by Parm Mann on 26 October 2006, 00:00

Quick Link:

Add to My Vault: x

The PSU, or Power Supply Unit, is one of several devices that one could argue is the most important component within a computer. Sufficient to say, without a PSU, the computer won’t work, so they are pretty important. Even more significantly, with a faulty PSU installed, all sorts of other problems can manifest themselves. It’s essential to have a PSU that can supply enough power, reliably.

In this guide we’re going to explore how a PSU works so that we can better understand their importance within a PC.


The modern ATX PSU is designed to provide a few hundred watts of power to the devices within (and in some cases attached to) a PC. All PC components require DC (direct current) power of some form, but the required voltages vary. So, a PSU must be able to provide several different voltages and deliver sufficient current at those voltages.

For a quick science lesson, a watt is a measure of power, while voltage is the potential difference across a component and current is a measure of the flow of electric charge through a component. Multiply the current flow through a device by the voltage across it and you have the power being consumed by that device.

A PSU accepts mains power from a household (~230 volts, alternating current here in the UK) and must then produce DC electricity that can be used by the PC components. The first thing that happens to the mains voltage is that it’s stepped down to a lower voltage through a transformer. Next, it’s converted to DC through a rectifier circuit.

Once in DC form it can be regulated to several specific voltages. Devices are wired up to the PSU to give them the voltages they need to operate.

So, a PSU takes mains power and converts it into several DC voltages. Simple as that… ish.

More detail

Interested to find out a bit more about the PSU? Stick with us in this section, then.

The ‘several DC voltages’ produced by a PSU are divided up into a number of ‘rails’. The main voltage rails you’ll find operate at 12V, 5V and 3.3V. Most devices take in power from one or more of these voltage rails. However, the amount of current needed on these rails, particularly the 12V rail, has increased to the point where more than one rail is required.

A modern high end ATX PSU has several 12V rails, as many as four, in fact. Each individual rail has a limit to the amount of current that can be provided on it. The uses for the 12V rails are split. For example, the +12V 4-pin power connector that you’ll find on most motherboards, tends to be fed by the 2nd 12V rail.

The maximum current each rail can provide doesn’t necessarily tell us the maximum power the PSU can output. The design of the PSU often means that it won’t be possible for all rails to output at their full capacity simultaneously. There is a sticker on a PSU detailing the currents available on certain rails and it will often also detail the maximum power output of a group of rails (often 5V and 3.3V have a limited combined output power.)

Aside from the ‘basics’ of providing power, there are a few extra wires on a PSU, that connect with the PC’s motherboard, which serve other purposes. One wire (green on an ATX connector) is used to control whether the PSU is on or in standby. This allows the motherboard to turn the system off by itself, and is why we can wake computers from standby without pressing a button. Another wire provides a standby current. It’s always present and provides limited current to a few devices, normally those that need to be kept alive, listening for a wake up signal.

Further wiring includes a PWR_OK signal wire, used to indicate whether there’s power coming into the PSU and whether the 12V, 5V and 3.3V rails are running at high enough voltages. There are also -5V and -12V power feeds.


Converting AC mains into low voltage DC wastes some power. The PSU’s components get warm; the power is wasted as heat. The hotter components get, the more resistive they generally get, which makes them hotter still and so even less efficient. So, an efficient PSU needs to have well specified components that are up to the job they are being asked to do and those components need to be kept cool.

The efficiency of an average PSU is roughly between 60% and 70%. That means for every watt of power provided to the PSU, 0.6 to 0.7 watts of useable electricity come out of it. Some PSUs are more efficient, 80% or more. They’re often more expensive.

What wattage?

Just how much power does a system need? PSU requirement calculators exist, but they’re usually inaccurate or a bit over zealous on power requirements.

For a modern mid-range system, a PSU rated at around 400W sustained output should be ample. High end CPUs, graphics cards and numerous hard drives will all push the requirement up. PSUs above and beyond 700W exist, but very few systems require this much power.

One thing to beware of when purchasing a PSU is whether its rating is peak or sustained. If the rating is for sustained output, then the manufacturer is saying that the PSU will be able to constantly provide that much power (although usually at a specific temperature). Peak ratings are not much use, but the PSU will only be able to provide that much power for a short time.

Do note that if you buy, say, a 500W PSU, then it will only use as much power as it needs. A 500W PSU can provide 100W quite happily, so you won’t be using ~650W of power all the time (remember that PSUs aren’t 100% efficient).


If a PSU cannot provide sufficient power then all sorts of things can happen. Rail voltages might drop, leading to one of more devices within the PSU operating erratically. As such, a problem that may seem like graphics card related might actually be PSU related, for example.

In other scenarios, the over-current protection of the PSU may kick in, making the unit power down. This makes it slightly more obvious where the problem resides (although a system might power itself down for other reasons, too!)

The worst case scenario is where a component within the PSU fails. In some cases, voltage spikes may occur, damaging not only the PSU, but the components it’s attached too as well. If a PSU goes bang, it won’t necessarily do any harm to the rest of the PC, but it’s better to do everything to avoid it going bang in the first place.

The Market

As a rule of thumb, if a particular PSU looks too cheap compared to similarly rated products, chances are it’s not a good purchase. It’s likely its rating is peak rather than sustained, or it might just be poorly constructed. Still, you don’t have to pay over the odds for a decent PSU.

Some PSUs come with certain features, like temperature controlled fans for quiet operation, or no fans at all. For neat freaks, modular PSUs are popular, whereby the power cables are removable, so that only the required cables need be kept within the system.

As PCs increase in performance, so to does the amount of power they consume, which is why we’ve seen PSU manufacturers come out with higher and higher rated models. However, as we’ve already stated, few systems actually require the super-high rated PSUs that you’ll see.

The Players

Some PSU manufacturers are respectable, others are atrocious. Names such as Antec, Enermax, OCZ, PC Power & Cooling, Coolermaster and FSP are well respected and known for producing high quality products.

There are a great many PSU manufacturers in the market. Some are re-branded versions of other PSUs, perhaps with slightly different features. The good thing about the number of players is that there’s a lot of competition, which hopefully means the prices we see are competitive and the products ever improving.

So, to summarise. PSUs are a rather complex device that ensure the components in a PC get enough current at the right voltage. A good PSU will avoid problems like excessive current draw and incorrect voltages, while being efficient in operation. Get a bad PSU, or one that cannot provide enough power to your system and the system may become unstable, or the PSU might fail and take something with it. A bit of sensible product selection will ensure you get a PSU that keeps your system well fed with juice.

Sponsered by SCAN