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Current limiting

Discussion in 'Electronic Basics' started by Nobody's Real, Sep 21, 2003.

  1. N00b Question:

    I have an old 150 watt atx computer power supply which I would like to
    use to power something. The label shows that at 12v the max output is
    4.7A, but I would like to limit that to a max of 1A, what would be the
    best way to do this?

    Thx.
     
  2. John G

    John G Guest

    Why would you bother?
    The device , if designed to run from a 12volt supply will use whatever it
    needs.
    The 4.7amp rating on the power supply is only what it can supply if required
    without being overloaded.

    Of course experimenters (professional or amature) use current limited power
    supplies so as not to burn something when they get their design wrong. But
    reading between the lines I do not think that is what you are asking for.
     
  3. I have an old 150 watt atx computer power supply which I would like to
    1A fuse.

    - Owen -
     
  4. happyhobit

    happyhobit Guest

    An LM317 in current limit mode with a 1-ohm resister (2 watt minimum) will
    limit current to 1.25 amps or use a 1.25-ohm resister for 1 amp output. Don'
    t forget a heat sink.

    http://www.national.com/ds.cgi/LM/LM117.pdf

    Jay
     
  5. Bill Bowden

    Bill Bowden Guest

    The output will only be about 9 volts using a 317 with 12 input,
    and there will be no voltage regulation using constant current mode.
    The 317T will self limit to 1.5 amps, but you still need 2 volts
    more going in than coming out.

    -Bill
     
  6. happyhobit

    happyhobit Guest

    Hi Bill,

    I agree with everything you said. So? He wanted a current limit, not
    voltage regulation.

    Do you have a better suggestion?

    Jay
     
  7. CFoley1064

    CFoley1064 Guest

    I agree with everything you said. So? He wanted a current limit, not
    If I absolutely had to have +12V regulated at a current limit of 1A from a PC
    power supply, I guess I'd have to use a step-up switcher to boost to a higher
    voltage (say, 18VDC or so), and then use a linear regulator to step down to 12
    with a current limit.

    It's possible that the "free" solution could be the most expensive one here.
    If I wanted to do this on the cheap, I'd use a laptop +18VDC supply (just look
    around, they seldom fail, and people can't bring themselves to throw 'em out
    when the laptop dies) and then use a linear post-regulator on a perfboard to
    get that [email protected]

    Good luck.
    Chris
     
  8. Bill Bowden

    Bill Bowden Guest

    Maybe he could try a "Low dropout" regulator such
    as LM2940 that only requires 0.5 volts overhead and
    limits to 1 amp. That will give him 11.5 regulated
    and limited to 1 amp.

    But he may also need a minimum load of 5 or 10 watts
    just to turn on the PC supply. They don't work
    at 1 or 2 watts. Maybe use a few 12 volt automotive
    tail lamps across the 5 volt output for a minimum load.

    -Bill
     
  9. John G

    John G Guest

    Wouldn't it be nice if "Nobody" told us what he really wants to do instead
    of people suggesting outlandish solutions to an undefined problem.
    Or is it that he is so out of his depth that he cannot define the
    requirement?
     
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