Also known as: PSU, Power supply
A power supply unit (PSU) converts alternating-current mains electricity into the regulated low-voltage direct current that a computer’s components run on.1
Overview
Most PCs use a switched-mode power supply (SMPS), which rapidly switches the incoming power at high frequency to regulate its output; this makes it far smaller and more efficient than the older linear designs it replaced. It first rectifies the AC mains to a high DC voltage, then switches and steps it down to the low voltages the machine needs.
The output is delivered as several DC rails — typically +12 V, +5 V, and +3.3 V — routed to the motherboard, drives, and add-in cards through standardized connectors. A unit is rated by total wattage (how much power it can deliver at once) and by conversion efficiency (how little it wastes as heat), with certification schemes such as 80 PLUS grading the latter.
The rails
Different components expect different voltages, so the PSU provides each on its own rail:
| Rail | Typical loads | Notes |
|---|---|---|
| +12 V | CPU, GPU, fans, motors | Carries most of the power in a modern PC |
| +5 V | Drives, USB, some logic | The traditional workhorse rail |
| +3.3 V | Chipset, memory, low-voltage logic | Lowest of the three |
Modern designs put nearly all the power on the +12 V rail and derive the lower voltages from it on the board, because high-current parts like the CPU and GPU dominate the budget.
Where it fits
The PSU sizes the whole machine: a power-hungry GPU and CPU demand more wattage headroom, and whatever power is wasted becomes heat the cooling system must remove. Small computers skip a full PSU — a Raspberry Pi running a GopherTrunk capture node takes regulated 5 V straight from a USB adapter — but the job is identical: turn wall power into the clean, steady rails the chips expect, and a noisy or undersized supply can quietly corrupt an SDR capture.
Sources
-
Power supply unit (computer) — Wikipedia, on PC power supplies, switched-mode design, voltage rails, and efficiency ratings. ↩