Electronics

PSU — Power Supply Unit

Every component on this page's neighboring pages — CPU, GPU, motherboard — needs steady DC power to run at all. The wall only ever provides AC. A PSU is the device that turns one into the other, built directly on the voltage/current fundamentals in Electronics Fundamentals.

AC to DC

Mains power is alternating current — voltage that reverses direction many times a second. Almost every electronic component needs steady direct current instead — see Electronics Fundamentals for what voltage and current actually are. A PSU's whole job is that conversion: rectifying AC into DC, then regulating it down to the specific steady voltages components expect.

Voltage rails

Wattage and efficiency

A PSU's rated wattage is its maximum DC output capacity, not how much power it draws from the wall — it always draws somewhat more than it outputs, because no conversion is perfectly lossless. The 80 Plus certification tiers (Bronze through Titanium) measure exactly how much extra is wasted as heat during that conversion — a real, ongoing factor in both electricity cost and how much heat a system's cooling has to remove.

Real-world applications

• Every desktop/server PSU — the case this page describes directly.
• Laptop and phone chargers — the same AC-to-DC job, done externally, at much lower wattage.
• Embedded device power supplies — a small embedded system often uses a simple AC adapter or an onboard regulator doing the same AC-to-DC or DC-to-DC conversion at a tiny fraction of the scale — see Embedded.

Gotchas