A desktop power supply converts the alternating current from the wall socket into three steady direct voltages that the computer uses: 12 V for the processor and graphics card, 5 V and 3.3 V for the rest. It does that with the same five stages as any switching converter — rectification, switching, a transformer, output rectification and feedback — arranged so that the hazardous side is sealed away from the side you plug into. The label on the outside summarises how efficiently it does the job.
Safety warning before you open anything
Do not open a power supply. The capacitors on the mains side store energy at a high voltage, and they can hold a dangerous charge for minutes or longer after the unit is unplugged and the computer is off. Unlike the low-voltage side of a computer, there is nothing inside that a hobbyist can safely probe. Every diagnostic in this article can be carried out with the unit closed, and that is deliberate.
The reason for the warning is worth understanding rather than merely obeying. A capacitor is a store of energy, and the input capacitor of a supply is charged to the peak of the mains voltage every time the unit runs. When the supply is switched off, that energy has nowhere to go until the circuit discharges it, which some designs do quickly and others do not. Touching the wrong point completes the circuit through you.
The five stages of a PSU
The first stage rectifies the incoming alternating current and smooths it into a high direct voltage, and it is here that the large input capacitor sits. The second switches that voltage on and off at a high frequency under the control of the regulation loop. The third transfers the energy across a transformer, which also provides the isolation barrier. The fourth rectifies and smooths the outputs on the safe side and produces the several voltages the computer needs. The fifth measures those outputs and adjusts the switching to hold them steady.
The comparison that fits is a water system with a pressure reducer. The incoming supply arrives at high pressure, a valve opens and closes rapidly to fill a small tank, and a regulator holds the output pressure steady no matter how much water the house draws. The valve is the switch, the tank is the transformer and its windings, and the regulator is the feedback loop.
Why it starts with AC rectification
The switching stage needs a direct voltage to chop, so the first thing the supply does is convert the alternating input into direct current. This is done with a bridge rectifier and a smoothing capacitor, and it is the stage where the design has to cope with the worst conditions: the highest voltages, the inrush current at switch-on, and the variation between a 110 V and a 230 V mains supply. A supply rated for both uses the same components and adapts the switching rather than the rectification.
The rectifier stage is also where a supply’s tolerance to a poor mains supply is decided. A brief dip in the input voltage is absorbed by the input capacitor, which is why a supply with a larger input capacitor is more tolerant of a momentary sag and takes slightly longer to discharge after switch-off.
Switching and the transformer
The switching stage is what makes the unit small. Working at tens of kilohertz rather than at the mains frequency allows a much smaller transformer to transfer the same power, and it also allows the output voltages to be produced by controlled windings and post-regulation rather than by separate linear regulators for each rail. The switches themselves are power MOSFETs, chosen for their resistance in the on state and their switching behaviour.
The transformer is also the isolation barrier, and it is the reason the low-voltage side of a computer is safe to touch while the input side is not. Everything about the physical construction of the supply — the spacing between the two sides on the circuit board, the insulation on the transformer, the slots cut into the board — exists to maintain that barrier. When a supply fails in a way that damages other components, it is usually because the barrier was breached or the output regulation lost control rather than because the mains reached the computer directly.
The 12 V, 5 V and 3.3 V outputs
A desktop supply produces three main voltages, and in a modern computer almost all of the power goes to the 12 V rail. That rail feeds the processor through a regulator on the motherboard and the graphics card through its own connectors, and each of those converts the 12 V down to the very low voltage the silicon actually needs. The 5 V and 3.3 V rails supply storage, USB and the smaller logic on the board.
Two consequences explain most practical questions. First, a supply’s wattage figure is the sum of what its rails can deliver, so a supply can be nominally large while having a modest 12 V capability, which is the rail that matters. Second, the split of the 12 V output across several connectors is a distribution detail rather than a set of independent supplies, and a device plugged into the wrong group can trip the supply’s protection without anything being broken.
Reading an 80 PLUS label
The label programme for desktop supplies uses tiers named after metals, and the tiers differ in how demanding the efficiency requirement is at defined load points, usually measured at low, medium and full load. The point of measuring at several loads is that a supply spends little of its life at exactly full output, so a design that is efficient there and poor at low load is not very useful.
What the label does not tell you is whether the supply is well built. It says nothing about the quality of the capacitors, the noise of the fan, the protection features or the hold-up time, and it is not a warranty. A supply from a lower tier with good components can outlast a higher-tier unit built to a price, which is why the label is a starting point rather than a verdict.
What actually fails after five years
The components that fail first are the electrolytic capacitors, and the reason is the same one that applies inside an LED bulb: their life depends strongly on temperature, and the rule of thumb in the industry is that it roughly halves for every ten degrees of additional operating temperature. A supply running hot in a poorly ventilated case ages faster than the same supply in a well-ventilated one, and the fan that keeps it cool is itself a mechanical part with a finite life.
The symptoms follow from the mechanism. A capacitor that has lost capacitance causes the output voltages to wander, which can appear as random reboots under load, a machine that fails to start in cold conditions, or instability that only shows when the graphics card draws power. A fan that has stopped causes the supply to run hotter and accelerates everything else, sometimes with an audible change in pitch before any instability appears.
Buying checklist
Five things are worth checking. The 12 V output current, because that is what feeds the parts that draw the most. The efficiency tier, understood as a statement about load behaviour rather than about quality. The protections listed in the specification, since over-current, over-voltage and short-circuit protection are the features that stop a failure from spreading. The physical size, because a supply that does not fit the case is not a supply. And the warranty, which is the manufacturer’s own statement about how long it expects the unit to keep working.
FAQ
How do I tell if my PC power supply is bad?
The behavioural signs are random restarts under load, a machine that fails to start when cold, an inability to power on with the graphics card connected, and a fan that has become noisy or stopped. A supply can also be fine and the problem can be elsewhere, so the useful approach is to test the easiest substitution first and to watch the voltage readings while a load is applied, rather than to open the unit.
What components are inside a PC power supply?
A bridge rectifier and a large input capacitor on the mains side, switching transistors controlled by a regulator circuit, a transformer that also provides the isolation barrier, output rectifiers and capacitors on the safe side, and a control circuit that measures the outputs and adjusts the switching. The mains-side capacitors hold a charge after the unit is unplugged, which is why the inside is not a workspace.
Can you run a PC without a power supply?
No, because every component in the computer needs a direct voltage at a level the mains supply does not provide. The alternatives that exist are different kinds of power supply rather than replacements for one: an external brick for a small form-factor machine, a bench supply used for testing, or a battery system with its own converters. In each case something converts and regulates the power.
What is the lifespan of a power supply?
It depends on temperature, load and the quality of the components, and the electrolytic capacitors usually set the limit. A supply that runs cool at moderate load generally lasts longer than an identical unit running hot near its maximum, so ventilation and the choice of a unit with some headroom both extend the useful life. The warranty is the manufacturer’s own estimate of the minimum.
Does a higher wattage rating make a supply better?
It makes it larger, and it gives headroom for transient peaks that a smaller unit would treat as an overload. It does not make the components better, and a large supply running at a very low load can be less efficient than a smaller one running near its optimum. The useful figure is the 12 V output current, with a margin over the actual requirement.
What to do next
Write down the 12 V current your processor and graphics card need, compare it with the figure on your supply’s label, and check that the case has clear airflow. Those two checks prevent more problems than any diagnostic. The engineering detail of the same unit is in rectifier roles and 12 V rails inside an ATX supply, the efficiency comparison is in switching versus linear supplies, and the thermal mechanism is in why diodes get hot.
Background is available in our overviews of switched-mode supplies, alternating current, rectifiers, capacitors and direct current, with energy material from the US Department of Energy, electrical standards from IEC and assembly expectations from IPC. Efficiency and safety background is published by NIST, ISO and the Semiconductor Industry Association.
Tell Good-Ark the input range, the output rails and the load profile, and we will point you at the rectifiers, switches and protection devices that suit the specification.
Email sales@goodark.com, or start from the application design centre.
This article is published by Good-Ark, a manufacturer of discrete semiconductor devices used in power supplies. It advises against opening any mains-powered supply because of the charge held by the input capacitors, and it gives no instruction for doing so.