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Power Supply Units (PSU)

11/10/08

Power Supply Units (PSU)

Permalink 12:17:59 pm by Eugene Gardner, Categories: Articles

When asked to produce a (no cost no obligation) specification for a new custom PC to suit a particular client I have a small questionnaire completed to let me know the basic requirements in non-technical terms. The two most important questions are in red: one asks which programs will be used, the other asks for the general attitude to buying equipment.

In answer to the second question some clients opt for cheap as possible in the short term, others are willing to pay more initially but want to know what the difference is between two apparently equally powered computers where one costs 10% - 20% more than the other.

A very important but often ignored component is the PSU and this exemplifies why I so often encourage clients to spend more initially. When choosing a PSU it is necessary to consider how much power each main component will require, both initially and allowing for future upgrades.

A good PSU will convert 240 volts AC to, at a minimum, 3.3, 5 and 12 volts DC on different rails. The ?cleaner? the supply from each rail, the longer the supplied sub components will last. And cheap PSUs may allow a fluctuation of up to 20% in the output power.

Another feature not found on cheaper PSU is the ability to switch between AC and DC before frequency smoothing. Switched supplies convert to DC immediately and then use an oscillator to produce a cleaner supply than is possible with regular transformers acting on AC power before it is converted to the DC that computer components require. This prolongs the life of the powered components.

The switching PSUs are inherently more efficient and therefore cooler than cheaper models. This is significant as the heat that is a by-product of cheaper models is not only wasted energy, but energy that has to be dissipated with the help of increased fan speed, power and noise.

Which leads to the biggest difference between modestly priced and well specified PSUs: efficiency. The amount of input power that is required to produce the required output varies considerably. Some PSUs have a power factor of 50%, meaning that half the input energy is wasted, but more expensive units can achieve over 80%.

The difference between 50% and 80% efficiency can be seen in the costings. If you pay 12p per unit of electricity and your PC requires 250 watts. A cheap 500W PSU that is 50% efficient will cost 6p per hour to run. Assume it runs for 40 hours a week, and that will cost 124.80 a year. However, an 80Plus rated PSU would require just 312 watts and the annual cost would be just 78 ? a saving of 46.80 which is, coincidentally, about the difference in price between a bottom of the range PSU and a top notch model.

Assuming the new PC serves for 6 years, the better specified PSU will cost about 46.80 more initially, but 234 less to run over the lifetime of the computer.

The voltages are measured as part of the routine preventive maintenance service. A varience of more than 10% on any circuit is noted.

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