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 is small and efficient compared with older linear designs. It delivers several DC rails — typically +12 V, +5 V, and +3.3 V — to the motherboard, drives, and add-in cards through standardized connectors. A unit is rated by total wattage (how much it can deliver) and by conversion efficiency (how little it wastes as heat), with schemes like 80 PLUS certifying the latter.
Where it fits
The PSU sizes the whole machine: a power-hungry GPU and CPU demand more headroom, and wasted power 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.
Sources
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Power supply unit (computer) — Wikipedia, on PC power supplies, rails, and efficiency. ↩