3.1.1 Power Supply Units
What is a PSU?
The power supply unit (PSU) delivers low-voltage direct current (DC) power to PC components. It converts alternating current (AC) from the wall outlet into usable DC power.
Internal Components
- Rectifier - Converts AC from the building into DC voltage
- Transformers - Step down to lower voltages
- Regulators & Filters - Ensure consistent output
- Fan - Dissipates heat
Form Factor & Compatibility
- Most desktop PSUs use the ATX form factor
- Size and shape affect compatibility with the case and motherboard
- Key fit points: screw locations, fan locations, and power connectors
Input Voltage by Region
| Region | Typical Voltage | Tolerance Range | Line Type |
|---|---|---|---|
| North America | 120 VAC | 100-127 VAC | Low-line |
| UK / Europe | 230 VAC | 220-240 VAC | High-line |
| Data Centers | High-line preferred | 220-240 VAC | High-line |
Note: AC voltage supply varies by country and distribution circuits, so PSUs have a wide tolerance range: 100-127 VAC for low-line and 220-240 VAC for high-line.
Voltage Switching Types
- Autoswitching (dual voltage) - Detects input voltage automatically; most common
- Manual switch - User must set low-line or high-line before use
- Fixed voltage - Locked to one line type only
Always check the input voltage markings on the PSU before plugging it in.
Key Terms
- AC (Alternating Current) - Power supplied from the building outlet
- DC (Direct Current) - Power delivered to PC components
- Low-line - ~120 VAC (North America)
- High-line - ~230 VAC (UK, Europe, data centers)
- ATX - Standard desktop PSU form factor
- Rectifier - Converts AC to DC