3.3 voltage from PSU

Discussion in 'DIY PC' started by Ian D., Mar 2, 2006.

  1. Ian D.

    Ian D. Guest

    I have never really bothered to learn much about PSUs. I've always
    bought the expensive ones and that strategy has payed off for 10
    years.
    Until now.
    So: Is it the responsibility of the PSU to deliver the 3.3V? I think
    it is, yes?
    So, If I have 4V instead, the PSU should be faulty, right?

    The PSU feature protective circuitry and shuts down to protect
    computer components, right?
    So, If I have Bios and MB stutdowns disabled, and the computer just
    suddenly instantly shuts down black, it's likely to be the PSU that
    shuts down, right? (monitors 3.3V alarm squeeking, = 4V)
     
    Ian D., Mar 2, 2006
    #1
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  2. Ian D.

    Paul Guest

    In article <>, Ian D.
    <> wrote:

    > I have never really bothered to learn much about PSUs. I've always
    > bought the expensive ones and that strategy has payed off for 10
    > years.
    > Until now.
    > So: Is it the responsibility of the PSU to deliver the 3.3V? I think
    > it is, yes?
    > So, If I have 4V instead, the PSU should be faulty, right?
    >
    > The PSU feature protective circuitry and shuts down to protect
    > computer components, right?
    > So, If I have Bios and MB stutdowns disabled, and the computer just
    > suddenly instantly shuts down black, it's likely to be the PSU that
    > shuts down, right? (monitors 3.3V alarm squeeking, = 4V)


    It depends on the era of motherboard. Some motherboards used
    to make something called "VIO", back when memory ran at
    3.3V. The motherboard might have an option to boost the VIO
    voltage to 3.5 or 3.6V. But the VIO was likely not monitored
    by the hardware monitor chip on those motherboards, and
    the monitor chip was connected to the real 3.3V coming from
    the PSU. (The 3.3V might be used by PCI cards, for example.)

    Yes, it is the responsibility of the PSU to deliver 3.3V.
    You will find at least one pin on the ATX power connector,
    that has two wires going to it. One of the wires on that
    pin, is a remote sense, intended to allow the PSU to more
    closely monitor the output of 3.3V. In theory, that should
    allow it to be regulated more closely (for example, nulling
    out the voltage drop in the cable).

    PSUs can have a variety of features, and a feature rich
    PSU might list overvoltage and overcurrent protection. But
    the overcurrent is hard to fit to the cheapest way of
    designing a PSU, and the current values listed suggest the
    power supply is detecting an overload thermally, instead
    of using a precise electronic method.

    Overvoltage is a bit easier to do. Since overvoltage happens
    so seldom, I cannot really tell you if your experience is
    typical or not.

    One thing to consider about a PSU, is how well it is able
    to protect itself. What I'm referring to here, is whether a
    PSU is "push-pull" or only "push". Say, for example, I connect
    a low value resistor from the 5V rail, to the 3.3V rail.
    A power circuit that was "push-pull", would simply sink
    the extra current coming from the 5V rail, and the output
    would remain at a steady 3.3V. If the supply could only
    "push" current, a +5V to +3.3V fault would result in the
    power supply stopping the pushing of current, but the
    voltage on the output would continue to rise. Such a power
    supply then, has no way to defend itself from a rail to
    rail fault. And the overvoltage circuit, if one is present,
    is the only opportunity for the fault to be stopped (by
    shutting down all outputs).

    Another condition for power supplies, is their minimum
    load requirement. Some supplies list 0 amps as the minimum
    current. Other supplies might list 2 amps or so as the
    minimum. What that means, is the output is not tightly
    regulated, if the load doesn't manage to meet the minimum
    value. So the voltage might go mildly off from the
    desired value, or the voltage could exceed the +/- 5%
    or so value, printed on the label on the side of the supply.
    Generally, you would not expect the design to rise to
    dangerously high values, if no load was present. But you
    wouldn't go out of your way to encourage the supply to
    do it either.

    So, where does that leave you ?

    To test the supply, disconnect it from the motherboard.
    Apply a minimum load to it. I use maybe $30 worth of
    load resistors to do that. Then I connect a voltmeter
    to the outputs, while the load is in place, and
    verify my PSU voltages. Having a minimal load on the
    supply, helps to ensure the regulation is going to work.

    Even without that care and attention, if it looks like
    you'll be throwing out the supply anyway, try running
    it without a load, connect PS_ON# to COM to turn it
    on, then measure the voltages. It is possible it will
    measure normal in this condition. If it does measure
    normal, it could be that your motherboard has a
    phantom connection between 3.3V and 5V or 3.3V and 12V.
    If a supply voltage more positive than 3.3V, leaks
    current into the 3.3V plane on the motherboard, then
    the voltage could climb to 4V. If the supply has OVP,
    then the supply could shut itself off, in response to
    seeing the voltage so high.

    So you still don't know for certain yet, where the
    fault lies.

    As for the motherboard end of things, I don't know if
    all the BIOS out there, would see a high value of
    voltage on a rail and shut down. I would expect
    beeping or some other sound effect, but I don't know
    if a motherboard would shut down as a response.

    I just checked one power supply spec, and on it,
    the OVP was set to 4.1V on the 3.3V rail. So that
    is roughly consistent with the power supply
    initiating the shutdown. If you can find a spec
    for the PSU, it might list the OVP value for
    the design.

    There is a real supply schematic here, if you want
    to see how an older supply works:

    http://www.pavouk.comp.cz/hw/en_atxps.html

    HTH,
    Paul
     
    Paul, Mar 2, 2006
    #2
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  3. Ian D.

    DaveW Guest

    The PSU is at fault.

    --
    DaveW

    ----------------
    "Ian D." <> wrote in message
    news:...
    >I have never really bothered to learn much about PSUs. I've always
    > bought the expensive ones and that strategy has payed off for 10
    > years.
    > Until now.
    > So: Is it the responsibility of the PSU to deliver the 3.3V? I think
    > it is, yes?
    > So, If I have 4V instead, the PSU should be faulty, right?
    >
    > The PSU feature protective circuitry and shuts down to protect
    > computer components, right?
    > So, If I have Bios and MB stutdowns disabled, and the computer just
    > suddenly instantly shuts down black, it's likely to be the PSU that
    > shuts down, right? (monitors 3.3V alarm squeeking, = 4V)
    >
     
    DaveW, Mar 3, 2006
    #3
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