Is this really a 300W PSU?

Posted on 2004-10-31
Last Modified: 2008-03-17
I have a generic "300W" PSU, but i'm no longer sure it is 300W. Here is my system:

ASUS P4P800 Mainboard
Intel Northwood-B 2,66GHz 533Mhz FSB D1 Stepping oc'ed to 3.02GHz (stable at stock voltages)
Sapphire ATi Radeon 9800XT
2 x Exlixir 256 MB DDR-400 RAM in DUAL Channel mode, @ 376MHz: 2.5,3,3,6
Seagate st3120026as (120 Gb Barracuda 7200.7 SATA)
Western Digital wd-400eb (40 Gb Protege 5400 PATA)
AOpen 48x12x50x CD-RW
2 x 80mm Case Fans
CNET Soft56K Internal Voice Fax Modem

Now, here are my voltages, as given the Winbond W83627THF  (ISA 290h) sensor on the board

12V     : 12.767
5V       : 5.375
3.3V    : 3.328

Are these values normal for a 300W PSU connected to the above components?
Question by:yolunga2000
    LVL 95

    Expert Comment

    by:Lee W, MVP
    I might worry about the 12V reading a little, but generally that looks ok - Are you having problems?  Your PS should be capable of outputting 300 Watts, but during normal operation, it probably won't need to output more than 150-200 watts.  Bootup, when everything wants power, is when you really need that 300 Watt capability.
    LVL 2

    Author Comment

    Everything boots fine, then under load, the voltages drop only marginally. But what is surprising is that on my friend's AOpen 300W PSU, the 12V is 11.7V the 5V is 4.7V and the 3.3V is 3.2V when connected to my system. Why is this and is this necessarily a bad thing?
    LVL 4

    Expert Comment

    Generally there are some differences in power supplies. Good quality will have more stability and usually cost more than (no-name) type power supply. It’s not un-common to see readings as you got but your reading are in the high rage and the aopen power supply are in the lower range. As long as they don’t fall out of range under load you should be fine. If voltage is too high you can burn the chips that are sensitive to high voltage. To make sure the voltages your reading are correct, use a good quality volt meter and measure it for yourself. I don’t recommend this if you’re not familiar with working on live circuits. You can damage your board if you short something.
    LVL 8

    Assisted Solution

    Here is from the ATX spec.

    sorry but this forum does not support any graphics so the PDF doc is fu.... up

    3.2. DC Output
    3.2.1. DC Voltage Regulation
    The DC output voltages shall remain within the regulation ranges shown in Table 2 when
    measured at the load end of the output connectors under all line, load, and environmental
    conditions. The voltage regulation limits shall be maintained under continuous operation
    for any steady state temperature and operating conditions specified in Section 5.
    Table 2. DC Output Voltage Regulation
    Output Range Min. Nom. Max. Unit
    +12V1DC ±5% +11.40 +12.00 +12.60 Volts
    +12V2DC (1) ±5% +11.40 +12.00 +12.60 Volts
    +5VDC ±5% +4.75 +5.00 +5.25 Volts
    +3.3VDC (2) ±5% +3.14 +3.30 +3.47 Volts
    -12VDC ±10% -10.80 -12.00 -13.20 Volts
    +5VSB ±5% +4.75 +5.00 +5.25 Volts
    (1) At +12 VDC peak loading, regulation at the +12 VDC output can go to ± 10%.
    (2) Voltage tolerance is required at main connector and S-ATA connector (if used).

    So I guess you are right your voltage readings are out of spec

    here is the link, tolerances on page 12
    LVL 5

    Assisted Solution

    First, you've got psu's and you've got psu's. there can be a lot of differences between psu's of different brands. some 300w may perform like a 420, while others will behave like a 250w psu. The voltages seem fine to me. The ATX standard allows a 5% margin on voltages (including measurement errors). If your voltagages drop below or come above this, I would start to worry and buy a new one. When shopping for a new psu, pay attention to the max. load on +12, -12 and +5 (in amps)!

    Blue Rishi
    LVL 91

    Assisted Solution

    i think you are having many devices, and thus a big load. It can be worth to calculate your power needs :      
    LVL 2

    Author Comment

    So even though the voltages are slightly on the high side, shoulfd i worry if the system is stable? Also, my system works stably when i connect more hdd's and fans than should theoretically be possible with a 300W PSU.
    LVL 2

    Author Comment

    According to the calculator at the link supplied by nobus, my power needs at startup should be 300W ?!
    When connected to my system with an extra CD-RW drive, my friends AOPen PSU just blew altogether, but mine is still going with another Seagate HDD (For RAID 0) and another CD-RW drive ?!
    LVL 91

    Accepted Solution

    well, Yolunga2000, consider yourself lucky.
    The different brands of PSU's  differ in evrey possible way : voltage margin, power margin, how much they deliver on each rail, and so on. The main thing is : i f your needs are 300w, then you are lucky to have a PSU that delivers everything you need, and not falling short on a rail, eg +12V.
    You realize, that adding, even a minor load to your system, can bring it down. I hope not, but those are the facts.

    Now, considering your PSU did a good job up to now, you can go 2 ways : you keep it, till you go over the edge, and accept that (keep making backups !) or you put in a bigger one, allowing some extra horns and whistles on your system

    Up to you ! and good luck, whatever you choose....
    LVL 8

    Expert Comment

    Hey Yolunga2k

    Its all a matter of 12 volts forget the watts

    Just a example

    An enermax 365 watt is capable of 26 amps on the 12 volt rail

    Some generic 550 watts is capable of 15 amps on the 12 volt rail

    In a modern PC CPU and GFX is powered solely by 12 volts fans and drives as well uses 12 volts.

    Pentium extreme is 100 watts, GFX is 60 to 90watts 9800/5900/6800/ x800

    Now divide those figures with 12 and you get the amps needed

    CPU 100/12= 8 amps GFX 70/ 12 =6 =totally 14 amps then add 0,5 maps for every fan and drive

    then in your case i will assume something like 16 amps as all drives not are running all the time

    Can you PSU deliver 16 amps on the 12 volt rail ?

    LVL 2

    Author Comment

    Well it must be able to, i mean the system boots up fine and runs stably. BTW, my CPU takes 5 amps, my Graphics 6 amps, 2 sytem fans which take a combined 1 amp, and two hard drives both running at the same time, one st31120026as and a wd400eb. Also, i have a dvd-rom and cd-rw.
    LVL 8

    Expert Comment


    Sorry calculation with the wrong cpu
    and It was just an example

    and there should be a sticker on your PSU telling max amps on each rail

    and i guess your system will at least require 14 amps on the 12 volt rail

    voltages out of range can indicate a PSU on the edge

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