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How to turn on an AT-style power supply?

Discussion in 'Electronic Components' started by, Jul 23, 2004.

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  1. (I was trying to use an ATX supply before, but it seemed more complicated,

    There is a switch on this AT-style power supply that I have now (of course).

    I stripped off the ends of one of the hard drive connectors, making sure
    don't touch when I hit the power switch.

    I hit the switch, and nothing happens.

    Do I have a bad power supply? Or do you have to have a load of some kind for
    it to work? I'm just trying to turn it on with nothing - so I can measure
    the voltage
    with a VOM. Starting at the beginning, as it were.

    The other day, I was diagnosing a friend's bad PC - it turned out to be a
    bad power supply. (ATX)
    The way I found out? I just plugged the AT power supply to the hard drive
    (which wasn't spinning up, BTW) and turned it on - voila! The hard drive
    spun up. By this I guessed that the power supply was bad (turned out to be
    the case).
    My point is, I had just 1 hard drive as a load, and the thing powered up
    just fine.
    (I'm pretty sure it's the same power supply, too)

  2. Dave Platt

    Dave Platt Guest

    Bingo. PC switching supplies have a minimum load (usually referenced
    to the +5 line, but some supplies may have a minumum load on +12 as
    well). If you don't have enough of a load on them, they won't
    regulate and (as a safety issue) usually will not start up.
  3. Fred*

    Fred* Guest

  4. Hans Summers

    Hans Summers Guest

    They need a minimum load. I have had a 100% success so far whenever I have
    tried it, by using a 10 ohm resistor across the +5V and Ground. This takes
    0.5A. Power is 2.5W so you need to choose a resistor rated for at least this
    power consumption, I suggest 5W or more. If no suitable resistor is
    available, I've also used a car headlamp bulb which works just as well.

  5. alpha_uma

    alpha_uma Guest

    Please note that Matthew is talking about an AT power supply, not an ATX
    power supply.

    I have gone through a dozen or so AT PSUs in ages past, and none of them
    needed any external load to be activated. If you connect the AT PSU to the
    AC wall socket, and then push the power button that comes with it, the fan
    of the power supply should spin right away. If it does not, then the AT PSU
    is not good (or at the very least, its fan could be "dead"). If the AT PSU
    fan is spinning, use a voltmeter to verify the voltages on the yellow line
    (12V) and the red line (5V). If they check out fine (could be anywhere from
    11.8V to 12.4V, and from 4.8V to 5.3V respectively), then power off the PSU,
    and connect the HD. Then power up. If the HD does not seem to be spinning
    up, then the problem is likely with the HD.

    As for an ATX PSU, shorting the green wire (pin 14) and a black wire (such
    as pin 16) is equivalent to toggling the "soft" power switch on a typical
    ATX computer case. In addition, most ATX PSUs these days would require some
    minimum load before they would power up properly (I've seen some exceptions
    in earlier days but that is another story). To test an ATX PSU, at the very
    least, connect an external system fan to the PSU or a floppy drive (if you
    have one handy) before shorting the green wire to the black wire. It it does
    not turn on, the PSU is probably bad (or at the very least, the fan inside
    the PSU is dead).

    I recommend reading the following pages to fully understand the differences
    between an ATX PSU and an AT PSU and the differences between the two form

  6. I have seen a lot of AT supplies, where powering them up without a load,
    will blow the input fuse. These have a thyristor crowbar, set so if the 5v
    rail goes above about 6v, the thyristor is switched on, and shorts out the
    input. This is so quick, that unless you happen to be watching the fan at
    the moment the power button is pressed, you will see nothing (the blades
    usually 'twitch' about 1/4"), and once it is done, the supply will be
    dead, until the input fuse is replaced. No supply of this sort should be
    operated without any load at all. However most will operate OK, with a
    fairly 'minimal' load.
    Best Wishes
  7. Some may need a load, some may not, some may need a load to stay in
    regulation, some may blow fuses (as Roger pointed out). It's probably
    a good idea to have some minimal load just to ensure that you aren't
    chasing load-related problems.

    FWIW, I've got an AT supply running a VHF marine radio (as 12V load
    only), and the 12V supplyis really noisy and unstable without a 5V
    load. I added some random 5V load and it works fine.
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