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300 W computer power supply

Discussion in 'Electronic Repair' started by [email protected], Oct 27, 2007.

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  1. Guest

    I have this power supply we stripped out of a pc we're junking. It
    has a 12V 10A section. I would like to use it as a 12V bench supply
    for testing and running equipment. I have no plans for the other
    supplies. Can I run it and the other supplies unloaded? And can I just
    use the 12V supply without using the others? The unit presently won't
    come on but there is a green wire in the bundle with the mother board
    connector that I suspect needs some kind of proper bias on it but I'm
    not sure. Does anyone have any information on this? Thanks, Lenny
    Stein, Barlen Electronics.
  2. There are two kinds of PC power supplies. The original or "AT" supplies
    which turn on and off by a switch and the newer ones called "ATX" which
    turn on and off by the computer.

    If the power plugs for the motherboard are seperate, you have an AT
    supply, if they are one 20 or 24 pin plug, you have an ATX. Some have

    In order to turn on an ATX power supply, you need to short two leads on
    the plug. This is well documented, STFW for "testing ATX power supply".

    I have not done this with ATX supplies, but I have used AT supplies as
    5 and 12 volt power supplies. Most of them require a load on the 5 volt
    side, when I used one to power a 12 volt radio, I put an automobile tail
    light bulb across the 5 volt leads. It drew enough current to keep the
    supply active.

    Note that while the DC voltage is quite clean, it is a switching power
    supply and many of them have lots of RF noise on the DC leads. I had no
    trouble using it for a VHF rig in a mixed HF/VHF environment, but many
    people have complained about using them on HF rigs.

  3. James Sweet

    James Sweet Guest

    Look up ATX pinout and you should find the info you need to fire it up.
    Connect the power on wire to the standby wire and that should do it.

    On most you'll need to load the primary output before the secondary outputs
    will regulate. On old AT PSUs it was the 5V, but on yours it's probably the
    3.3V output. A power resistor or automotive lightbulb will work.
  4. It's amazing what you can find with Google:
  5. Guest

    Thanks a lot for finding this for me! I really appreciate it. Lenny.
  6. Franc Zabkar

    Franc Zabkar Guest

    No, connect PS_ON* to any ground wire.

    IME most PC PSUs regulate by sensing a weighted average of the +5V and
    +12V rails.

    I believe this example is typical:

    - Franc Zabkar
  7. Franc Zabkar

    Franc Zabkar Guest

    You can convert your AT or ATX PSU to a tightly regulated 13.8V supply
    by changing the +12V sense resistor and removing the +5V sense
    resistor. You may also need to modify the overvoltage sense zeners, or
    whatever circuit is used to monitor this condition.

    - Franc Zabkar
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