Estimating PSU needs?

By foycur
Dec 16, 2004
  1. I have a home built system (see details below) for which I am using a PSU from CompUSA that is 300 W. When I have ASUS Probe up and running in the background, I am getting two things. A consistent elevation to 15 Volts on the +12V lead, and the +3.3V drops occasionally to 2.9-3.0 (the 15 volt reading has been there from day one; it's always 12 V in the BIOS, but once I boot up the OS it jumps to 15 and stays there). As a relative newcomer to building systems, I have three questions:

    1) is there an easy way to 'eyeball' what wattage would be needed for given components?
    2) could a defective power supply cause all of these goofy numbers?
    3) Should I believe BIOS or ASUS Probe?

  2. Mictlantecuhtli

    Mictlantecuhtli TS Evangelist Posts: 4,345   +11

    No software can adjust the voltage that comes out of PSU, as far as I know. There are a lot of applications that modify the mainboard voltages (AGP, CPU, memory), but not PSU's 5 or 12V rails.

    Therefore I think it's a software problem. If you have steady hands and a multimeter, you could measure the voltage straight from any molex connector, yellow wire is 12V, red 5V and black (either) is ground.

    One PSU size / wattage calculator is here:

    I don't know how reliable that is but it should give some directions.
  3. foycur

    foycur TS Rookie Topic Starter Posts: 114

    thanks for the info

    Thanks a lot esp. for that website. I think my 300 W power supply is inadequate, the program suggests a power supply no less than 310 watts, and I've got a 300 watt power supply. Time to get a new supply!

    The Molex connectors are the ones that go to each individual piece of hardware, like HDD and vid cards?
  4. Mictlantecuhtli

    Mictlantecuhtli TS Evangelist Posts: 4,345   +11

    If the current one works, why?

  5. foycur

    foycur TS Rookie Topic Starter Posts: 114

    well, the 3.3 V rail seems to be dropping off to 2.8 or 2.9, and i'm getting 15 V on the 12 V rail. Haven't had the time to get out and pick up a little voltmeter. And lately the MoBo has been giving me problems with rebooting spontaneously, etc. I just think the 300 W PSU is just not enough to do the job with all I've got on there. I've got a 9800 pro card, two 7200 RPM hard drives, a PCI fan card beneath the vid card, two case fans, and soon to be adding a sound card. The MoBo is another issue because the BIOS got corrupted and I cannot flash it, so it's heading back to ASUS for look see. That may be why it's rebooting spontaneously, but I figure a bigger power supply can't hurt esp when it comes time to upgrade, or if they send me a sweeter MoBo.
  6. nein

    nein Banned Posts: 109

    Yes, time for a new PS ... Click on quoted words below for pertinent link.

    "There're signs of power-supplies being overloaded for one reason or another. One of the most common signs for marginal/in-sufficient 3.3V + 5V combined power is the inverse level up shifting of 12V instead of drooping.

    When 3.3V and 5V get loaded down and dropping downward, 12V level will shift upward (-5V and -12V will be more negative!!!)"
  7. foycur

    foycur TS Rookie Topic Starter Posts: 114


    Nein, just to update you on what happened....I ended up switching out my 300 Watt PSU for a Thermaltake 480 W PSU and the 3.3 V rail problem seemed to get solved. ASUS PC Probe keeps showing my 12 V rail at 15.5 V, but when I go into BIOS it says it's running around 12.2 V. Thanks for everyone here who gave me advice, it was much appreciated. I can't imagine that 480 Watts would be insufficient for my system and cause this 15 volt error. It makes me wonder if ASUS Probe is sort of screwy.
  8. RealBlackStuff

    RealBlackStuff TS Rookie Posts: 6,503

    Look on the ASUS website, they may have an updated PCProbe.
  9. foycur

    foycur TS Rookie Topic Starter Posts: 114

    Updated Probe is what I needed

    Yep, that seems to have solved it. Thanks to everyone who helped me out on here. I have a home built system that I've been tweaking by myself for some time without the benefit of other people to ask questions, so I thank all of you. You have saved my marriage. My wife is tired of me tinkering with the computer, but she doesn't understand that it's a sickness!! Must upgrade, must fiddle with motherboard.........
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