Power Distribution within Six PCs

Power | The Silent Front
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POWER DRAW OF INDIVIDUAL COMPONENTS

In addition to looking at differences in power consumption between idle and load, we also looked at how idle power consumption changed when individual components were added or removed from a system. This allowed us to judge roughly the power overhead for various components. Several different parts were tested: Two different kinds of RAM, two different video cards, a PCI Ethernet card, and an optical drive. Hard drives were not tested, as we already have a reliable method of determining their power consumption.

For each component, the power consumption on each voltage line was measured with the system at idle. These measurements were then compared to the relevant idle measurements without the component installed. The results below represent the net change between these two measurements. Differences of 0.2A or less were assumed to be within the margin of error for our testing equipment, and are therefore not included in the results below.

RAM

Two types of RAM were tested: Regular PC3200 DDR and 533 MHz DDR2. In each case, a single 512 MB stick of RAM was added to an existing system configuration. The P4 Socket 478 system was used to test the PC3200, and the Pentium D Dual Core system was used to test the DDR2.

To stress the RAM modules while we measured them, we used Memtest86 to drive the power consumption up. Because Memtest86 runs under its own OS, not Windows, a new idle baseline was needed for accurate comparison. Measurements for each type of RAM, at idle and under load, are given below. The total power draw of the system is only given for the 512 MB configuration at idle. All other configurations are given as differences in power relative to this baseline. Note that the power consumption for the +12V2 line was not listed; it was assumed that the CPU was the only load on the +12V2 line and therefore not used by the RAM.

POWER CONSUMPTION: RAM
RAM Type
Size
Load
+12V1
+5V
+3.3V
Rise from Baseline
PC3200 DDR
512 MB
Idle
0.5A
0.6A
3.0A
n/a
Memtest86
No Change
No Change
+0.7A
+2.3W
1 GB
Idle
No Change
No Change
+0.6A
+2.0W
Memtest86
No Change
No Change
+1.0A
+3.3W
533 MHz DDR2
512 MB
Idle
0.5A
3.6A
0.5A
n/a
Memtest86
No Change
+0.4A
No Change
+2.0W
1 GB
Idle
No Change
No Change
No Change
No Change
Memtest86
No Change
+0.9A
No Change
+4.5W

The power consumption of a single 512MB stick of SDRAM, either regular DDR or DDR2, was too small for the coarse resolution of our measuring equipment. The largest measured increase was only marginally larger than the potential error in the system: 0.2A * (12V + 5V + 3.3V) = 3.9W. Let me state this in another way: It is statistically possible that the total power consumption went down for almost every test case, which is a result that should be impossible given proper test conditions.

That said, the very fact that our potential for error is so high implies that the power required by RAM is basically insignificant. Even in the worst possible case, DDR2 under load, the total increase over the baseline measurement remains under 10W.

A tentative estimate of the power draw for a single 512 MB stick of SDRAM can be gleaned by comparing the rise in load when a stick of RAM is added. For ordinary DDR, this change was a paltry single watt, and the higher speed DDR2 was limited to 2.5W. When idle power requirements were compared, the situation was reversed: Ordinary DDR increased power draw by two watts, while DDR2 did not appear to draw any power. As mentioned above, these numbers are too rough to draw any firm conclusions, but the average power consumption appears to be in the 1-3W range per 512MB stick.

PCI Ethernet Card

The P4-2.8 GHz system was used as a baseline for comparison. There were no identifying marks on the card itself, but the controller chip was marked "RTL8139B", which identifies RealTek as the OEM.

POWER CONSUMPTION: PCI ETHERNET CARD
Load
+12V (total)
+5V
+3.3V
Net Change in Power Draw
Idle
No Change
No Change
+0.5A
1.6W
Network Data Transfer
See Text
See Text
+0.6-0.8A
2-2.6W

Like the RAM test, only a small change in the load on the +3.3V line was observed after the Ethernet card was installed. The net increase was ~2W: Barely significant.

An attempt was made to measure the power consumption during a sustained data transfer. This produced a small increase on all voltage lines, but it is unlikely that the increases on the +12V and +5V lines could be attributed to the PCI card itself. Most likely this power was needed by the hard drive to copy the data used during the transfer. Hard drives do not consume power from the +3.3V line, so it seems safe to attribute this increase to the add-in card.

Optical Drive

The baseline system is completely irrelevant to the measurements of the optical drive (Creative Labs 52x CD-ROM). Because the optical drive is powered by an individual IDE power connector, the current through these wires could be measured directly.

POWER CONSUMPTION: OPTICAL DRIVE
Load
+12V
+5V
Total Power Draw
Idle

0.0A

0.3A
1.5W
Typical Read
0.3A
0.4A
5.6W
Full Speed
1.1
0.5A
15.7W

At lower speeds, the power draw for the optical drive was fairly small, but at full speed and spin-up the sustained power draw was about 15W, mostly from the +12V line. This is enough to be worth considering when sizing a power supply.

Video Cards

Three different video cards were also tested: A Matrox G550 and an ATI Radeon 9600XT. AGP cards were tested on the P4-2.8 socket 478 system, and the AOpen Aeolus 6800GT PCIe card was tested on the Intel Pentium D dual core system. Only differences at idle were examined, as trying to gauge differences at load proved to be near impossible because too other components are brought into play (particularly the CPU).

POWER CONSUMPTION: VIDEO CARDS
Video Card
+12V (total)
+5V
+3.3V
Change in Power
Matrox G550
No Change
+0.3A
+1.7A
+7.1W
ATI Radeon 9600XT
+0.3A
+0.4A
+0.3A
+6.6W
Aeolus 6800GT
+3.0A
No Change
No Change
+36.0W

The Matrox and the ATI both drew less than 10W at idle. The Matrox card drew most of its power from the +3.3V line, while the Radeon seemed to draw power from all three main voltages.

The Aeolus 6800GT, on the other hand, drew much more power at idle — about five times as much. All of the power came from the +12V line; neither of the other two lines were affected. To put it in perspective, the entire A64-3200+ socket 754 system used less power at idle than this video card.

Although the 6800GT uses a PCIe connector to draw power directly from the power supply, most of its power seemed to be coming through the PCIe slot on the motherboard. Of the total 3.0A load on the +12V line, 2.1A was drawn through the motherboard, and 0.9A came from the direct connection to the power supply. Note that this is only at idle; it may change during high load.

From the measurements presented here, it is hard to generalize about the power consumption of video cards. The power requirements for each card are unique; there do not seem to be any similarities between the models we examined. That said, it is well known that the most powerful (and recent) cards draw from the +12V line. This can be seen by examining the external PCIe connector: It has only +12V and ground wires. It is important to remember that, while this generalization might be true for powerful cards, the more mainstream cards do not necessarily echo this power profile.

THE HEAVIEST LOAD

As a matter of curiosity, a system of the most power hungry components in our lab was put to the highest load we could devise. The AOpen Aeolus 6800GT was installed in the Pentium D Dual Core system, and both the CPU and the GPU were stressed by running two instances of CPUBurn simultaneously with 3DMark05. Peak power consumption was noted on each voltage line.

In addition to the usual measurements, the +12V wires in the ATX motherboard connector were measured separately from the rest of the +12V wires, as were the PCIe wires. This allowed us to measure the power drawn by the video card from the power drawn by the CPU. The +12V ATX wires were also measured without the video card installed so that a baseline could be established; no current seemed to be drawn from these wires unless the PCIe slot was in use.

POWER CONSUMPTION: HEAVY LOAD TEST
Load
+12V (total)
+12V1
+12V2
+12V (ATX)
+12V (PCIe)
+5V
+3.3V
Total Power
2x CPUBurn +
3DMark05
16.9A
6.4A
10.4A
3.4A
2.5A
3.6A
0.7A
223W
91%
34%
56%
18%
13%
8%
1%

Under this extreme load, the total power draw peaked at about 220W. Neither the +5V nor the +3.3V lines drew appreciably more power during the test. About 90% of the power was drawn though the +12V line.

Although the overall power draw was much higher than the original CPUBurn test, it is interesting to note that the CPU seemed to consume slightly less power when 3DMark05 was added to the mix. Since CPUBurn should have been using all idle processor cycles, it is possible that the CPU operations demanded by 3DMark05 do not require as much power as the ones used by CPUBurn.

The total power draw of the Aeolus 6800GT at load can be estimated at about six amperes on the +12V line, or around 70W. At first glance, this seems considerably higher than the 55W measured for the 6800GT by X-bit Labs. However, our loads and measuring tools are different from the ones used by X-bit Labs. In addition, our measurement also includes efficiency losses that may occur in the power circuitry in the motherboard.

CONCLUSIONS

It is important to keep in mind that the measurements presented here are continuous loads. Our test equipment does not have the resolution to measure peaks, which may last for 10 ms or less and may be much higher than the continuous load. Most power supplies are rated for a continuous load with allowances for higher peaks, but the internal protection circuits may still be tripped by an exceptionally high peak. It is wise to leave perhaps 30% headroom for peaks when sizing a power supply.

With these caveats, some broad, predictable conclusions can be drawn:

1) It seems to be highly unlikely that a modern system will ever overload either the +5V or +3.3V lines of a ATX12V 2.x compliant power supply. In our systems, neither of these lines ever drew more than 5A under any circumstance, and many power supplies rate them above 20A. The power draw on these lines tended to be quite stable and did not fluctuate much with load.

2) The +12V lines, on the other hand, are very heavily used, especially under load. For Intel-based systems with no external VGA card, this power comes almost exclusively from the +12V2 line. Adding a high powered VGA card may add some load to the +12V1 line, although not all cards use +12V. The systems with AMD CPUs tended to draw power more evenly across the two +12V lines, mainly because they do not consume as much power as Intel CPUs.

3) Looking at the total power draw alone, it would appear that all of our systems could easily be handled by a 300W power supply. Given that as much as 90% of that power comes from the +12V lines, it is likely that that the ratings for the +12V lines matters more than the total wattage. If these lines are inadequate, the power supply may not provide enough power even if its "wattage rating" exceeds the total power draw of the system. It would be wiser to qualify our statement thus: When it comes to adequate power delivery, all of our test systems could easily be handled by a 300W power supply that conforms to ATX12V v2.xx. Conversely, an older PSU rated honestly for 300W output may not be adequate for the most powerful system examined here because of the much lower 12V current capacity on models that comply with v1.3 and earlier versions of the ATX12V spec.

NOTE: None of the above conclusions are meant to suggest that power delivery alone are the only criteria by which a PSU should be chosen. We are only considering adequate power delivery. We have not touched on noise, efficiency, cooling, voltage regulation — in short, all of the other relevant criteria we examine in our PSU reviews.

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