Understanding power supply units

Many of us upgrade our PCs regularly: we want faster processors, dual graphics cards and Raid systems. New power supplies though? To most of us they’re just not sexy, but you overlook the PSU at your peril.

In the body of your PC, if the CPU is the brain then the PSU is certainly the heart. Often overlooked, a ‘dicky ticker’ can be the cause of many computer problems, from baffling and seemingly random crashes through to sudden deaths, accompanied by loud explosions. And as well as increased system stability, a decent PSU may bring quiet or silent operation, better overclocking and more capacity for upgrades.

Why a new power supply?
If you like to keep your PC up to date, you’ll know very well how much of a boost to performance there can be from new graphics cards and dual-core CPUs. With these upgrades consuming so much of your hard-earned cash, it’s easy to overlook the increasing demands these products may make on your power supply.

It’s true that in many cases newer products are designed to consume less energy than their predecessors, but an upgrade in performance within any given technology will usually result in increased power demands. You can’t just take it for granted that your power supply will cope with the extra load, or even come with the requisite number of connectors.

As PCs have evolved, so have PC power supplies. Modern motherboards, graphics cards and hard drives require different PSUs to older ones; in fact, if you’re upgrading a motherboard you may have no choice but to buy a new power supply, as the old one may not have the right connectors.

And even if you do have one with the right connectors, it may not be up to the job; see later for an explanation of how you can perform some basic tests.

When choosing a new power supply there are some key things that you need to consider:

• The total available power
• The power available to individual components
• The number and type of available power connectors
• The amount of noise generated by the power supply
• The efficiency of the power supply (if you care about the environment or your electricity bills)
• The cost of the power supply itself.

Estimate your power requirements
With so many different components available to anyone building a PC, an exhaustive list of component power needs is virtually impossible to put together; finding out the details usually involves rifling through product data sheets and even then it’s rare to find a straight answer to the question, ‘How much power does this component need?’ However, one thing’s very clear – the two biggest consumers of PC power are CPUs and graphics cards.

CPUs
Intel and AMD don’t quote power consumption figures directly. Instead, they quote the processors’ Thermal Design Power or TDP, and to make matters worse, they don’t calculate these figures in the same way.

TDP is a measure of how much heat the processor will dissipate under load. This is intended to help you to specify the correct cooling system for your CPU. While AMD tends to quote a figure based on the maximum current the processor can actually draw, Intel’s estimates are more conservative.

Typical TDP specifications range from about 62W for an AMD Sempron to over 130W for a dual-core Pentium Extreme Edition, based on the Prescott core. Overclocked CPUs can require well over 300W. You can find detailed processor specifications at Intel’s and AMD’s support websites.

Graphics cards
Some graphics cards, such as Nvidia’s workstation Quadro range, quote power consumption figures explicitly. These range from 105W to 136W per card. Obviously, you’ll need to double up if you’re using a dual-card system.

Nvidia and ATI also certify power supplies for use with dual-GPU SLI and Crossfire systems.

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