Required wattage confusion

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fippil

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Dec 13, 2015
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To make this short, I built a PC about 6 months ago that worked perfectly for 5 months until one day it decided to just not start after unplugging it and plugging it back in. It gave me some half seconds boots and that's it. I left it alone for a while and now I came back to trouble shooting it. I unplugged a lot of stuff and plugged it back in, shorted the PSU hot and ground to see if it worked and it does. It seems that everytime I unplug my PC and plug it back into an outlet, it won't start and will maybe start if I leave it sitting for an hour, but that's not gauranteed. I bought a new PSU (750W down from my 850W) and now it doesn't start unless I remove the GPU. I tried my old PSU and everything fired up the first try, which doesn't match it's previous behavior of not starting at all after being unplugged.

Now I understand how PCs work, and I calculated my estimated wattage and it came out to about 550W, so a 750W PSU should work, but it clearly doesn't.

My specs

CPU: 4690K intel
Mobo: Asus Z97 Gamer Pro
GPU: R9 290x RT Sapphire
Ram: 4g x 2
PSU Old: 850W XFX 80+ Bronze
PSU New (doesn't lite off PC with GPU in): 750W 80+ Gold
HDD: 7200 rpm
SSD: x1

I used outervision to calculate. I'm pretty confused on what my PC is even doing right now. I'm suspecting a mobo issue, but I just don't know anymore.
 
Solution
T
There is a lot more to a PSU than a wattage rating and the 80+ certification. The 80+ certification only attests to the PSU's efficiency of converting AC wall power to DC power for the components. It doesn't, however, say anything about the quality of the PSU which some people think it does.

Unplugging the PSU from the wall outlet resets some circuits in PSU. Leaving it plugged in and just powering down the system is a better way to take the machine down. Here's a link about unplugging the machine:
http://boards.straightdope.com/sdmb/archive/index.php/t-104562.html

I double checked your calculations and they seem to be OK and the 850W PSU should have enough "headroom" to make it stable unless you have a lot of peripherals...
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fippil

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Dec 13, 2015
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I actually just finished putting in my old PSU and I'm replying from my desktop. Everything is running fine and games run normally like they usually do. I mean if the GPU was bad, I imagine it would have trouble running games, especially under a load.
 
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thx1138v2

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Jun 18, 2011
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There is a lot more to a PSU than a wattage rating and the 80+ certification. The 80+ certification only attests to the PSU's efficiency of converting AC wall power to DC power for the components. It doesn't, however, say anything about the quality of the PSU which some people think it does.

Unplugging the PSU from the wall outlet resets some circuits in PSU. Leaving it plugged in and just powering down the system is a better way to take the machine down. Here's a link about unplugging the machine:
http://boards.straightdope.com/sdmb/archive/index.php/t-104562.html

I double checked your calculations and they seem to be OK and the 850W PSU should have enough "headroom" to make it stable unless you have a lot of peripherals attached. The only thing I see about that PSU (if I'm looking at the right one - XFX P1-850S-NLB9) is that it has a single rail +12V supply which might make it iffy for heavy graphics use but the headroom should cover that.

Do you know how clean your AC power from the wall socket is? That PSU has some over voltage protection but I didn't seen anything about under voltage protection and under voltage can be just as bad for circuitry as over voltage. If you've been having inclement weather and possibly brown outs that could be a cause of your problem. Using an uninterruptible power supply and leaving it plugged in but powered down might help.

I have had a similar problem at times and found that just letting it sit for say 30 seconds and then pressing the reset button on the case and holding it down for 5 or 10 seconds and releasing it would get the machine to boot up. I was thinking of replacing the PSU but that only happened a couple of times in very hot weather and then it has just gone away since then. You might try that.

That seems to be a good quality PSU but all manufacturers put out lemons on occasion.
 
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fippil

Honorable
Dec 13, 2015
21
0
10,510


Thanks for the lenghty response! As for the quality of my AC power source, I don't know, I don't have a DMM to check. I live in Florida and power outs happen during storms, but I've never experienced a brown out, usually power just cuts out immediately.

I still don't understand how the 750W isn't still enough to run the system. I'm bummed out because I really want to use it because it's modular. Only thing I can thing of is that I'm using a 40 in TV as the display, but that has its own power source, it would only make the gpu maybe work harder, which wouldn't add that much more to the watt count.

Anyway, thanks again, I'll take a look at that link, I didn't think unplugging a desktop had more to it than the obvious. I actually had a new mobo about to ship out, but I cancelled it last minute.

 
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