Power Supplies (PSUs)

CompTIA A+ Core 1 (220-1101) • Domain 3.0 Hardware

Key Concepts: Wattage Connectors 80 PLUS Modular Rails

What Does a PSU Do?

The Power Supply Unit (PSU) converts AC power from the wall outlet to DC power that computer components require. It also regulates voltage to provide stable, clean power.

AC to DC Conversion

          Input (AC)

          110-120V (US) or 220-240V (EU)

          From wall outlet

          Output (DC)

          +3.3V, +5V, +12V, -12V

          For components

A+ Exam Tip: The +12V rail is most important for modern systems - it powers the CPU and GPU. Most components now use +12V rather than +5V or +3.3V.

Voltage Rails

Rail	Uses	Wire Color
+12V	CPU, GPU, fans, drives	Yellow
+5V	USB, SATA logic, older components	Red
+3.3V	RAM, chipsets, some SATA	Orange
-12V	Legacy (serial ports)	Blue
+5VSB	Standby power (wake features)	Purple
Ground	Common reference	Black

PSU Connectors

Main Power Connectors

24-pin ATX Main motherboard power

8-pin EPS12V CPU power (4+4 split)

8-pin PCIe GPU power (6+2 split)

Peripheral Connectors

Connector	Pins	Use
SATA Power	15-pin	HDDs, SSDs, optical drives
Molex	4-pin	Legacy drives, fans, accessories
Berg/Floppy	4-pin (small)	Floppy drives (legacy)
12VHPWR	16-pin	High-power GPUs (ATX 3.0, up to 600W)

80 PLUS Efficiency Ratings

The 80 PLUS certification indicates a PSU operates at a minimum of 80% efficiency, meaning at least 80% of AC power is converted to DC (less wasted as heat).

80 PLUS Standard

80%

80 PLUS Bronze

82%

80 PLUS Silver

85%

80 PLUS Gold

87%

80 PLUS Platinum

90%

80 PLUS Titanium

94%

Why Efficiency Matters: A 500W system with 80% efficiency draws 625W from the wall. At 90% efficiency, it draws only 556W - saving electricity and reducing heat.

Modular vs Non-Modular

          Non-Modular

          All cables permanently attached

          Cheapest, more clutter

          Semi-Modular

          Main cables fixed, others detachable

          Good balance

          Fully Modular

          All cables detachable

          Best airflow, premium price

Warning: Modular cables are NOT interchangeable between brands! Using wrong cables can damage components.

PSU Form Factors

Form Factor	Dimensions	Use Case
ATX	150mm × 86mm × 140-200mm	Standard desktops
SFX	125mm × 63.5mm × 100mm	Small form factor PCs
SFX-L	125mm × 63.5mm × 130mm	SFF with larger fan
TFX	85mm × 65mm × 175mm	Slim/low-profile cases

Calculating Power Requirements

Calculate total wattage needed by adding component power draws, then add 20-30% headroom.

Typical Component Power Draw

Component	Typical Wattage
CPU (mid-range)	65-125W
CPU (high-end)	125-250W
GPU (mid-range)	150-200W
GPU (high-end)	250-450W
RAM (per stick)	3-5W
SSD	2-5W
HDD	5-10W
Motherboard	25-80W

Example: CPU (125W) + GPU (300W) + Motherboard (50W) + RAM (10W) + Storage (10W) = 495W base. Add 30% headroom = 650W PSU recommended.

Knowledge Check

Q1: What color wire carries +12V in a PSU?

Q2: What is the minimum efficiency for 80 PLUS Gold certification at 50% load?

Q3: How many pins does the main ATX motherboard power connector have?

Q4: Which voltage rail powers modern CPUs and GPUs?