Building your PC:
The Power Supply Unit (PSU)

Introduction

In general, a Power Supply Unit (PSU) is defined as: 'a device or system that supplies electrical or other types of energy to an output load or group of loads.' Most commonly the term PSU is used in reference to the power supplies of electrical or electronic devices.

PSU:

Power Supplies and Computers

In computer terms the power supply is a transformer that converts the 110V/240v alternating current of the domestic power supply into the 12v direct current supply required to drive all a computer's components.

In general, the power supply comes as an integral component of the computer case but it is often only attached by five or six screws and if you are upgrading it can be swapped for a more powerful power supply unit.

In common with all electrical devices how powerful a power supply is measured in terms of their wattage. Low-end power supplies usually start at 350W and truly high-end power supplies can reach 600W. Given this range, what is the right power of PSU for you?

Computer Power Requirements

Being electronic devices all the components of the modern PC require electrical power. Everything from the DVD and CD-rom drives, the motherboard itself, the fans, and most especially the CPU and the graphics card.

Power supplies come in two main flavours: AT and ATX named after the design of the motherboard and the power connectors on those motherboards. Though the AT motherboard is now a legacy system power supplies for this type of motherboard (with the single row connector, far left) are still sold. Modern motherboards, however, all come with an ATX connector (dual row connector immediate left). Please note that the two connectors are not interchangeable and a PSU designed for an ATX system will not work in an AT system (and vice-versa).

Considering that motherboards are now using two power connectors (one for the motherboard and a four-pin connector for the CPU), video cards now need additional power connectors, and not to mention all the additional components, it's not surprising that today's computers simply need more than 350W to run everything effectively. As a minimum you need a 400W power supply and you are probably best buying a case with a 450W power supply. Indeed, a 450W power supply is becoming an increasingly-common component in most modern system cases.

It is also the case that if your power supply isn't strong enough to support all the components in your PC you will soon know about it. The machine may work erratically, you may suffer system errors and crashes, the video display may freeze and some of your components such as DVD drives and hard drives may simply fail to work. If this is the case, and there is no other obvious cause for these errors you should upgrade your power supply. (This is really not as daunting a procedure as it sounds. All you need is a screwdriver and a little confidence).

Power Supplies and Case 'Modding'

'Modding', or 'modifying' is a relatively new phenomenon in PC terms. Here components are bought or upgraded because they will add something to the overall appearance of the PC. Often they will include LEDs (light emitting diodes) that display coloured lights of patterns of lights. A good example being the 500W power supply shown here on the left. This has a number of blue LEDs built in and can be used with a transparent or semi-transparent computer case so that when switched on the entire inside of the case glows a blue colour.

Power Supply and Cooling

The power supply is one of the noisiest components in a modern computer and this is mainly due to the design of the ATX case. Indeed, in the ATX case specification, cool air is drawn into a typical case from vents in the front panel. The incoming air helps cool components as it moves through the case, becoming warm in the process. It is evacuated through the PSU and out the rear by the PSU fan. So the loud, fast fans do help to keep a case cooler. Manually varying a high airflow PSU fan can cause CPU temperature to be affected as much as 5–6°C. The problem is that the ATX specification was designed in the 1980s when computers ran at no more than 30W and a relatively slow turning (and therefore quieter) CPU fan was sufficient to cool the entire machine. These days modern PCs typically run at 130W and to cope with the additional heat generated CPU fans need to run much faster. This causes more vibration and more noise. If noise is a real concern for you you may be able to use a PSU with a slower-spinning fan (which are generally available) as long as your fit enough additional fans in your machine's case.

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