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If you’re thinking about upgrading or building a computer from scratch, the important parts that first spring to mind are almost certainly the processor, motherboard and hard disk.

But there’s one part without which none of the others will work – the power supply unit (PSU). Yet all too often, it’s an afterthought; you put together a PC, painstakingly choosing the right components, then simply accept the no-name power supply that comes with the case.

If you do think about the PSU, it’s usually in terms of whether or not it has enough oomph for all the things you’re fitting in the case. And upgrading? Well, that will probably only happen if the old one blows up.
The power supply is one of the most crucial elements in your PC, but it is also one of the most ignored, except perhaps by a hard core of upgraders and tweakers.

In this article, we explain why you should pay more attention to your PSU, and what you should look for when choosing a new or replacement one.

Power behind the button
Before looking at the technology inside PSUs, it’s worth remembering how crucial they can be. If you’ve ever dabbled with overclocking, or read about it in PCW's Hands On section, you’ll know that one of the key parts of making a system run well when overclocking is adjusting the voltage of different parts of the memory and/or processor – sometimes in quite small increments. This gives you a big clue that getting the right amount of power to the right parts of your PC is vital. And it doesn’t apply to just overclockers.

If you don’t have a stable source of power, you’ll suffer random crashes and freezes. In a modern PC, there can be quite a lot of demands on the power supply - from hard disks spinning up to high-end graphics cards. In high-end systems, there may also be sophisticated coolers that require pumps, fans and thermo-electric elements.

A good power supply must ensure that your PC’s motherboard has all the power it needs without voltages fluctuating. In a modern PC, the PSU provides a range of voltages – 5V, 3.3V and 12V. If your experience of PSUs is limited to the sort of hefty transformers used for model railways, you may well be surprised at the relative lightness of a modern PC supply.

Switch on
PCs use a type of PSU called ‘switched mode’ (also known as switching or switch mode). In a more traditional power supply, a large transformer first steps the mains voltage down, then it’s converted from alternating current (AC) to direct current (DC), and smoothed and regulated to ensure the correct stable voltage is delivered. And in this type of power supply, a proportion of the energy drawn from the mains is lost as heat in the transformer.

Switched mode supplies work rather differently – although they still have a transformer, it’s not connected to the AC mains. Instead, the power that arrives is converted directly to DC. Often the rectifier circuit that does this can also work as a voltage doubler; the switch between US and European voltages in effect turns this on/off, so the rest of the design will work identically anywhere in the world.

Since it’s created from the alternating current mains, the DC power will rise up and down, so it’s smoothed out and used to feed a high-frequency oscillator (a transistor that switches on and off rapidly, hence the name switched mode).

That might sound a little odd (converting AC to DC and back again), but the oscillator makes a square wave – one where the new AC current is off or on, rather than rising and falling like the mains. And it takes advantage of one of the useful properties of transformers; for a particular size of transformer, the amount of power it can handle increases with the frequency, so instead of a huge mains transformer operating at 50Hz, a switched mode PSU can use a smaller transformer operating in a range from 10s to 100s of kilohertz.

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