulator on the motherboard — also called the ATX12V or EPS12V connector. High-end boards may require two 8-pin connectors (16 pins total). Required for all modern motherboards — without it, the CPU has no power and the system won't POST.

PCIe power connectors: 6-pin (75W) or 8-pin (150W) connectors for discrete graphics cards. High-end GPUs may require multiple 8-pin connectors (two 8-pin = 300W from PCIe power) plus 75W from the PCIe slot = 375W total. GPU instructions specify required connector count.

SATA power: L-shaped 15-pin connector for SATA HDDs, SSDs, optical drives. Molex (4-pin): legacy connector for older drives, fans, and accessories — increasingly rare in modern builds. PCIe power, SATA, and Molex all output 12V, 5V, and 3.3V rails (Molex provides 12V and 5V only).

PSU failure symptoms: system won't power on (click then nothing), random shutdowns under load, system powers on then immediately off, burning smell, high-pitched whine from PSU fan. Test: use a PSU tester or substitute with a known-good PSU. Multimeter: test voltage rails — 12V should be 11.4–12.6V; 5V should be 4.75–5.25V. Out-of-spec voltages indicate a failing PSU.

Key exam facts — A+

24-pin ATX: main motherboard power; 8-pin EPS: CPU power — both required for POST
PCIe 6-pin: 75W; 8-pin: 150W — for discrete graphics cards
SATA power: 15-pin L-shaped; Molex: 4-pin legacy connector
80 PLUS ratings: Bronze (82%), Gold (87%), Platinum (90%+) efficiency at full load
PSU failure: random shutdowns, won't start, burning smell — test with PSU tester or swap
Calculate required wattage: sum all component TDPs + 20–30% headroom
ATX form factor for standard desktops; SFX for mini-ITX; TFX for slim towers

Common exam traps

A higher-wattage PSU uses more electricity

A PSU only draws the power that components actually need — a 1000W PSU in a system that draws 200W will only pull ~230W from the wall (accounting for efficiency losses). Oversized PSUs don't waste power beyond efficiency curves. However, a 1000W PSU operating at 20% load (200W) may be less efficient than a 500W PSU at 40% load — stay within 40–80% of PSU capacity for best efficiency

Practice questions — Power Supplies

These questions are representative of what you will see on A+ exams. The correct answer and explanation are shown immediately below each question.

Q1.A desktop computer with a new high-end GPU starts fine but shuts down unexpectedly after 10–15 minutes of gaming. The system runs normally with light tasks. What is the most likely cause?

A.The GPU is defective

B.The PSU wattage is insufficient for the system under full GPU load

C.The operating system needs reinstalling

D.The RAM is faulty

Explanation: Shutdown specifically under GPU load (gaming) strongly points to PSU capacity — the GPU draws significantly more power under 3D rendering than idle. If the PSU wattage cannot sustain the GPU's full power draw, it triggers a safety shutdown. The system runs fine at idle because light tasks don't stress the PSU. Solution: calculate actual system wattage requirements, verify the PSU is rated for sufficient wattage with 20%+ headroom, and upgrade if necessary. Check with a PSU tester or monitor voltages in HWiNFO.

Frequently asked questions — Power Supplies

What is the difference between modular, semi-modular, and non-modular PSUs?

Non-modular PSU: all cables are permanently attached — unused cables must be bundled and stored inside the case, reducing airflow. Semi-modular: essential cables (24-pin ATX, 8-pin CPU) are permanent; additional cables (SATA, PCIe) are detachable. Fully modular: all cables detach — only connect what you need, cleaner cable management. Modular PSUs are preferred for clean builds and good airflow but cost more. Performance is identical — cable management is the only difference.

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