Differences in PSU quality
Posted By matt on July 18, 2006
The PSU (Power Supply Unit) is probably the most underrated component in most computer systems. Quality of power affects heat, system stability, and uptime.
About a week ago, the power supply in my server died. When I pulled it out, it was the generic no-name PSU which came with the system, which I bought off ebay some years ago. Here are the specs:
| Voltage | Current |
| 3.3V | 40A |
| 5V | 46A |
| 12V | 24A |
| -5V | 1A |
| -12V | 1A |
| +5V sb | 1.8A |
This power supply is rated at 550W, which is typically a “sustained” rate, as opposed to the “peak” rate which is what the individual voltages are. Doing the math, it's 676W, peak. (I think the label called it 650W peak, which is fine).
Now, this may look all well and good on the surface, but it's not. See, servers need lots of drives. Drives are power hungry, especially at startup on the 12V line. My drives pull 2A per on that line on startup, then settle down to 0.5A. There are 9 drives, which means that the PSU is peaking out at 18A, just for the drives! When I bought it, there were 12 drives, which means it was pegged.
Now, contrast this with the Turbo-Cool 510ATX PSU from PC Power and Cooling
| Voltage | Current |
| 3.3V | 30A |
| 5V | 40A |
| 12V | 34A |
| -5V | 0.3A |
| -12V | 2A |
| +5V sb | 3A |
Running the numbers on this, we find that this PSU, despite being rated at 510W, actually weighs in at a peak of 747.5W.
Now, in a postmortem of the other PSU, I think what happened is that the 12V lines are actually 2 12A lines combined into one 24A line, and one of them died. When it died, the whole server died, and wouldn't boot. Unplugging half the drives allowed the machine to boot.
Some thoughts:
(1) You get what you pay for.
(2) Cheap power supplies are cheap.
(3) PC Power and Cooling Power Supplies are way overspeced/way underrated (this is discussed at this page on PC Power and Cooling's website).
I also notice that PC Power and Cooling has even more powerful 850W and 1000kW PSU's. Sweet! If I'm ever building a new box and need a beefy PSU, these will fit the bill.
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