PSU exceed +12V

Posted on 2003-02-20
Medium Priority
Last Modified: 2011-10-03
My system has a 400Watt PSU, 6 fans, 3 Hdds, 1 cd writer.
When i power up the system & run Asus Probe, the dafault +12V value would be 12.896 plus minus.
Then when i run the torture test (Prime95 http://www.mersenne.org/prime.htm), it will exceed till 13.3 plus minus.
The +5V, +3.3V and VCore are under normal values.
I've already blew up my 2 350Watt PSUs.
May i know what is the problem that causes the +12V went so high? Will there be any harm to my system?

Please advice.
Question by:cukcuk
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Expert Comment

ID: 7987836

Don't be afraid. It's normal ... 12V is not "regulated". Only 5V and/or 3.3V. The 12V output "follow" the 5V output.

When you run a torture test, CPU needs more current from +5V or +3.3V output. It's not the case for 12V output.

The 12V output is just for harddrive and cdrom electric motors, fans and old serial ports.

Author Comment

ID: 7990954
My +5V and +3.3V are under threshold while +12V is over threshold. Shouldn't the +5V and +3.3V also be over threshold for the +12V to be over threshold?

BTW, what would be the cause for my 2 350Watt PSUs to blow up?

I really don't want to blow up another of my PSU anymore.

Expert Comment

ID: 7995774
Frache is right to a degree.  

It depends of course on your PSU and how it regulates the power.

12V is used for newer (XP and P4) processors these days.  It used to be that cpu's used the 5v/3.3v rail but they havn't done for a year of so now.

You need to make sure that you have enough power on the 12V rail for modern processors, plus 3.3 and 5 v for modern graphics cards.

Just to say that it's a 450W PSU says nothing about its characteristics.

What cpu are you using, what does it say on your PSU outputs

ie 3.3v=30A
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Expert Comment

ID: 7995779
Oh, can you give the voltage values as well? under normal and under stress?

Author Comment

ID: 8002231
I'll get back to you after i check the values.

Author Comment

ID: 8002532
Kingsize: Under normal will be +12.8**V & under stress will be +13.3**V. Here are my PSU's Specs:

ATX-400 Watt

+3.3V = 16A
+5V = 30A
+12V = 15A
-5V = 0.5A
-12V = 0.5V
+5VSB = 2A

Accepted Solution

Kingsize earned 100 total points
ID: 8004763
Not all PSU's are the same.

So thats 52.8W power on your 3.3v
150W on your 5v:
200W 3.3v and 5V combined - should be fine.

180W on your 12V - should be fine as long as the PSU can handle a peak load of 18A.

AMD's white paper on PSU's


You might have blown your old PSU's because they had insufficiant power for the 12V line.  

Older 350W PSU's tended to have more power on the 3.3 and 5v lines because that is where processors used to draw the power from.  The 12V line used to be just used by Fans/mobo and HDD's - and at lower Amps.

Good tolerances on the 12V supply are 12V +/- 5%

=i.e 12.00V normal load to 12.60V full load

Power supplies are "tuned" by their capacitors to have a specific voltage.  In your case this seems a little high.

i.e 12.80V

12.80 V + 5% acceptable tolerance = 12.80 to 13.44V

So your PSU is within acceptable tolerance, but it's baseline is a bit poor - although I have run systems on higher 12V rails and they've been fine.

My guess is that your PSU isn't from a reputable brand name - e.g. enermax, antec and therefore isn't too perfect on the 12V rail...  I doubt it will explode quite yet.

Do check, however that there arn't any shorts on the case or those 4 pin molex connectors that might be straining the 12V suppy.  12V = usually yellow lead on the molex, there is a 12V supply on your ATX connector and the 4 pin connector.

I hope this helps.


Expert Comment

ID: 8004803
Oh... I notice you have 6 Fans + 3 HDD.  This may explain the load on the 12V feed.

If you look on the fan itself it may give you the Amps per fan - although the figure is small, 6 fans I would guess would probably safely need about 18A on the 12V line to compensate.

Disconnect all but 2 case fans and see what effect it has on the 12V rail.

Superfluous fans are a bad idea - especially delta ones that drain a lot of current.  If you really want those fans, buy a PSU with a high 12V ampage.


Author Comment

ID: 8008310
It was the same after i disconnected all fans that i have.
Now i guess it should be my PSU problem. I'll try to get an Antec PSU with +12V of at least 18A.
Maybe that should help.

Thanks Kingsize, your explanation was clear.

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