Power Distribution in Three PCs (2012)

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Power Distribution in Three PCs (2012)

February 26, 2012 by Lawrence Lee

A few years ago we analyzed the power consumption of six desktop PCs to uncover just how much energy they consumed and how the power was distributed within each machine along the power supply's various rails. These computers were based on Pentium 4, Pentium D, Athlon, and Athlon 64 processors, dinosaurs by today's standards. The results of our testing gave critical insight into power supply requirements at the time.

Since then energy efficiency has become an important factor in hardware selection. Today, a CPU, GPU, motherboard, hard drive, or complete PC review isn't complete without power consumption measurements — it has become standard operating procedure for most tech sites. Not only does this help end-users pick out the right PSU, it also allows them to weigh the financial and environmental ramifications. Given this new climate we thought it would be interesting to revisit the subject with a set of modern PCs to see how things have changed.


An Extech True RMS Power Analyzer.

The first piece of equipment we'll be using for our tests is an Extech True RMS Power Analyzer/Datalogger. This is an expensive tool that displays four readings (watts, power factor or VA, voltage or frequency, and current) simultaneously using True RMS voltage and current measurements of sine, square, triangular and distorted wave forms. It's overkill as we'll only be using it to measure the total AC power draw from the wall, but it does deliver better accuracy than simple wall meters like the Seasonic Power Angel and Kill-A-Watt.

This measurement will tell us exactly how much electricity is needed to use the system as well as give us a rough estimate of the output required for a power supply to run it. A higher draw not only means a heftier electric bill but also translates into a larger impact on the environment (from the resources consumed by the utility company to generate the extra energy) and a louder cooling system to deal with the increased waste heat of the various components inside.


A Fluke 36 Clamp Meter.

Another key to our testing is a Fluke 36 Clamp Meter. This device measures the electromagnetic field around any wire carrying electricity and translates it into a current readout in Amperes. It's not a precision lab tool, being sensitive to RF fields, with a margin of error of about 1.9% (far less accurate than that of a common digital multimeter), but it is a safe and easy way to isolate the power being delivered along each rail (+12V, +5V, and +3.3V) to the various components of the system. Clamping the meter around each set of power cables is certainly more convenient than exposing wires to measure the current directly.

Determining the power draw along the three main rails will tell us whether the distribution within modern power supplies is appropriate. When we did this test six years ago, 75~90% of the total power consumption was on the +12V line, but most power supply units at the time had outputs just about even in terms of current between the three main rails (+12V, +5V, +3.3V). Today the current available on the +12V line is frequently twice that of the other two, more so on high capacity units.



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