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PWM in ATX / Fan

Discussion in 'General Electronics Discussion' started by davelectronic, Dec 24, 2011.

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  1. davelectronic


    Dec 13, 2010
    PC problems today, just by chance i noticed a temperature rise in the PC i am currently using.
    Its a Siemens Fujitsu 3.2 GHZ P4 HT, 3 GB Ram, any way i needed to replace the two cooling fans to the CPU cooler, they happen to be 4 lead wire axial fans, for the bios smart cooling i expect, put two new 3 lead fans in, OMG going like a jet engine on full re heat after burners, way to fast, bios wont slow it down.

    After some thought i decided to run them of the ATX 5 volts rail, all ok as there is a two fan configuration, but then i noticed the PWM for the ATX was failing, Ahhh what now, so i looked in to the ATX opened it up, way to many SMD on the PWM board to work out whats going wrong, so what to do, i tried 12 volt rail for the ATX fan, F16 again on full re heat, so i went for the 5 volt rail again, this really is a leap into the unknown as hard system use could mean insufficient cooling IE fixed speeds.

    I know i could have used a series resistor, but i wanted it as quiet as possible, so long term experiment, there was no room in the unit for add on PWM circuits, so i go with all fans running 5 volts rail, it works out about 5.35 volts, so far there spinning nicely, time will tell if ive sealed its fate for good or the bad, the latter meaning PC pop / bang. :)

    PS. its an eBay purchase, i think theses second user machines are more trouble than there worth.





    Nice spray job on the fascia if i do say so myself, it was a yuk chocolate brown colour, hope it lives a while.
    No more second user machines for now, i think eBay is a lottery, and ive not won yet. :(
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    As you've no doubt figured out, 2 wire fans just spin, 3 wire fans spin and report their speed, 4 wire fans can have their speed controlled.

    The problem with running the fan at a lowe voltage to get a slower speed is that *if* your computer needs the fan to run faster, it can't.

    You'll also find that some fans move more air than others, so choosing another fan may also be a good idea.

    Running from 5V, the speed may not be reported correctly either.

    However, it will probably work OK, certainly better than a fan that's seized!
  3. davelectronic


    Dec 13, 2010
    Hi Steve.
    Yes ive worked most of that out, i am using fans new ones that offered the greatest CFM a minute, so not totally ideal at that voltage, but bios reports temperature problems and monitors them, set to shut down at a preset temperature, but ive not run anything massively demanding yet, the misses plays on line FB games, there fairly demanding, so i will see later when she uses the machine.

    So far so good.
    Merry Christmas Steve. :)
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