What is -12V for? Usually -12V shows no voltage at startup

Discussion in 'Asus' started by Herbert Chan, Aug 4, 2004.

  1. Herbert Chan

    Herbert Chan Guest

    Hello,

    What is it for? I think CPU fan runs on +12V. How about -12V? My computer
    always reports that there is no -12V at startup. There will be -12V (like at
    this moment, it is -12.27V) again when the computer has been on for a while.

    I want to know what components actually run on this voltage. Does it mean
    that if there is no -12V, those components won't run during startup until
    there is a voltage?

    Thanks.

    Herbert
     
    Herbert Chan, Aug 4, 2004
    #1
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  2. RS-232 serial ports require it. On some motherboards, that voltage is
    generated on the motherboard for that purpose only, and -12v from the
    power supply isn't used.
     
    Barry Watzman, Aug 4, 2004
    #2
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  3. Herbert Chan

    Herbert Chan Guest

    Actually my CPU fan is not running, and I think it runs on +12V.

    There is a reading of 11.308V for +12V according to Asus probe. So I
    suppose the power supply to this voltage is working. So it must be
    something wrong with the CPU fan itself, right?

    My board is Asus CUSL2 and CPU is Intel III 733MHz.

    Thanks.

    Herbert
     
    Herbert Chan, Aug 4, 2004
    #3
  4. The -12v rail, was historically used for two things. On early machines
    some memories required a -ve bias (hasn't applied for perhaps 10
    years...). The other user, is the RS232 ports. A few PCI cards use the
    supply too, but these are now very rare.
    Many modern boards use a chip like the MAX232 to supply the RS232, and no
    longer use this rail at all.
    Even if the rail is used by such drivers, they will still wake up and
    'run', but just be unable to pull the line properly 'down' when sending
    RS232 data.

    Best Wishes
     
    Roger Hamlett, Aug 4, 2004
    #4
  5. Herbert Chan

    Ed Guest

    Try a different known working fan on the CPU fan header, it's probably
    just a dead fan and needs to be replaced, or it could be the fan header
    on the mobo went bad.

    Ed
     
    Ed, Aug 4, 2004
    #5
  6. Herbert Chan

    Paul Guest

    I have just discovered to my shock and horror, that my P4C800-E Deluxe
    uses a TI GD75232 for at least the COM2 port, and that is not a MAX
    type device. The MAX232 style has a charge pump built in, and converts
    a motherboard voltage to make the necessary negative voltage. The charge
    pump uses external capacitors to work. It seems the GD75232 is one
    of the old style parts, running off +12 and -12V. So, there are
    still boards that need the -12V supply.

    http://www-s.ti.com/sc/ds/gd75232.pdf

    For the following, see pg.17 and C1, C2, C3, C4, which are parts of
    the voltage doubler and voltage inverter. +5V makes something like
    +10 and -10V.

    http://pdfserv.maxim-ic.com/en/ds/MAX220-MAX249.pdf

    When a GD75232 costs $0.50 and a MAX232 type costs $1.85, it is not
    difficult to see which they will buy.

    Paul
     
    Paul, Aug 4, 2004
    #6
  7. Herbert Chan

    Herbert Chan Guest

    The power fan header and the chassis fan header are free. (I do have a
    power fan within the power supply unit, but nothing to plug onto the
    motherboard. And I do not have a chassis fan.) The power fan header is
    within reach of the CPU fan wire, while the chassis fan header is not. I
    have tried plugging the CPU fan wires to the power fan header, and the fan
    still does not turn.

    So I can suppose the fan has gone dead? I don't have any other fan to try.

    I had better lift the lever and take the CPU + heatsink + CPU fan off to
    replace the CPU fan, right?

    Thanks

    Herbert
     
    Herbert Chan, Aug 5, 2004
    #7
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