YOUR GUIDE TO PC COMPUTER POWER SUPPLY

Needless to say, the voltage available from a wall outlet is a poorly regulated AC ranged worldwide from 90V to 240V, while electronic circuits require a well stabilized low-voltage DC. That's why all electronic equipment obviously needs some form of power conversion and regulation. In PC these functions are done by a power supply unit (PSU)- an internal device that converts an input AC voltage to a set of regulated DC voltages needed by personal computer.
In the process a PSU also provides primary to secondary safety isolation. Since the introduction of IBM PC/XT, there have been about a dozen different desktop PC types. They differ by their structure, form factors, connectors and volt/amp ratings. The output rating of a modern computer PSU ranges anywhere from 185 W to several kilowatt. The units over 450W are used mainly for servers, industrial PCs and to supply the desktops with high-end video applications
The traditional standard ATX PSU generates the following DC voltages: +5V, +3.3V, +12V1, +12V2, -12V, and a standby 5V. Some very old models may also have minus 5V. Additional "point of load" (POL) buck converters on the motherboards step down 12V to the CPU core voltage and to other low potentials needed for internal components. Each PSU rail in theory should have an individual current limit. This is required to meet 240VA safety requirements of IEC 60950 and UL 60950-1. However, in practice, all 12V rails often have a single combined current limit. To meet PCI Express requirements, in the ATX2 computers the legacy 2x10 main power connector has been replaced by 2x12 one. An additional 2x2 power cable is used for the second 12V rail. It supports the processor's voltage regulator. There are also peripheral, floppy drive, and serial ATA connectors. The PSU for high-end discrete graphics cards have additional 2x3 or 2x4 connectors to supply extra power to the graphics that require more than 75 watts. For detailed info see our ATX2 power supply pinout guide. In 2022 Intel released ATX 3.0 standard with a new auxiliary power connector 12VHPWR to support next-generation hardware and technologies like PCIe Gen 5.0. Note that some brand name PCs have propriatory pinouts of their PSU that differ from generic ATX.

In order to increase PC PSU efficiency and meet so-called alternative sleep mode requirements, in 2019 Intel introduced a totally different single-rail standard ATX12VO (which stands for ATX 12V only). The ATX12VO spec replaces the 24-pin connector with a 10-pin part providing single 12V output. All other voltages, including 5V and 3.3V will be produced on the motherboard by POL regulators. This architecture also significantly reduced the cost of PSU, but increased the cost of motherboards, which now have to provide additional power conversion functions.

Today's power supplies for computers utilize switching mode technology (read more about SMPS). Modern units usually include a PFC "front end" boost followed by downstream half-bridge or forward converter (see SMPS topologies). Most of today's models meet ENERGY STAR® requirements. In the past it just meant they consumed <10% of rated power in standby mode. However, in an active mode the efficiency of old cheap models used to be 60-70%.

An incentive program called 80 PLUS® required PC and server power supplies to demonstrate efficiency >80% at 20% to 100% of rated load with the power factor >0.9. Later on they've added Bronze, Silver, Gold and Platinum labels for higher levels of efficiency (up to 92%) with PF up to 0.95. The updated ENERGY STAR Desktop Computer Specification Version 5.0 sets similar requirements for internal PSU. In spite of the new regulations, PC PSUs remain inexpensive: you can buy a standard off-the-shelf model for about $0.10/watt. When buying a replacement unit be sure to match not only its form-factor and net wattage, but individual current ratings of all outputs.

Troubleshooting

. The first thing to check if your computer stopped working is its power supply. The main causes of PSU failure are overheating, voltage surges in the input line, and dried out electrolytic capacitors. All this may result in a catastrophic failure of one or more transistors or rectifiers. This in turn usually opens the input fuse (see this tutorial for block diagram and theory of operation).

Needless to say, you should not do a troubleshooting unless you have a proper electronics training and know how to work safely with high voltage circuits. If you should decide to open up a PSU, always unplug its power cable first, and then wait at least five minutes before removing your PC cover to let all the capacitors discharge. You will likely need to unplug all the cables coming out of the PSU and take the unit out of chassis. The first thing to check is the input fuse- get an off-shelf tester and connect its leads across the terminals of the fuse. If it reads "short" it means the fuse is good. If it's open-- the fuse failed. In most cases the fuse is not failing alone- usually you will find one of the semiconductor devices failed too. "Buzz" the power transistors and the rectifiers. If you did not find any failed parts, or found and replaced them, now try to power up the PSU. To enable a stand-alone PSU outside of the PC chassis, you need to ground PS_ON# pin (see connection diagram to the left for the test setup). In a standard ATX-2-compliant model it means shorting out pins 15 and 16 on the 24-pin connector. You can do it by using a small piece of a copper wire. In an older 20-pin unit you need to short out pins 13 and 14. Note that some manufacturers, such as Apple, HP and Dell used proprietary non-standard connector sizes and pinouts: see more info here. After enabling the unit you can turn on the input power and check output voltages one at a time. To measure any voltage connect a voltmeter between a respective bus pin and any common pin. You can use the same tester set to DC voltage to simplify this process.

Below you will find some schematic diagrams, reviews, pinouts, specifications, and other useful information for repair and electronic projects.

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