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


This page of the Build Your Own PC section of this website deals with the ATX12V 2.0 power supply unit, often called the PSU, used in ATX PC cases to power all of the PC's components. The power supply is vital part of the desktop PC and should be given serious consideration when buying one. If you can't discover what the make/model of the PSU is before you buy a PC from its specifications, provided by the seller or manufacturer, you should ask the seller, otherwise you will have to open up the case to find out if it is a quality unit or one of poor or bad quality that you should replace. This page provides information on how to determine if a power supply is a unit of quality or not. Remember that there are cases of bad-quality PC power supplies having caused house and office fires. Information is also provided on Uninterruptible Power Supply Units (UPS) used as a backup source of power should the mains power fail.

This article consists of six main sections. You can access the other five sections by clicking the links below.

Power supplies/Power Supply Units - 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 can destroy your power supply unit or at the very least blow a fuse in it. - 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.

Click here! to go to more detailed information on this website on power supplies than is provided on this page.

Click here! to see an annotated image of an ATX power supply unit on this website.

The power supply unit (PSU), usually just called the power supply, is the component that provides the computer's components with power. It is connected to the mains electricity by a power cable and is itself usually connected to the PC's motherboard via a main power connector and an auxiliary power connector. (If the auxiliary power connector is not connected, the PC won't boot - a mistake that is often made by people building their first PC.) When the Power button is pressed in the front to the case with a desktop PC and the power button on a laptop PC, the motherboard turns the power supply on, its internal fan starts working and power is supplied to the components connected to the motherboard.

Some of the PC's components are only powered from the motherboard, such as an adapter card (network card, sound card, etc.), and others are powered only from the power supply (hard disk and optical disc drives). Some high-end graphics cards installed in adapter-cards slots are powered from the motherboard and from one or two power connectors from the power supply.

Since the motherboard has to be switched on via the PC's Power button before the power supply is turned on, if the power supply's fan works while it is not connected to the motherboard then the power supply is faulty. If the power supply doesn't work when the Power button is pressed, then either the power supply is not connected properly to the motherboard or the motherboard is faulty.

You can program what happens when the Power button is pressed whe the PC is working via the Power Options category in the Windows Control Panel. You can set the PC to go into hibernation, standby mode or switch off when its Power button is pressed.

The power supply 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.

In 2005, I bought an Advent desktop PC from PC World for office use that came with a decent power supply unit. When it gave up the ghost, I looked at the specs and bought another Advent desktop PC from PC World, but when I opened the case it had a very light 300W switching power supply made by ISO that can be purchased for $15, so I replaced it with a decent 400W model made by Corsair, because if the power supply goes it can take the other components with it. The power supply wasn't listed in the specs. This is the one component that PC manufacturers can skimp on without the buyer noticing it, so you have to check what is installed yourself. Even reputable case manufactures, such as Foxconn, are using these cheap ISO power supplies in their cases.

Here is a video review of an "Alpine Black 700W Silent Quiet PC ATX PSU" priced at £17.65, which an self-builder should avoid like the plague:

http://www.pcbuyerbeware.co.uk/images/Low-quality-power-supply-review.mp4

It was unable to provide the 460W required by the reviewer's newly-built PC. When he replaced it with a higher-quality make/model, his PC worked properly. Many other purchasers found it inadequate, while others had no complaints, probably because their self-built PCs were built around a motherboard and processor that provided integrated video and sound, or they were using it to replace the power supply in an old, low-spec PC.

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.

Some power supplies have a switch to change between 230V (used in the UK) and 115V (used in the USA), but other models have automatic sensors that switch input voltage automatically, or are able to accept any voltage between those limits. If the power supply uses a switch, you must make sure, having just installed it, that you have it set to use the correct voltage before you switch the computer on. The following Q&A discusses what can happen if you have the 115V switch selected in the UK., which uses 230V mains. In that case, only the fuse in the power supply, which can be replaced easily, probably blew.

Choosing a power supply for your self-built desktop PC is an important factor that can easily be underdone or overdone, depending on your computing needs. A gaming computer requires much more power tha an office computer. The following power calculator allows you to enter the components. When all of the components are entered, the calculator tells you the wattage you require, which allows you to choose a power supply that provides that wattage plus some overhead just in case all of the components are being used at the same time. The power used by the motherboard is factored into the calculation. Search for Enermax Power Supply Calculator to find a good one.

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.

Power supply rail [Page valid Aug. 2012] - http://en.wikipedia.org/wiki/Power_supply_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.

Note that adapter cables are available that bridge the previous and current power supply standards. The following webpage shows which adapters are available:

Power Cable Adapters [Page valid Aug. 2012] -

http://www.akasa.com.tw/update.php?tpl=product/...

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.

Some PCI Express video cards require to be connected to the power supply unit (PSU) via a special PCI Express power connector. If you use two such cards, you need a power supply unit (PSU) with two such power connectors - one for each card. An excellent example is the Akasa Power80+ 500W unit. (Akasa also make excellent cases). It has "Two CPU 12V connectors" that combine to make an 8-pin connector (required by some motherboards) or spit in two to make a 4-pin connector (required by some motherboards) and "PCI-E connectors for multiple dual GPU", which translates as PCI Express connectors for multiple graphics cards.

Note that the very latest ATI (now called AMD) 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 recommendations 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 PC Power Supplies on this page for more information:

Certified NVIDIA SLI-Ready Components -

http://www.geforce.com/hardware/technology/sli

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

http://sites.amd.com/us/game/Pages/game-home.aspx

Note that the video/graphics card manufacturer, ATI, which was purchased by AMD, is now called AMD.

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 (Expert Reviews on the web). 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 supply, priced at £76, was given a Budget Buy award by Computer Shopper (Expert Reviews on the web). 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. I have a Jeantech 350W power supply (that cost me about £30) in a desktop PC that runs an AMD 64 X2 3800+ dual-core processor, 1GB of RAM memory, which has been running flawlessly since 2005. It was recently upgraded to run Windows 7 without having to upgrade any of it hardware (graphics card, memory, etc).

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 supply to the motherboard. The ATX12V 2.0 standard uses a 24-pin power connector. The image below shows the two power connectors, with the smaller 20-pin connector on the left.

Showing the 20-pin ATX 1.3 and ATX12V 2.0 power connectors that connect the power supply to the motherboard

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.

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.

A web search will find local vendors and reviews of any of the products mentioned above.

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.

Watch the following video if you want to see a demonstration of how to install an ATX power supply in a case and connect its cables to the devices to an ATX motherboard and inside the computer.

How to install a Power Supply [the power connectors for the various devices - motherboard, Molex, SATA hard drive, PCI Graphics card, etc., are shown/discussed] - http://www.youtube.com/watch?hl=en-GB&gl=GB&v=xvClS_M-09o

Choosing a power supply for a self-built desktop PC or one that you intend to buy and then add components

Choosing a power supply for your self-built desktop PC is an important factor that can easily be underdone or overdone, depending on your computing needs. A gaming computer requires much more power than an office computer. The following power calculator allows you to enter the components. When all of the components are entered, the calculator tells you the wattage you require, which allows you to choose a power supply that provides that wattage plus some overhead just in case all of the components are being used at the same time. The calculator linked to below takes into consideration the power used by the motherboard.

http://www.extreme.outervision.com/psucalculatorlite.jsp

Modular power supplies from Enermax and Antec

Modular power supplies allow the user to attach only the power connectors in use instead of having them all connected to the power supply whether they are in use or not. They come with a full range of cables. Molex cables connect to IDE hard and CD/DVD drives, PCI Express (PCI-E) cables connect to PCI Express video/graphics cards, and SATA cables connect to SATA hard drives. The cables connect to the devices and to dedicated ports on the power supply unit itself.

The following 500W modular power supplies are products made by Antec and Enermax. There will be more recent models the information about which can be accessed on antec.com and ernermax.com, but these will do as examples here.

The Antec Neo HE500 is supplied with six Molex, two PCI-E, and four SATA cables that connect to five modular ports on the power supply. If you want to use a mixture of Molex and SATA cables, you have to use more than one cable.

The Enermax Liberty ELT500AWT is more flexible than the Antec Neo HE500. It has six modular ports on the power supply for both Molex and SATA cables. It has two dedicated PCI-E ports. The Molex and SATA connectors are one one cable. The unit comes with a neat bag in which to store the spare cables.

If you want more information on these power supplies, look them up on the Antec.com or Enermax.com, or use the make and model as the search query in a web search engine.

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.

PSUs made by Enermax (top) and Antec

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.

A cheap adapter power cable can convert a Molex plug into an SATA power cable that is connected to SATA hard-disk or optical drives (CD/DVD/Blu-ray drives). Adapters are also available that convert a Molex plug into a PCI Express power cable that is connected to a graphics card to provide additional power. The connection points for two PCI Express power cables are shown in the image below of AMD Radeon HD 6790 graphics card.

Where two PCI Express power connectors from the PC's power supply power are connected  on an AMD Radeon HD  6790 graphics card

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 or you have to buy an adapter. An ATX 4 Pin Male to 8 Pin Female EPS Power Adapter Cable, available from Amazon for £2.50, is attached to the 4-pin auxiliary power connected to convert it to an 8-pin connector. It is also possible to use a 4-pin Molex to 8-pin 12V EPS Motherboard ATX Power Cable Adapter to convert a standard Molex power cable used to power non-SATA devices (IDE hard drives, etc.). When I looked for one on Amazon.co.uk, they were out of stock.

The extra 12V power connector from the PSU required to power Intel Pentium 4 and AMD Athlon 64 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.

Note that some power supplies provide an 8-pin connector that can be split into a two 4-pin connectors, as shown in the image below, allowing it to be used with motherboards that require a 4-pin or an 8-pin connector.

An additional 12V power connector that splits into two so that it can be used as a 4-pin or an 8-pin connector

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 donít 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.

The power requirements of the power supply unit (PSU)

The maximum amperage delivered by the +12V rail of a power supply unit (PSU) is of particular importance with modern high-performance systems. The PSU unit itself always has the power details of its different rails printed on it.

Most motherboard manuals tell you what the amperage of the +12V rail has to be in order to power the motherboard properly.

For example, the manual for the MSI RS480M2-IL motherboard, which has an inbuilt video chip and therefore doesn't require a video card to be installed, says that the +12V rail must supply at least 18A (amps) of current. Many cheap off-the shelf PSUs provide only 15A or less of current on the +12V rail, and therefore can't be used with that motherboard if you want a stable system.

If you want to install a high-end video/graphics card the +12V rail would have to deliver between 20A and 30A. Therefore, you should always make sure that the PSU in a system can match the power requirements of a high-end video card before you buy or install it.

For example, it's the middle of June, 2005, and NVIDIA has just released its long-awaited 7800 GTX graphics chip. A single video/graphics card using the chip requires the minimum of a 400W PSU with a +12V rating of 26A, while the SLI, dual-card configuration, requires a 500W PSU with a +12V rating of 34A as the minimum. Most standard PSUs don't usually deliver either 26A or 34A on the +12V rail, so a special PSU is required to run both of those options.

High-end PCI Express video/graphics cards have to be connected directly to the PSU.

If a particular PSU doesn't have a PCI Express connector and one is required of it, it's possible to buy an adapter that is fitted to a standard power connector of the kind that fits to standard IDE ATA (non-SATA) disk drives.

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, Power Supplies and Cases 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.

If want in-depth details of the specifications of a power supply, read this article:

Power Supply 101: A Reference Of Specifications [Dec. 14, 2011] -

http://www.tomshardware.co.uk/...


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 make excellent units that can come with excellent software. Even the cheapest APC UPS shut the system down or put it into hibernation automatically.

Select your [UPS] protection needs -

http://www.apc.com/tools/ups_selector/

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.

A web search for uninterruptible power supply 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 to conduct a web search should bring up links to sites with all kinds of information on them.


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 an Intel Core 2 Duo dual-core or Core 2 Quad quad-core, or an 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.


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