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Computer power supply

Discussion in 'Electronic Basics' started by Mountain Dwe, Jan 25, 2005.

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  1. Mountain Dwe

    Mountain Dwe Guest

    Can I use a OLD computers power supply box as a power supply unit for my
    circuits?
    I don't have a motherboard or anything for the power supply so id it
    possible to jus hook up the power supply to the wall and use the 5V, 12V and
    grounds?
     
  2. JeffM

    JeffM Guest

  3. Mountain Dwe

    Mountain Dwe Guest

    ok..
    That sounds more trouble than convenience.. so let me ask this simple
    question.....
    Is the step down transformer in the power supply just a normal step down
    transformer???
    I mean can I salvage it and make my own general propose bench power supply?
    I would assume I could use the rectifier diodes on the board too??

     
  4. peterken

    peterken Guest

    The stepdown transformer is a switched mode power supply
    (switched primary, rectified secondary, usually pulse width modulated
    circuit)

    You *can* make a general 5V supply out of it, but beware of the "power good"
    line
    If using it, just use it "as is" since many of them are rather critical in
    layout and used components
    (especially if resonant-mode types)
    I personally wouldn't dream of disassembling it to build another, I'd just
    design another then
     
  5. Art

    Art Guest

    Also, you may need to provide a load to the supply to keep it from shutting
    down (protect mode). Many PC Supplies will not continue to run unless
    properly loaded, either with a M/B or a "Dummy Load".
     
  6. Kitchen Man

    Kitchen Man Guest

    As a minimum, buy some terminal boards and build yourself a proper
    interface to the voltages, maybe mount everything on a spare piece of
    wood. Understand the current and power limits of the supply. Fuse the
    outputs appropriately, and realize that you will not have an indicator
    of the status of the voltage and current without building your own
    displays. You might want to integrate some switches for better control.
    It all depends on the scope of what you want to do.
     
  7. JeB

    JeB Guest

    Also ... I think all the computer supplies need some sort of load on
    the 5V line to power up. A 6v automotive bulb will work.
     
  8. Is it worth the trouble, how expensive are the power supplies your looking
    at. If you are a novice, do you really want to be playing about with 240V
    AC?


     
  9. Kitchen Man

    Kitchen Man Guest

    Yeah, there's more to it than just plugging it in, I kinda overlooked
    some of the finer details when I posted. But, where there's a will,
    there's a way. It's a handy, pre-built switcher with common voltages
    ready to go. 5V and 12V will let you build some interesting projects,
    fun with op amps and low power auto stuff. Looks like there's plenty of
    info on the web about how to get the thing running, and there must be a
    gazillion of them laying around in junk piles.
     
  10. But it's also tremendous overkill for a small bench supply. The fact
    that the supply likely needs a minimum mode should be a giveaway, you
    will be wasting more current on the load than you use on the op-amps.

    The only time such supplies are really useful is if you have a need for
    high current.

    They do make nice cases for small projects, though. You can pull out
    the circuit board, strip some of the parts, and then build a linear power
    supply inside. Grab a suitable transformer out of a junked consumer device,
    use the bridge from the switching supply, get some electrolytics from
    somewhere (since switching supplies work by generating a signal at a much
    higher frequency than 60Hz, they don't need as much capacitance at the output
    and thus they'll be too small for a 60Hz supply), and likely pull a 3terminal
    regulator out of the same consumer device, or another.

    Such a supply will not supply nearly as much current as the switching supply,
    but you won't be wasting power on a load to keep the supply running, and it
    won't be as noisy for those analog devices, and the regulation may be better.

    Michael
     
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