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Build Your Own PC: Computer Power Supplies/Power Supply Units/PSUs

Last updated on 30 June 2008

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This article consists of six pages. You can access the other five pages by clicking the relevant hyperlinks below.

The safest way to upgrade RAM: Use the UK and US Crucial Memory Advisors

Paul Mullen, the highly-respected computer guru of the Helpfile at ComputerShopper.co.uk - "I have recently been buying my memory only from Crucial Technology. I would rather pay the extra cost than waste time trying to track down the obscure program faults that bad memory can cause."

The memory requirements of the versions of Windows Vista

Most of the versions of Windows Vista require more RAM memory to run optimally on a computer that doesn't use memory-hungry applications than Windows XP. A video-editing application is an example of memory-intensive software. Only Windows Vista Home Basic has a recommended minimum amount of memory of 512MB, which is the same amount recommended for Windows XP. Windows Vista Home Premium, the most popular version, and Windows Vista Ultimate require a recommended minimum of 1GB (1024MB) of memory, which is twice the amount of minimum memory recommended to run Windows XP. For more information on computer memory, read the RAM pages of this site.

For example, if your computer has an Asus motherboard, open the menu, scroll down to ASUS, and click GO. If, say, you have a Dell computer, scroll down to DELL, and do likewise. You will be taken to the relevant information on Crucial's website.

If you don't know the make and model of the motherboard installed in your computer, here is a good free utility - Belarc Advisor - that creates an analysis of the hardware and software on a personal computer. Look under FREE DOWNLOAD - http://www.belarc.com/. Another utility that also provides detailed information on the memory itself is CPU-Z.

Power supplies - PSUs

NEVER OPEN A POWER SUPPLY UNIT. ITS CAPACITORS CAN CARRY A LETHAL CHARGE LONG AFTER IT HAS BEEN SWITCHED OFF!

NOTE WELL - BEFORE TURNING A POWER SUPPLY UNIT (PSU) ON, ENSURE THAT THE VOLTAGE SELECTION SWITCH ON THE PSU IS SET CORRECTLY (230V in the UK -115V in the USA).

Failure to ensure this switch is set to the correct voltage will probably destroy your PSU. - And do not use the power supply unit unless it has a load attached to the motherboard, such as the video card and hard disk drive. Turning the power on with only the motherboard attached to it, could also destroy the power supply unit, which must have a load to function without burning out.

The power supply unit (PSU) is one of the most overlooked components in a PC. I suppose the reason is that most people simply believe that if a power supply unit has the physical capacity to be connected to the other components via the motherboard then it must be able to deliver the quality of power that they require.

Unfortunately, that is often not the case, because the power supply in a computer, or the power supply that comes with a PC case, can be underpowered or of low quality. If that is the case and you install a high-end graphics card or a dual-core processor, the system could become unstable or might even fail to boot.

Note that most power supplies should fit into a standard ATX case, but some of them, such as the Zalman ZM500-HP, Hiper Type M 670W, and Tagan Dual-Engine 700W, have more depth than is usual, so make sure that you can fit a power supply in your PC case before you buy it.

In the UK, a power supply uses the 230V AC mains supply and tranforms it into power at the correct voltages for the computer's components.

Different components require different voltages, so a power supply is really several power supplies in one unit. Each supply is called a rail. All power supplies have three main rails - 3.3V, 5V, and 12V - two secondary standby rails - 12V and 5V. A power supply's maximum wattage (e.g., 700W) is the total power it can supply across all of its rails. Some power supplies can have up to four 12V rails. These are used to power PCs running powerful components so that they can each have their own rail.

An ATX12V 2.0 power supply has several four-pin Molex and SATA connectors/plugs. Molex plugs are connected to IDE ATA CD/DVD optical drives and IDE ATA hard disk drives, and some AGP graphics cards. SATA connectors/plugs connect to SATA hard disk drives and optical SATA CD/DVD drives. Molex-to-SATA adapters that make an Molex plug into an SATA plug are readily available, but SATA-to-Molex adapters are not.

Powerful high-end PCI Express graphics cards connect to a six-pin power connector from the power supply, so if you buy such a card your PC's power supply must have one. The most powerful PCI Express graphics cards require two such connectors, which are also required for dual-card setups.

Note that the very latest ATI graphics cards use a six-pin and an eight-pin PCI Express connector. The Enermax Infiniti 650W power supply provides the correct eight-pin plug.

A dual-core processor can draw up to 130W on its own, and nVidia currently provides certified reccomendations on the power supplies that should be used for its high-end graphics cards in SLI modes. The power output can vary from 750W (750 watts) to 1500W, depending on the number of GPUs (Graphics Processing Units) being powered. Look under Power Supplies on this page for more information:

Certified NVIDIA SLI-Ready Components -

http://www.slizone.com/object/slizone_ecosystem.html

You can find the same information for ATI graphics cards using CrossFire technology on this page:

http://game.amd.com/us-en/crossfirex_about.aspx

If you plan on using dual graphics cards that use either nVidia's SLI technology or ATI's CrossFire technology, make sure that your PC has a power supply that explicitly states in its specifications that it supports the dual-card technology that you want to install.

For example, the Tagan Dual Engine 700W ATX power supply, priced at £100, was given a Best Buy award by Computer Shopper in June 2007. It is certified for dual nVidia GeForce 8800GTS (SLI) graphics cards, but it doesn't have CrossFire certification.

The Hiper Type M 670W ATX power suppy, priced at £76, was given a Budget Buy award by Computer Shopper in June 2007. It is CrossFire certified for high-end ATI Radeon X1900 and X1950 graphics cards, and SLI certified for nVidia 8800GTS and 7950 GX2 graphics cards.

Recommended makes of power supply? - The image above shows power supplies made by the major manufacturers Enermax (top) and Antec. Both companies make cases with power supplies, and separate power supplies. Akasa, Cooler Master, Zalman, Seasonic, and Tagan are reputable major manufacturers of power supplies.

There are many smaller, less well known manufacturers of quality power supplies, such as Jeantech, that makes cases and power supplies. The best-known supplier of Jeantech products in the UK is PC World.

The new power supply standard: ATX12V 2.0

A new ATX standard for power supply units has replaced the previous ATX 1.3 standard. It's called ATX12V 2.0.

The old ATX 1.3 standard has a 20-pin power connector that connects the power suppply to the motherboard. The ATX12V 2.0 standard uses a 24-pin power connector.

An adapter is required to be able to use the new 24-pin plug of a ATX12V 2.0 power supply in motherboards that have the current ATX 1.3 20-pin motherboard connector. Most power supply manufacturers include one with their new ATX12V 2.0 power supplies, because 20-pin connectors are still being used on many motherboards. A 20-pin power-supply cable also fits into the adapter, making it possible to use an old-style power supply unit with a motherboard that has the new 24-pin connector.

Note that the motherboards for Intel Pentium 4 and AMD Athlon 64 and the current AMD and Intel processors have what is called an additional ATX 12V Power Connector. It is a small square box on the motherboard for a four-pin plug from an ATX12V 2.0 power supply unit.

Here is what a motherboard manual from MSI says about the connector:

"MSI Reminds You... 1. These two connectors connect to the ATX power supply and have to work together to ensure stable operation of the mainboard. 2. Power supply of 350 watts (and above) is highly recommended for system stability. 3. ATX 12V power connection should be greater than 18A."

Most motherboards with this connector won't boot unless it is connected. You should not use a standard ATX 1.3 power supply unit, because it doesn't not provide such a connector.

Note that some motherboards that run Intel Core 2 Duo and Core 2 Quad processors require the use of 1x12V(8pin) power connector, which not all power supplies provide. However, not all motherboards require an 8-pin power connector. The MSI 945GCM-F motherboard that can run Intel Core 2 Duo processors only uses the additional 1x12V(4pin) power connector.

You should always read the motherboard's user manual all the way through before you start buying or installing the components in the case. The power requirements for the processor and the graphics card(s) will always be made clear in it.

Most of the major motherboard manufacturers recommend power supplies suitable for use with a particular model of motherboard.

You can make use of the Google search box at the top of this page (with its Web radio button enabled) to find local vendors and reviews of any of the products mentioned above.

Look under the PRODUCTS heading on the Tagan site for images of the different power connectors, some of which are also provided further down this page.

Power supplies are often recommended or ridiculed on computer forums.

ATX is a power standard as well as a motherboard standard, as is the newer BTX (Balanced Technology Extended) standard, which was intended to replace the ATX standard for power supply units and motherboards. However, although Dell used it extensively, it has failed to take over from the ATX standard.

In June 2007, AMD announced its own DTX form factor, which is similar to Intel's BTX form factor. However, at the time of writing this (June 2008), the ATX standard still rules.

The cooling of the case is a crucial requirement, especially with high-speed Intel Pentium 4, Intel Core 2 Duo, Intel Core 2 Quad and AMD Athlon 64 x2 dual-core and AMD Phenom quad-core processors.

In any case, always remember to check from the manufacturer's website what level of cooling and power supply is required for a OEM (Original Equipment Manufacturer) processor before you buy it. An OEM processor is supported by the vendor instead of the manufacturer. If it is faulty, you return it to the vendor, not the manufacturer. An OEM processor is usually sold on its own. If so, you would have to buy a cooling unit for it. The boxed, retail product comes with a heatsink and fan unit.

Note that Intel does not allow retailers to sell its processors on an OEM basis; AMD does. The product will always be labelled as an OEM product.

Heat dissipation occurs most rapidly when there is a large temperature difference between the heatsink and surrounding air. It is pointless to invest in a good cooling unit, and then allow heat to build up in the case.

Heat also shortens the life of other components, in particular the video/graphics card and hard disk drives. Consequently, it is important to keep the air well circulated. If the power supply unit's fan is the only source of ventilation, the addition of a second fan is advisable, since many new cases are designed to accept a 60mm intake fan at the bottom front area of the case, where you should see a series of ventilation holes from the outside of the case.

The image above shows power supply units made by Enermax (top) and Antec.

The clusters of wires to the left and right of each unit have the various Molex plugs (or headers) on their ends that are required to connect drives, the motherboard, heatsink and fan units, and sometimes even adapter cards, such as some FireWire cards, etc., to the power supply unit.

If you run out of Molex drive headers, you can use a power Y splitter, which adds two headers by plugging it into one header, but, note well that because they can put a greater load on the power cable than it was intended to carry, they should only be used to power devices that don't draw much power.

If you run out of drive power headers, you can use a power splitter, which adds two headers by plugging it into one header, but they can put a greater load on the power cable than it was intended to carry.

Newer power supplies have the ATX 12V header introduced for use with Pentium 4 and Core 2 Duo motherboards. Such power supplies are often labelled as being "Pentium-4 Compatible", even though they work with other ATX platforms, such as Socket A, Socket 754, Socket 939, and Socket AM2 motherboards that run AMD's Athlon XP and Athlon 64 processors.

See the Processor Sockets page on this site for more information on them.

AMD Athlon 64/Phenom and Intel Pentium 4/Core 2 Duo/Core 2 Quad processors require a motherboard with a power supply with an extra 12V connector that is connected to a 4-pin header on the motherboard, as shown in the image below. Some motherboards have an 8-pin 12V connector. You have to have a power supply that provides the 8-pin connector to use it with such a motherboard.

All fairly recent power supplies have the extra power connector. However, if you have a power supply that doesn't have the extra connector, and which also supplies the correct maximum amperage that can be delivered by its +12V rail (or rails, because some of the latest power supplies have up to four +12V rails), you can't use it to power the above-mentioned processors.

As you can see, the keyed connector can only be connected one way to the motherboard. The hook on the connector fits over the protrusion on the header on the motherboard.

Download the user manual for the MSI 945GCM5-F Socket LGA 775 motherboard, made by MSI, to read the illustrated installation instructions. It can run the latest Intel Core 2 Duo processors and only uses the additional 12V four-pin connector.

It can be very frustrating if a power supply's cables are too short to reach the top or the bottom drive installed in a case, or if the ATX power leads don’t reach to the motherboard when, say, it is installed in a full tower case. Lead extenders are available, but it's always best to buy a fully satisfactory power supply in the first place. Most quality brands of power supply provide plenty of cable length, while many of the cheaper brands do not.

You can purchase individual 430W, 550W, 600W, 700W, 750W, 800W, 1000W, 1200W, 1300W, and 1500W power supplies, or PC cases that come with that magnitude of power supply. Power supplies higher than 700W are used for systems that have up to four graphics processor units (GPUs) in multiple-graphics-card configurations. A standard office PC won't need a power supply higher than 400W.

If you are ignorant on the subject, the above article should be read, because the latest AMD Athlon x2, Phenom x4 and Intel Core2 Duo and Core 2 Quad dual-core and quad-core processors draw most of their power from the +12V rail(s), which was not the case with earlier processors, which draw most of their power from the +5V rail. Therefore, if a power supply designed to run earlier processors is used to run the latest processors, the +12V rail might not be up to the job.

Click here! to go to more information on the second Motherboard page on this site about standard ATX 1.3 power supplies, the new ATX12V 2.0 standard, and how serial-ATA (SATA) drives connect to them. Use your browser's Back button to return to this point on this page.

The Uninterruptible Power Supply Unit - UPS

Note that if your electricity supply is irregular or subject to storm interference, etc., you should invest in an Uninterruptible Power Supply unit (UPS) that you plug the whole system into. The UPS is connected to the wall socket. It has batteries that it keeps fully charged, and when there is a power failure it automatically keeps the power supply to the computer running until either a person shuts it down, or the software it runs shuts the system down automatically.

UPS units vary in price depending on the use. A unit for a single PC will be cheaper than one designed to cover a network. They are available from most of the large vendors of computer components.

Battery technology is improving all the time. The latest UPS is lighter, smaller, and cheaper than its predecessors. For as little as £30/$50, you can buy an amazingly small, entry-level UPS that has a built-in power surge suppressor. It provides enough battery power to give you the time to shut the system down properly in the event of a power failure.

The more expensive UPS run software that comes with the unit, which monitors the system and shuts it down properly, saving unsaved data first so that nothing is lost, or puts it into hibernation so that it can be resurrected without losing data. For mission-critical systems, the software an UPS runs is therefore as important as the UPS unit itself.

UPS can even come with generous insurance policies that pay for the replacement of any equipment that is damaged by electrical problems that the UPS has failed to filter. The longer the run time, the higher the surge ratings, and the greater the insurance protection, the more the UPS unit will cost.

Note that as long as the UPS has the plugs for it, you can plug in any device into the unit that you want to protect against power-supply problems.

APC - http://www.apc.com - make excellent units that can come with excellent software. Even the cheapest APC UPS shut the system down or put it into hibernation automatically.

Click here! for additional information, such as to find out how to do the calculations so that you select the correct UPS for your situation.

Using the Google search box at the top of this page (with its Web radio button enabled) will provide plenty of information on this subject.

Adding additional PC case fans

If you are building a PC with plenty of high-end components, you will want to consider adding extra cooling fans to the case, or buy a case with extra cooling fans. using a search phrase such as pc + case + fans in the Google search box at the top of this page (with its Web radio button enabled), should bring up links to sites with all kinds of information on them.

Using that term, I found the following site that should provide you with all of the information you need on case fans, case modding, (case mod supplies) and power supplies.

Computer Case Fans and Cooling Fans - http://www.xoxide.com/

Some further considerations on this subject...

ATX cases made to accommodate the latest ultra-fast processors are equipped with more than one fan, and the processor and video-card fan can come together to produce a great deal of noise. Because of the noise, you can purchase cases designed to be quiet, such as Enermax Whisper cases, which, of course, cost more than regular, much noisier cases. The Enermax Whisper II power supply, for example, balances maximum power with minimum noise.

The more powerful the power supply in the case, the longer it will be before you have to upgrade it.

If in doubt about the power supply requirements for a particular processor refer to the Intel and AMD websites.

A new PC case will usually come with the connection cable to connect it to a power outlet, and all of the stand-off screws and washers required to mount a motherboard, plus an assortment of mounting screws for drives and adapter cards.

A power supply is easy to install in the case. It just screws into its bay that has a cutting in the back of the case for the fan to blow out of, and is attached to the motherboard by a specially keyed plug - the largest of them - so that it can only be inserted one way round.

In an ATX case, the wires for the power-on and reset buttons, and LED (Light-Emitting Diode) lights, at the front of the case, have plugs that connect to the motherboard that are marked with a short description of their purpose. For instance, HDD written on a connector means that it is connected to the LED light that flickers on and off to indicate hard-disk-drive activity. Note that this light will also flicker when an IDE CD/DVD drive is in use, because it indicates IDE activity, and most CD/DVD drives are IDE devices. However, note that SATA CD/DVD drves are available. A plug marked Reset, will be for the Reset switch, etc. Where the plugs connect to the motherboard will be shown in its manual.

Note that the case might provide more LED plugs or other plugs than can be fitted to the motherboard. This is because the case provides all of the plugs that can be supported by a wide range of motherboards.

An old-style AT case usually has a power supply with a thick black cable coming from it that is attached to the power-on button in the front of the AT case, thereby making the replacement of the power supply much more difficult than it is for an ATX case that has a power supply switched on via a power-on button attached to the motherboard by a cable made up of only two wires. An AT power supply in an AT case has two plugs that are attached to an AT motherboard. They are correctly installed when the black wires on both plugs are next to each other so that all of the black wires are in the middle when plugged into the motherboard. Some older motherboards have both ATX and AT power- supply connection sockets.

An ATX case is unlikely to come with an instruction manual, because you only need to know how to attach five or six plugs to the motherboard, the instructions for which will be provided by the motherboard's user manual, and you can easily work out which of the other power plugs will be attached to a floppy disk drive, a CD/DVD drive, or a hard disk drive. Moreover, the plugs are all keyed so that they can only be plugged in one way round.

The largest keyed plug on an ATX power supply powers the motherboard. It can only fit in the socket the correct way round.

Remember never to use force to fit anything. If the installation of a component seems to require force, the chances are that it is not being installed properly, or is not the right way round.

Socketed processors, for instance, look very strongly built, but they can easily be cracked in half by rough handling when fitting the heatsink and fan unit. And any kind of pin can easily be bent, including those on the processor. All of the pins have to fit the holes meant for them or the device will not work. (Note that Intel's processors no longer have pins, just metal contact points.)

Finally, always remember to make sure that the case and its power supply unit can accommodate the processor and all of the hardware that you want to use in the system.

The more hardware you want to install, the more powerful the power supply will need to be. You cannot just buy any ATX case (with a power supply) and use it for any ATX motherboard and the processors it supports. If you purchase a standard ATX case with a 230W or 300W power supply with the intention of installing a motherboard fitted with the AMD Athlon 64 X2 dual-core or Phenom quad-core processor, a 512MB video/graphics card, and 2GB of RAM memory, plus a huge, fast hard disk drive or two, you won't be able to get it to work.

Google searches

If my descriptions are not clear enough for you, you'll be able to find many sites offering illustrated information on how to install computer components by entering a phrase such as "install a xxxx" (where xxxx stands for the device, such as RAM, hard disk drive, etc.) in the Google search box at the top of this page (with its Web radio button enabled).

PC Buyer Beware! Copyright © Eric Legge 2004-2008. All rights reserved.