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PC PSU for microcontroller experimenting?

Discussion in 'Electronic Basics' started by Robbs, Jul 14, 2003.

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  1. Robbs

    Robbs Guest

    What steps need to be taken to use a standard (AT?) PC power supply for
    experimenting with micros/basic circuits? Hell, is it even safe/advisable?
    In the discussion about using two PSUs for devices in the same system, it
    sounded like such units won't activate without a suitable load across the
    motherboard connector.

    I'm new to electronics, so a basic explanation would be appreciated.


    rob burns
  2. Some power supplies will operate without a sizeable load attached, others
    will not. For the ones that do work without a load, it is generally true
    that the voltage regulation will be inferior compared to the loaded
    condition. People always suggest connecting a 1 amp or so load to the 5V
    rail to correct for this condition, but since my ATX power supply works fine
    without a quiescent load attached, I don't bother. The load doesn't have to
    be connected to the motherboard connector, it could be from any of the red
    wires to any of the black wires.

    I personally find using a computer power supply to be an advisable thing to
    do, and so long as you aren't using it in a condition where AC mains
    isolation is needed, it is likely a very safe thing to do to. I'm not sure,
    but some power supplies may be mains isolated too. If you short circuit the
    power supply, you will likely discover that it will create a very unpleasant
    spark and shutdown, rather than internally overheat and risk start a fire.
    On the other hand, since computer power supplies are capable of supplying a
    lot of power, if you make a big mistake in your circuit and not detect it
    for a long time, it could easily melt breadboards and cook stuff externally.

    Anyway, I personally rarely use my ATX power supply, despite the fact that I
    have no bench power supply. I don't care for the noisy fan inside computer
    power supplies. The voltage provided by 4 AA size NiMH batteries suits my
    needs, and in a case such as [*&terms=12BH348/CS&Dk=1&D=12BH348/CS&N=0 ]
    , it is a very convenient and portable power solution.

    To power up an ATX power supply, you have to short the green wire on the
    motherboard connector to any of the black wires. An AT power supply should
    have a separate wire (with likely 4 conductors in it) with a physical switch
    attached which will turn it on. All of the black wires are a common ground.
    Red wires will likely be +5V, yellow +12V. If you have an ATX power supply,
    it will likely have orange wires on the motherboard connector, which are
    probably 3.3V. Blue reminds me of -12V. AT and ATX power supplies both
    provide a -5V supply as well, but I don't remember what color it normally
    is. The colors of the wires are recommended by the power supplies's class
    specification, but are not required, so my generalizations may be wrong and
    you should measure the various taps before using them.

    Howard Henry Schlunder
  3. piero

    piero Guest

    apart from the requirements of a minimal load , that kind of power supply can
    generate destructive currents. A source for 5V or less for experiments should
    be limited to a reasonable current.
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