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Operate computer power supply independently??

Discussion in 'Electronic Design' started by Jake W. K., Jul 14, 2003.

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  1. Jake W. K.

    Jake W. K. Guest

    I have a power supply from a previous computer I've since scavenged
    for parts. It is a Hipro power supply with a standard power supply
    connector for the motherboard. Is it possible to crosswire or rig
    this power supply to where it will operate independently, powering an
    additional hard drive or two which are linked by IDE cable to a mother
    board in an adjacent case? If it is possible, how or where can I
    find out which wires to crosswire to accomplish this?

    I've run out of space and power connections in the existing box, which
    already contains and powers four hard drives plus two disc drives.

    Any help would be greatly appreciated?

  2. Put a 5 ohm power resistor across the +5V output and ground. This will
    generally be a sufficient load to make the supply operate independent of the PC.
    An amp of current is enough in almost all cases to make the switching regulator
    continue to operate.
    Make certain your power resistor is rated at more than 5 watts. I would
    pick a ten watt one myself to make sure I had plenty of "head room".


    Chip Shults
    My robotics, space and CGI web page -
  3. rw

    rw Guest

    I would also connect the black wires of one supply to the black wires of
    the other supply. Both are signal-ground, just to
    have a reference other then the IDE cable, between the supplys.
  4. Well, that goes without saying- no common ground, no way to compare logic
    signals. But I suppose I assumed that he would know.


    Chip Shults
    My robotics, space and CGI web page -
  5. Doing that strikes me as a particularly risky and non-ideal thing to do. AT
    and ATX power supplies, in the interest of being cheap, do not regulate the
    power rails with exceptional tolerance. If one of them is a fair amount
    higher than the other (about 0.7 Volts), I can envision the I/O lines on one
    of the devices going high, and feeding power up through the internal CMOS
    diodes to the other device's power rails. In such a case, exceptionally
    high current may flow and lead to catastrophic destruction of either your
    hard drive(s) or your motherboard.

    That being said, I've connected such a configuration up once (when I needed
    data off of a hard disk in another computer, who's power connector didn't
    want to come off). I operated it for a few tens of minutes without
    destroying anything. If you were to attempt such a venture for a permanent
    solution, I would recommend connecting several of the black wires together,
    and the green wires (on the motherboard connector) of the power supplies
    should also be connected. This green wire needs to be pulled low by the
    motherboard to turn on the power supplies. As a safety feature, I would
    connect a comparator, transistor, and other necessary circuitry up such that
    the power supplies would automatically power down if the two 5V power
    supplies rails deviated by more than a certain threshold of say 0.5 Volts.
    You may need some circuitry to give it a delay before deciding to shut down
    to provide for temporary transient conditions (such as during power up).

    Howard Henry Schlunder
  6. Jake W. K.

    Jake W. K. Guest

    Please don't assume or presume that. Thanks for the information you
    did give...but did you see where I asked how or where can I
    find out which wires to crosswire to accomplish this?

    How do I determine which of the leads you are talking about?

  7. The black leads are your common ground. The red leads are the +5V outputs.
    The yellow leads are +12V. You can connect the power resistor from the red to
    the black and that should be sufficient to make the supply run. Then you can
    check any other voltages with a standard voltmeter.
    I installed a system with the standard power plugs for PC devices on my work
    bench and can troubleshoot them without having to plug them into a PC. Hard
    drives, CD-ROMs, etc. are pretty easy to play with this way.
    When you use this with a computer, you will only want to connect your common
    ground. If you hook the power leads from this supply to the power leads in your
    PC, the supplies will buck each other and something will smoke. Just connect
    the ground leads, no more.


    Chip Shults
    My robotics, space and CGI web page -
  8. Fred Abse

    Fred Abse Guest

  9. This may help:
    < >.

    Connecting pin 14 to ground is required to get the power-supply to turn on.
    Wired to a switch on the case, perhaps?
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