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jo blo

PSU measurements - OK or inconclusive

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I know I have a supply that does not meet the official 480W requirement, but I have always fealt that the 430W that I have is enough for my setup, especially since it is a high quality unit. But there always is that question in my mind, and someone usually brings it up when I post with a problem. I did the following measurements, and I'm curious to know if the results can be taken to mean that yes, this PSU is adequate (for my setup), it's not cutting it, or if there's more to it and no conclusions can be drawn.


Measurements taken with a Fluke meter, case on its side with side panel open, only 1 stick of ram (due to a now-resolved problem).


3.3V rail

Post: 3.265 - 3.267 (1% max error)

Boot: 3.263 - 3.266 (1% max error)

Win Startup: 3.264 - 3.267 (1% max error)

Idle: 3.265 - 3.266 (1% max error)

Load* : 3.264 - 3.265 (1% max error)

Shutdown: 3.266 - 3.266 (1% max error)


5V rail

Post: 5.064 - 5.079 (1% max error)

Boot: 5.062 - 5.065 (1% max error)

Win Startup: 5.057 - 5.053 (1% max error)

Idle: 5.061 - 5.067 (1% max error)

Idle after stress test: 5.073 - 5.074 (1%)

Load*: 5.062 - 5.071 (1% max error)

Shutdown: 5.062 - 5.072 (1% max error)


12V rail

Post: 11.99 - 12.01 (0.1% max error)

Boot: 12.02 - 12.03 (0.2% max error)

Win Startup: 11.99 - 12.05 (0.4% max error)

Idle: 11.98 - 12.04 (0.3% max error)

Load**: 11.99 - 12.01 (0.1% max error)

Shutdown: 12.00 - 12.03 (0.2% max error)


* load for 3.3V test was two instances of Prime95

** for the 5V and 12V rail test, I noticed that voltage fluctuated with HD access, so I threw in a HD component -- in addition to running two instances of prime95, I simultaneously started a 68GB copy of media files (10KB - 3GB each) from the RAID 0 array to the IDE HD, which took about half an hour to complete. Somewhere in the middle of this, I repeatedly opened both optical drives, put a disc in each, closed them at the same time, let the discs spin up, then ejected them.


After these measurements, I put the other stick of RAM in, closed the case, and ran Prime95 for 19 hours. At the end of that, the readings were

3.3v: 3.26V

5V: 5.07V

12V: 11.99V

(CPU @ 48deg, PWM @ 62deg, Chipset @ 44deg)


What do you all think? It seems stable to me. Do you ever have to worry about wall voltage fluctuating? I'm in an appartment with old 2-pole outlets (no ground) and glass fuses in the fuse panel.

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Although I've heard that a 480 watt is minimum for stability, I have also been told otherwise provided you aren't running dual vid cards, lots of drives, etc. If your system seems to be working ok, I wouldn't get too stressed about it, although I would follow the recommendations and find a better one when you can get around to it. However, I would be very concerned about the poor power system in your apartment, and I would definitely be running a UPS of some kind. I spend some time on an island where the power is produced with diesel generators which makes for dirty power in addition to frequent outtages, so I wouldn't live without one! APC is probably the most common, and my personal preference, but there are a number of other companies that produce them as well.

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Even if the rails read ok (the PSU is not dead), what I've seen is that the variations under load cause problems. You can only read static or snapshot voltages with the DMM -- see catastrophic failures. This is why I always recommend borrowing a PSU when there's any doubt. You CAN use the DMM to calibrate your MB's internal DMM and then use a program like MBM5 to create a log of the voltages over time and see how they vary. I've still seen this look ok and had the PSU not be stable enough. Bottom line is if you have stability problems, the only way to rule out a PSU that hasn't failed is to swap-in another known good PSU and see if the problem persists.


How big a pig is your video card? If it's lightweight, then you can probably get by with less (if you're stable now). If it's not, then I'd be saving for a good PSU...

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