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Decouple fans and hard disks supplies

Discussion in 'Electronic Basics' started by [email protected], Jun 15, 2006.

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  1. Guest

    Hello respected engineers!
    I've built myself a frame where I've put six hard disks for a RAID5
    system, and six 8x8 cm fans to cool them down (they live in a quite
    hot environment!). The PSU is a 500W one however I was concerned that
    the fans may pollute the hard disks' supply too much.. since they're
    motors after all!
    I've accidentally put a finger into a fan (what a pain!), and this
    stopped the fan for a moment and also caused hard disk errors, and
    thus convinced me even more that I need to decouple the fans' motors
    supply from the hard disks supply.

    But how? I'm just a hobbyst when it comes to electronics, so I may
    even intuite correctly, but I won't be really sure of my thoughts.
    Right now, they are that I need some big inductors (with as low DC
    resistance as possible), some (big?) capacitors and maybe also a big
    rectifier as well.

    The best scheme I've thought so far is to link all 6 fans together in
    parallel, then apply power through a circuit like this:

    6 fans--*-ind-*-ind-*-ind-*-ind-*-ind-*-ind---diode------*-----PSU +12V
    | | | | | | |
    cap cap cap cap cap cap 6 HDs---PSU +5V
    | | | | | | |
    Gnd ----*-----*-----*-----*-----*-----*------------------*-----Gnd

    of course in the above scheme I don't even need them to be six,
    it's simply the more the better, I just happened to have six of
    them on my bench (more or less a coincidence).

    But now the big question: what's the right size of the caps, also
    to not overload the PSU at start? Will the diode really help the
    hard disks to not get some bad bad bad back EMF?
    Notice also that the wire that feeds the fans is AFTER all the wires
    that feed the hard disks (otherwise the voltage drop caused by the
    fans would have affected the hard disks). Yes, at least I know this. ;)

    Is my scheme ok? What can I do to improve it? I really care about
    the hard disks and the data they contain!

    Thank you very much,
  2. Easy solution is to just get another seperate PSU to power the fans.
    Isolate them from the hard drive supply completely.

    Dave :)
  3. JeffM

    JeffM Guest

  4. Jasen Betts

    Jasen Betts Guest

    sounds like you caused to great a load on the 12v by stalling the fan.
    that may not help much. the diode will reduce voltage top the fans
    which will reduce the power draw from the fans which may help a little.
    but you're wrong. the fans will still reduce the 12V even if they are on a
    different cable to the disks.
    install a larger powersupply, or a separate supply for the fans.

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