)- an internal device that converts an input AC voltage to a set of regulated DC voltages needed by personal computer.
In the process it also provides primary to secondary safety isolation. Since the introduction of IBM PC/XT, there have been about a dozen different 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 400W are used mainly for servers, industrial PCs and to supply the desktops with high-end video applications.
Today's standard ATX PSU generates as a minimum the following DC voltages: +5V, +3.3V, +12V1, +12V2, -12V, and a standby 5V. Some older models may also have minus 5V. Additional "point of load" DC-DC converters step down 12V to the CPU core voltage and to other low potentials needed for motherboard components. Each output in theory should have an individual current limit. This is required to meet 240VA safety requirements of EN 60950 and UL 60950-1. However, in practice all 12V rails often have a single current limit. To meet PCI Express requirements, in the new ATX2 computers the old 2x10 main power connector has been replaced by 2x12 connector. 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 ATX power supply pinout guide
Today's power supplies for computers utilize switching mode technology (read more
about SMPS). Modern units usually include a PFC front end followed by 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.
. 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). To check your unit, first of all you need to unplug input power cord and wait 5 minutes to let all capacitors discharge before removing your PC cover. Then unplug all the cables coming out of the PSU. To enable a stand-alone PSU, you need to ground PS_ON# pin (see connection diagram to the left for the test setup). In an 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 pin and any common. You can use an off the shelf tester to simplify this process. If you should decide to open up a PSU, always unplug it first and then wait at least five minutes to let all capacitors discharge. You should not
do a troubleshooting unless you have a proper electronics training and know how to work with high voltage circuits.
Below you will find schematic diagrams, reviews, pinouts, specifications, and other useful information for repair and electronic projects.
The voltage available from a wall outlet is a poorly regulated AC ranged worldwide from 90V to 240V, while most electronic circuits require a well stabilized low-voltage DC. That's why all electronic equipment needs some form of power conversion. In PC it is done by power supply unit (