Weird PSU problem

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jacek

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Feb 4, 2009
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I have ASRock FM2A88X Extreme 6+ mobo with A10 6800K CPU - no graphics card. I have 3 24 inch (1920x1200) monitors hooked up to this machine - DVI, HDMI and DIsplayPort using Eyefinity.
At some point I decided to replace the PSU because the old junk was driving me nuts. I installed a modular Antec 450W PSU but it did not work with this motherboard. My first reaction was the mobo did not like the 24/8 PIN connectors combination (the Allied uses 20/4 PIN connectors) because the Antec PSU works fine in another PC. So I got the Corsair CS450M. The same story. The mobo would not post. The CPU fan would spin but no post. Finally I got it to work, sort of. The PSU works only if the monitor using the Display Port is not connected to the motherboard. I have 2 monitors connected now using HDMI and DVI and all works fine. The moment I try to hook up the Display Port monitor the mobo will not post. And keep in mind that by just switching back to the old Allied PSU all works as it should.
I suspect it has to do with the power output delivered on some of the voltages

The old Allied PSU was rated 400W but it provided more output for some voltages:
+3.3V - 24A
+5V - 35A
+12V - 12A
-12V - 0.8A
-5V - 0.5A
+5VSB - 2A

The CS450M delivers:
+3.3V - 20A
+5V - 20A
+12V - 35.5A
-12V - 0.8A
+5VSB - 3A

I just need to know which one is needed by the DisplayPort so I can get a PSU that will work with my setup. If it is 35A on +5V I may have a hard time finding PSU with such output.

 
Solution
InvalidError
Modern PCs (almost anything newer than 10 years) draws the bulk of their power from 12V. Unless you have far many more HDDs, SSDs, USB devices and other stuff that still draws a fair chunk of their power from 5V or 3.3V, you will not get anywhere near 20A on those rails. Your PC probably does not even draw 10A from those.

Also, while the DP spec may call for 0.5A at 3.3V available on the cable, that power is only used to power the display's EDID chip (maybe 20mA during access, less than 1mA otherwise) and amplifiers/transceivers when using active cables. So, most of the time, the DP port uses practically nothing on 3.3V.

Whatever your problem is, it is not that hypothetical 0.5A. I would try switching which display connects to what...
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InvalidError

Titan
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May 18, 2007
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Modern PCs get the bulk of their power from the 12V rail(s) since the CPU and GPU are the two single biggest loads and are usually powered almost entirely from 12V.

If your computer fails to boot when a specific display is connected to it, I would suspect that display might be defective.
 
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J

jacek

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Feb 4, 2009
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Not to be offensive but did you REALLY read my post?
If I replace the new Corsair PSU with the old Allied PSU EVERYTHING WORKS. There is some additional power requirement when the monitor is hooked up to the Display Port which causes the mobo to play dead. Based on what I found so far the Display Port specification calls for 3.3V 0.5A input but I can not believe the mere .5 A will cause such a problem. Where does the remaining 20A or so of 3.3V went to?
 
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InvalidError

InvalidError

Titan
Moderator
May 18, 2007
23,815
5,265
108,890
Modern PCs (almost anything newer than 10 years) draws the bulk of their power from 12V. Unless you have far many more HDDs, SSDs, USB devices and other stuff that still draws a fair chunk of their power from 5V or 3.3V, you will not get anywhere near 20A on those rails. Your PC probably does not even draw 10A from those.

Also, while the DP spec may call for 0.5A at 3.3V available on the cable, that power is only used to power the display's EDID chip (maybe 20mA during access, less than 1mA otherwise) and amplifiers/transceivers when using active cables. So, most of the time, the DP port uses practically nothing on 3.3V.

Whatever your problem is, it is not that hypothetical 0.5A. I would try switching which display connects to what port to see if problems are specific to Display Port or the monitor. If the same monitor causes issues on different ports, my guess would be that the monitor is defective, is spewing garbage over the DP cable (conducted EMI) and causes something in the PC to malfunction. Ex.: if grounding on that monitor is defective, there could be a mains fault current going over the display cable, through the PC and its power supply.
 
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