Tech Files
Power Supply Efficiency
You know what a power supply does for the PC, right? It takes AC electricity out of the wall socket and converts it to DC electricity that the motherboard and other internal devices require in order to function.
You probably also know that your devices require proper voltage (5-volt for logic, 12-volt for motors) to function correctly. You might know that every device uses a measurable amount of current, expressed as amperage used. Those two factors combine to determine the amount of wattage your PC power supply needs to produce so the PC runs and remains stable.
The formula is simple. Take the voltage and the amperage used by a device and multiply together to get the wattage requirements (VA=W). Here’s a sampling of typical component power needs.
Device |
Wattage1 |
Motherboard |
20-30 |
Athlon XP CPU |
60-70 |
RAM |
15 |
Hard drive (PATA/SATA) |
15/30 |
CD-ROM drive |
27 |
Video card |
30+ |
Sound card |
5-10 |
1. Most of these numbers are drawn from individual companies, such as AMD for the Athlon, Seagate for hard drives, and so on. Some numbers drawn from a PC World online article.
The numbers start adding up with a fairly complex system—like almost every enthusiast’s rig—especially if you start adding features like multiple system fans, cathode ray tubes for interior lighting, and more. If you overload a power supply, the PC will either fail to start at all or start and turn into a stability nightmare. Still, even with all these extra bells and whistles, a good 300-400 watt power supply will handle all without flinching… Right?
Not so fast!
Power Factor
Converting from AC to DC comes at a certain cost. You didn’t think humans could invent something efficient, did you? Not a chance. The average wattage lost in the conversion process is 30-50 percent! The number you really want to know when examining a power supply is the power factor (PF) number. Most power supplies have a PF number around 0.7, meaning a 30% power loss. Suddenly that nice, shiny new and oh-so-expensive 500-watt power supply looks a lot more enticing!
Active PFC
Some newer power supplies have added active power factor correction (Active PFC) circuits that smooth and tone the power conversion process, making it much more efficient. Active PFC power supplies have a PF upwards of 0.95, meaning only a 5% or less power loss in the conversion process. As an important added benefit, the Active PFC process reduces harmonics in the power supply—vibration that creates both physical noise (like a high-pitched whine) and electrical noise. To deal with harmonics, electrical components have to work harder and thus burn out more quickly. Using Active PFC puts a lot less strain on the internal components of the power supply.
Bottom line? An Active PFC power supply converts energy efficiently. As an added bonus, it’s quieter and will last longer than a power supply without it.
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Additional References
http://www.pcpowerandcooling.com