Power Distribution within Six PCs

Power | The Silent Front
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METHODOLOGY

The key to our testing is a Fluke 36 Clamp Meter. This is a device that measures the electromagetic field around any wire that's carrying electricity and translates it into a current readout in Amperes. If you've never seen one before, it's almost magical how a clamp meter works. Playing with electricity, one comes to assume that you have to expose bare wire and have good firm contact before you can take any kind of measurement. Not so with the clamp meter. Typical of such meters, the Fluke 36 is specified for about 1.9% accuracy, which is far less precise than, say, voltage readings from a common digital multimeter. Our meter has not been calibrated for over a year and is primarily an electrician's tool for measuring AC current. It is sensitive to RF fields, which means its accuracy may be adversely affected around lots of electronic gear. In short, it's not exactly a precision lab tool. But, for characterizing general tendencies in power distribution within a PC, our clamp meter is perfectly useful.


This Fluke 36 Clamp Meter was used to make current measurements.

Six different systems were available for testing in the lab:

P4 Socket 478 System

  • Intel Pentium 4 2.8 GHz (Northwood core)
  • AOpen AX4GE motherboard
  • 512 MB OCZ PC3200 RAM
  • ATI Radeon 9600XT AGP VGA card
  • 40 GB Seagate Barracuda IV HDD

P4 LGA775 System

  • Intel Pentium 670 (3.8 GHz)
  • Intel D915PBL motherboard
  • 512 MB Corsair DDR2 RAM
  • AOpen Aeolus 6800GT PCIe VGA card
  • 250 GB Western Digital Caviar SE HDD

Pentium D Dual Core System

  • Intel Pentium D 820 (2 x 2.8 GHz)
  • Intel D945GTP motherboard
  • 512 MB Corsair DDR2 RAM
  • 74 GB Western Digital Raptor HDD

AMD Socket A System

  • AMD Athlon 2500+ (Barton core)
  • MSI K7N2G motherboard (NForce 2)
  • 512 MB OCZ PC3500 RAM
  • 20 GB Seagate Barracuda IV HDD

AMD Socket 754 System

  • AMD Athlon 64 3200+ (Newcastle core)
  • Epox EP-8KDA3+ motherboard
  • 512 MB OCZ PC3500 RAM
  • Matrox MX440 AGP VGA card
  • 80 GB Samsung Spinpoint P80 HDD

AMD Socket 939 System

  • AMD Athlon 64 3500+ (Venice core)
  • DFI NF4 LanParty motherboard (NForce 4)
  • 2 x 512 MB OCZ PC4000 RAM
  • AOpen Aeolus 6800GT PCIe VGA card
  • 2 x 300 GB Maxtor DiamondMax 10 HDDs (in RAID)

The systems represent a decent cross-section of PCs in use today. None of them can be directly compared to the others; what we are looking for is not the differences between systems, but general tendancies of power usage that are true for almost any configuration. A wide variety of CPUs and chipsets were tested, as well as several different VGA cards. Each system was powered by its own PSU; there were a variety of PSU models. All the PSUs were capable of delivering more than 15A on the 12V lines (combined).

For each system, the measurement procedure was the same: The individual wires in the various cable sets from the power supply were separated and then recombined according to the voltage they carried. In this way the total current from each individual voltage line could be measured separately. The two +12V lines were also separated and measured independently. The -12V and +5VSB lines were not measured, as they carry so little current that they are insignificant (typically well under 5W).

Each system was measured at idle and then under load using CPUBurn (two instances were run for multi-threaded and dual core processors).

Ambient temperature at the time of testing was 26°C.

IDLE POWER TEST RESULTS

From a cooling perspective, the power draw at idle is largely irrelevant. A good cooling system must provide adequate cooling when the system is under sustained high load. Any system that can handle this will automatically be cool enough at idle. The same thing applies when sizing a power supply: If it can handle the peaks under heavy load, it should have no problems supplying the power required by a system at idle.

From the standpoint of conserving power, however, the idle power draw is very important because most systems spend a vast amount of time at or close to idle. The total power draw at idle is largely determined by the load on the +12V lines. In fact, with only one exception, the relative power draw on the +12V lines was a good predictor of the total power draw.

POWER DISTRIBUTION WITHIN THE PC: IDLE
System
+12V (total)
+12V1
+12V2 (CPU)
+5V
+3.3V
Total DC Power
AMD Socket 754 Athlon 64 3200+ (Newcastle)
0.8A
0.4A
0.4A
2.5A
2.6A
31W
Intel Socket 478 P4 2.8 GHz (Northwood)
1.8A
0.9A
0.9A
1.1A
3.3A
38W
AMD Socket A Athlon 2500+ (Barton)
4.6A
2.0A
2.6A
2.3A
2.0A
73W
Intel LGA775 Pentium D 820 (2 x 2.8 GHz)
4.6A
0.5A
4.1A
3.6A
0.7A
76W
Intel LGA775 Pentium 670 (3.8 GHz)
5.1A
2.3A
2.8A
2.9A
1.5A
81W
AMD Socket 939 Athlon 64 3500+ (Venice)
4.6A
4.0A
0.6A
3.0A
3.9A
83W
Listed in order of increasing total DC Power

Note that the systems were listed in the table above in order of increasing total power. That the A64-3200+ system should come in with the lowest power draw at idle was no surprise, given what we know about the relatively high efficiency of the A64. The modest idle draw of the P4-2.8 is a bit of a surprise, as is the high power draw of the Athlon 2500+. The latter certainly dates it as a pre-Cool 'n' Quiet AMD processor, but there may have been other factors, as discussed below.

The A64-3500+ system was arguably the most powerful system tested; it was the only system with two sticks of RAM and the only one with two hard drives. This is reflected in the relatively high current draw from both the +3.3V line (for RAM) and the +5V line (for the HDD). In fact, the combined power draw on these two lines totaled almost 30W, about 50% more than the other systems, which drew 18~21W from these lines at idle. This is enough to make this system the most power hungry at idle by a small margin.

So, why does an ostensibly power-efficient AMD-based system draw the most idle power of any system tested? The extra RAM and hard drive obviously contribute a little, but not enough to explain the 50W gap between this system and the Socket 754 system. Most of the power is being drawn on the +12V1 line, so it makes sense to figure out what is being drawn from this line. The likely suspect is the high-powered 6800GT VGA card, which consumes a significant amount of power.

However, this does not explain why the Pentium 670 system — with the same VGA card — draws so much less current from the +12V1 line. We can only speculate. Perhaps the power regulation circuitry on the LGA775 motherboard is more efficient, or maybe the nForce 4 chipset for the AMD system is especially power hungry. Another possibility is that two motherboards may divide up the power from the different voltage lines in slightly different ways. Ultimately, it doesn't really matter which component required the additional power. As far as the power supply is concerned, all that matters is the distribution across the various voltage lines.

The vanilla Athlon system also draws a substantial amount of power from the +12V1 line. This is unlikely to be needed by the CPU, which has its own voltage line, and it does not have a video card. So, the power seems to be required for the motherboard itself. The only other load on the +12V1 line is the hard drive, which accounts for just 0.3A. But, why should the motherboard require so much power? It's impossible to say for sure, but is it possible that the onboard video, based on the GeForce 4 MX, is the culprit?

Despite all of these minor differences, the general trend was quite clear: The +5V and +3.3V lines draw very little power. Furthermore, the total power required by these lines does not vary much between systems. Building a system that draws less power at idle seems to rely mainly on keeping the power draw on the +12V line as low as possible.



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