Switch on and save

The output of the transformer is converted to DC again and regulated to provide the appropriate voltages to supply the PC.

The outputs are also monitored and a feedback system uses the output voltages to control the oscillator, ensuring the output is kept within pretty tight tolerances.

Switched mode power supplies can be more compact and efficient than traditional ones. And since they don’t rely directly on the input, but instead on AC generated by the oscillator or ‘chopper’ transistors, they’re less susceptible to interference from the mains.

Conversely, a poorly designed switched mode PSU can feed back interference to the mains supply from the high-frequency switching. That’s why cheap power units, such as telephone chargers, sometimes cause problems with home powerline networking systems.

Efficiency
While all switched mode power supplies are based on the same principles, they’re far from equal. As well as things such as the amount of interference they can cause to the mains supply, there are other differences, too. One is called the ‘power factor’. A PSU with a lower power factor requires more energy from the electricity supply to deliver the same amount of power to the computer. The power factor isn’t the only thing to bear in mind, though. Just as with other sorts of power supply, the one in your computer generates some heat, although nowhere near as much as if it was used as an ordinary transformer. And some of the energy is lost through the other components in the power supply, too.

Don’t forget that in a modern PC, the power switch often doesn’t disconnect the mains, as it would in older ones. Instead, there’s a small amount of power used all the time, allowing systems to wake from sleep at the touch of a button, or be powered on and off remotely. Some subsystems run all the time - for example, to power system Ram in Vista’s hybrid sleep mode.

A few years ago, we might not have considered how important the efficiency of a PSU is, but with fuel bills soaring and at the same time some components, such as graphics cards, becoming more power-hungry, if you’re looking for a new PSU, you should consider how efficient it is. You might also be alarmed at how inefficient some may be. For example, PCW labs tested some that were only 46 per cent efficient; in other words, 54 per cent of the power taken from the mains was wasted, leaving users to pay for twice as much as the PC itself actually used.

In some circumstances, it can even be worse than that. An inefficient PSU creates more heat, and in an office or server room full of them, more energy is needed for air-conditioning to maintain a suitable temperature. So, whether it’s for a home PC or a whole office full of them, making the power supply efficient is becoming more important.

Star ratings
The latest versions of the Energy Star (www.energystar.gov) ratings in the US require a minimum efficiency of 80 per cent for power supplies, as well as specifying the maximum power consumption in sleep and standby modes. As they’re required for US Government purchasing, most main brands comply.

The 80Plus programme (www.80plus.org) is a labelling system for PSUs that fit the new criteria - being at least 80 per cent efficient at 20, 50 and 100 per cent of their rated load. That’s important because power supplies are often less efficient when they’re not fully loaded, so while it might seem tempting to choose a higher rated PSU when you’re building a computer, in case you decide to expand later, it could turn out to be costly.

Let’s say you pay 12p per unit of electricity and your computer components require 250W. Put in a cheap 500W PSU that’s only 50 per cent efficient when delivering half its rated load, and the computer will draw 500W from the mains, costing you 6p per hour. Using it eight hours a day, five days a week, will cost £2.40, a total of £124.80 per year. Fit an 80Plus-rated power supply on instead, and the computer will consume no more than 312.5W, equivalent to 2.5 units of electricity per day (30p rather than 48p) for a total cost of £78 per year, saving you £46.80.