Power for my PC

January 23, 2009 1:26 AM

Help me to understand the brain-melting world of computer power supplies, why my new PC is having issues, and what I should be looking for in a new PSU... see within for the usual technical guff.

Here's the system that I have:
AMD Phenom X3 8650 cpu
Asus M3N78 motherboard
Single 9800GT 512MB graphics card
4GB RAM
A couple of SATA HDD
Optical drive

Now, I thought that this Enermax 535W PSU would be enough to handle all that, but after plugging it all in and switching it on, I didn't even make it into POST. However, this Thermaltake 480W PSU gets things up and running all the way to Windows (though there are some stability issues as soon as I try to run anything too graphically taxing).

I've pretty much ruled out that either PSU could be faulty, as they both seem to run fine in a similar but lower spec PC.

I've tried looking over all the facts about voltages/amps/watts, and this talk about rails, and I think I've gotten a rough idea of how it all fits together, but my recent experience doesn't make much sense (the electrickery confuses me...)

So, the questions:
1.) Why did the 535W, dual 12V rail PSU not work, when the 480W, single 12V rail PSU did work?
2.) What should I be looking for in a new PSU?

Thanks in advance!

posted by malpractice to Technology (5 answers total)

According to this PSU calculator the Enermax should be fine, but the Tt might be pushing the 12V rail (which would explain your instability once the gfx card gets going).

My guess is the Enermax is faulty despite working in a lower spec PC.
posted by markr at 1:56 AM on January 23, 2009

I've never had a problem like that when I simply bought whatever brand capable of delivering sufficient wattage.

It's possible that you simply need higher wattage. Perhaps you were overworking both power supplies, but only the Enermax PSU was smart enough to refuse to go anywhere; the Thermaltake just gave up the amps.

Is your mobo being picky?
posted by Netzapper at 1:58 AM on January 23, 2009

It's quite possible for a PSU to have a fault that only becomes apparent when attached to a more demanding system.

I'd strip the system down to its bare minimum (mobo, cpu and ram, and using the integrated graphics) and see if it makes it to the POST. If it does, attach an HDD and get the system booted into an OS. If that seems stable, add the rest of the drives and test again. Then maybe take off the drives and try it with the graphics card. Also, make sure you turn off the integrated graphics from the BIOS if you can.
posted by le morte de bea arthur at 3:49 AM on January 23, 2009

Power supply wattage is starting to become like computer speaker wattage- almost meaningless from a real world usability standpoint.

The computer draws different wattages at different voltages. It's possible that while the power supply can deliver the listed wattage in total, but is unable to deliver the wattage the computer needs on one of the power rails. IE, it can deliver 1000 watts @ 12 volts, but it can only deliver 3 watts @ 3.3 volts. If the computer needs more than 3 watts of 3.3v power, the power supply will fail.
posted by gjc at 5:15 AM on January 23, 2009

Response by poster: Thanks for your help everyone. I've tried stripping the system down, and the Enermax will *just* make it into POST with absolutely nothing else connected (and running on the on-board graphics). I'm just going to write it off for this system.

Markr: thanks for the link to that calculator - almost reached information overload, but it makes enough sense for me to use it to do some further investigation.

Time to go buy a new PSU... Cheers everyone!
posted by malpractice at 12:43 PM on January 23, 2009