Power Distribution within Six PCs

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CPU LOAD POWER TEST RESULTS

POWER DISTRIBUTION WITHIN THE PC: CPU LOAD (CPUBurn)
System
+12V (total)
+12V1
+12V2 (CPU)
+5V
+3.3V
Total DC Power
AMD Socket A Athlon 2500+ (Barton)
5.4A
2.3A
3.2A
2.3A
1.8A
82W
AMD Socket 754 Athlon 64 3200+ (Newcastle)
5.2A
1.8A
3.4A
2.5A
2.6A
83W
Intel Socket 478 P4 2.8 GHz (Northwood)
6.5A
0.9A
5.9A
1.2A
3.7A
96W
AMD Socket 939 Athlon 64 3500+ (Venice)
7.8A
3.9A
4.0A
3.8A
3.8A
125W
Intel LGA775 Pentium D 820 (2 x 2.8 GHz)
11.4A
0.5A
11.0A
3.5A
0.6A
155W
Intel LGA775 Pentium 670 (3.8 GHz)
13.7A
2.2A
10.7A
2.8A
1.6A
183W
Listed in order of increasing total DC Power

In every system, the CPU load resulted in increased current draw on the +12V2 line. For the most part, none of the other lines were affected by the CPU load. Once again, exceptions were seen in the AMD based systems.

As expected, the Socket 754 system saw an increase of 3.0A on the +12V2 line. However, the current on the +12V1 line also rose by 1.4A; this increase was not seen on any other system. This suggests that the CPU in this system was drawing current from both +12V lines.

Likewise, the Socket 939 system also drew primarily from the +12V2 line, but power draw also increased by 0.8A on the +5V line. In the grand scheme of things, this increase is barely significant, but it does show that a given load on one system may not produce the same load pattern on a different system. We can only guess as to why the nForce4 board requires power from the +5V line when other boards do not.

Note how the total system power ranking order changed. There's no question here that under high CPU load, the Intel systems are the power hogs. The lower end AMD systems didn't even get close. Only the AMD A64-3200+ system exceeded the power draw of any Intel system, in this case, the much less capable P4-2.8.

Power demand on the +5V and +3.3V lines varied widely between different motherboards and chipsets. Only the combined power draw of the two lines stayed fairly consistent. Regardless of platform, whether at idle or at full CPU load, the combined power draw for these two lines almost always stayed in the 20-30W range. Once again, the amount of power drawn on the +12V line almost invariably determines the total system power draw.

POWER DISTRIBUTION BY PERCENTAGE: CPU LOAD (CPUBurn)
System
+12V (total)
+12V1
+12V2 (CPU)
+5V
+3.3V
Intel Socket 478 P4 2.8 GHz (Northwood)
81%
11%
73%
6%
13%
Intel LGA775 Pentium 670 (3.8 GHz)
90%
14%
70%
8%
3%
Intel LGA775 Pentium D 820 (2 x 2.8 GHz)
88%
4%
85%
11%
1%
AMD Socket A Athlon 2500+ (Barton)
79%
34%
47%
14%
7%
AMD Socket 754 Athlon 64 3200+ (Newcastle)
75%
26%
49%
15%
10%
AMD Socket 939 Athlon 64 3500+ (Venice)
75%
37%
38%
15%
10%
Based on power (watts) drawn from each line.

It is instructive to look at the load on each individual rail as a percentage of the total load. The most evident trend is that the vast majority of the power is drawn from +12V2 line. This is not surprising. After all, CPUBurn stresses the CPU almost exclusively, so it makes sense that the CPU voltage line dominates power needs.

It is also interesting to note the differences between the AMD- and Intel-based systems. The AMD systems drew about 75% of their power from the +12V lines. This is a lot of power, but not as much as the Intel systems, which drew as much as 90% of their total power from these lines. This difference reflects the higher power demand of Intel CPUs, especially the newer ones. The older Northwood-core P4-2.8 drew much less power than Intel's newer CPUs, and consumed proportionally less power. It behaved more like the AMD CPUs than the newer Intel processors.



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