post supply scare - all's well that starts up again!

Discussion in 'Asus' started by Lady Margaret Thatcher, Aug 24, 2005.

  1. Here is a problem that solved itself. I thought I would share it with
    the group because it is a "good news story," as a contrast to all the
    tales of woe that fill these groups.

    I was building up a new system using an ASUS board. Antec power
    supply. I've had good luck with both of them in the past, so I stay
    loyal to the brands.

    So I'm putting together this new system, and I use my "diagnostics"
    card that gives me a readout of the POST messages. The diagnostics
    card has an extension cable with a second display, in case you can't
    read the display off the card because it's buried inside the system.

    Last night, I booted up the system for the first time. Looks good.
    Then I tried to read that extension cable display. But instead, I
    yanked a bit too hard and the diagnostics card popped out of its slot.
    It is a small card and doesn't have a bracket mount.

    Suddenly the system shut down. And wouldn't restart. Even pressing
    the front power button didn't work. Switched the power supply off and
    then on again didn't help. Unplugging the power supply didn't help.
    The system was dead, dead, dead. Not just the motherboard, but even
    the case fans didn't spin up.

    Geez. What now. Nothing on the Antec web site that could help me.
    Maybe 15 minutes goes by. So I try again, and the case fans spin for
    about 10 seconds, and then shut down again. Tried again. Same
    result.

    Ok. I wait about 30 minutes, anxiously. I was so nervous you could
    think that I was waiting for a child to be born.

    This time, it all works. Case fans, power supply fans, motherboard
    green light, and of course the CPU heat sink fan. And the system
    boots up perfectly, into POST because there is no hard disk in the
    system yet. I have no problem starting up the system BIOS, where I
    check the power supply and fan/temp settings.

    And three days later, it's still running fine. Motherboard temp
    stable. CPU temp up only 2 degrees F from when the system first
    started up. Voltages were very close to nominal at boot up (less than
    1% off), and no change after three days.

    So, all's well that starts up again.

    --maggie-
     
    Lady Margaret Thatcher, Aug 24, 2005
    #1
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  2. Lady Margaret Thatcher

    abc Guest


    I had something similar happen to me with an Abit board.
    I was plugging in some HDDs, the PC wasn't running but was on at the wall
    (no on/off switch on the generic PSU). I think I was trying to diagnose HDD
    problems at the time.

    I think it might be some sort of protection built into the mobo or PSUs. I
    didn't short anything (wasn't using tools and I don't wear rings), so the
    only thing I can think of is that I may have touched the internals
    momentarily, (I'm talking milliseconds), before I earthed myself on the
    frame , and some static charge caused this to happen, Even then I just
    touched either a SATA cable or HDD power lead.

    I could be wrong but I believe most PSUs have a breaker inside them that
    acts like a thermostat, and as the breaker closes again you get a small
    amount of power-which is why the fans might spin, but nothing else. I'm sure
    someone will correct me (even if I am right ;) )

    Cheers.
     
    abc, Aug 25, 2005
    #2
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  3. Lady Margaret Thatcher

    Phil Weldon Guest

    'abc' wrote, in part:
    | I could be wrong but I believe most PSUs have a breaker inside them that
    | acts like a thermostat, and as the breaker closes again you get a small
    | amount of power-which is why the fans might spin, but nothing else. I'm
    sure
    | someone will correct me (even if I am right ;) )
    _____

    ATX type switching power supplies have 'crowbar' overload protection. If a
    short condition exixts, the power supply shuts down. This is by current
    sensing, not heat. The supply will not restart until the short is removed.
    Such supplies also have a more conventional overload protector that limits
    the total amount of power drawn (a fuse or thermal breaker.)

    Phil Weldon

    ..
    ..
    ..
     
    Phil Weldon, Aug 25, 2005
    #3
  4. To meet the ATX specs, the supply should have crowbar _overvoltage_
    protection, but not a crowbar on the overcurrent state. Normally if the
    crowbar circuit fires, it will blow the input fuse, and will not softly
    recover as described. The key to the fan spinning, is that ATX supplies
    have two complete powr supplies in the one case. the first is the 'main'
    supply, and this uses eletronic on/off control. The second is the
    'standby' supply, and this powers the circuit that controls the main
    supply. The actual decision whether to power up, when the external supply
    is switched on, is done in software on the board itself (hence the BIOS
    option on many boards for 'power fail restart'). It is fairly common for
    the standby supply to start first, and the circuitry it supplies, to then
    momentarily engage full power, while a decision is reached on whether the
    system should restart. This gives a fractional 'glitch' on the fans etc.,
    powered off this supply.

    Best Wishes
     
    Roger Hamlett, Aug 25, 2005
    #4
  5. Lady Margaret Thatcher

    Phil Weldon Guest

    'Roger Hamlett' wrote, in part:
    | To meet the ATX specs, the supply should have crowbar _overvoltage_
    | protection, but not a crowbar on the overcurrent state.
    _____

    #1. Not according to ATX 12V (March 2005):
    [From the document at
    http://formfactors.org/developer/specs/ATX12_BSDB_2_2_public_br2.pdf ]

    3.4.2. Short-circuit Protection
    An output short circuit is defined as any output impedance of less than 0.1
    ohms. The
    power supply shall shut down and latch off for shorting the +3.3 VDC, +5
    VDC, or
    +12 VDC rails to return or any other rail. The +12 V1DC and +12V2DC should
    have
    separate short circuit and overload protection. Shorts between main output
    rails and +5
    VSB shall not cause any damage to the power supply. The power supply shall
    either shut
    down and latch off or fold back for shorting the negative rails. +5 VSB must
    be capable of
    being shorted indefinitely, but when the short is removed, the power supply
    shall recover
    automatically or by cycling PS_ON#. The power supply shall be capable of
    withstanding a
    continuous short-circuit to the output without damage or overstress to the
    unit (for
    example, to components, PCB traces, connectors) under the input conditions
    specified in
    Section 3.1. The maximum short-circuit energy in any output shall not exceed
    240 VA, per
    IEC 60950 requirements.

    3.4.3. No-load Operation
    No damage or hazardous condition should occur with all the DC output
    connectors
    disconnected from the load. The power supply may latch into the shutdown
    state.


    3.4.4. Over-current Protection
    Overload currents applied to each tested output rail will cause the output
    to trip before
    reaching or exceeding 240 VA. For testing purposes, the overload currents
    should be
    ramped at a minimum rate of 10 A/s starting from full load.
    **
    #2. The 'Power Good' signal is not a 'software' function; it is a hardware
    function, and does not depend on the BIOS (depending on the BIOS would
    defeat the purpose, as the BIOS is of no use without a CPU, and the whole
    purose of the 'Power Good' signal is to prevent operation at incorrect
    voltages.)

    #3. The 'Standby Power Supply' is always on (as long as AC power is
    applied.)

    Phil Weldon
     
    Phil Weldon, Aug 25, 2005
    #5
  6. A crowbar circuit is not the same as the overcurrent shutdown on the
    power supply. A crowbar circuit actually shorts out the power supply's
    output in order to either force the overcurrent to trip or blow the
    input fuse. This prevents excess voltage from damaging components if the
    output voltage rises too high due to some malfunction.
     
    Robert Hancock, Aug 28, 2005
    #6
  7. Lady Margaret Thatcher

    Phil Weldon Guest

    'Robert Hancock' wrote:
    | A crowbar circuit is not the same as the overcurrent shutdown on the
    | power supply. A crowbar circuit actually shorts out the power supply's
    | output in order to either force the overcurrent to trip or blow the
    | input fuse. This prevents excess voltage from damaging components if the
    | output voltage rises too high due to some malfunction.
    _____

    Your description is of older, linear power supplies, not of modern switching
    power supplies such as ATX specification power supplies. The classical
    crowbar function is not necessary or desirable for switching power supplies;
    the function is provided by overvoltage sensors and short sensors (now
    usually part of the controller chip) that remove the drive to the input
    switching transistors, shutting down the power supply. There is no need to
    force an AC supply side fuse to blow since the same results are much better
    obtained by removing the drive. The fuses/breakers on on an ATX power
    supply protect against excessive current draw on the line voltage side. On
    the low voltage, high current side, sensors protect each of the rails, one
    or more of which might go into a over current condition without causing
    excessive AC current draw.

    As an experiment you could use a bare ATX supply, shorting one or more of
    the outbut rails. The supply will shut down, but no fuses will ever be
    blow, nor any breakers tripped.

    What you describe just doesn't happen for switching power supplies.

    Phil Weldon
     
    Phil Weldon, Aug 28, 2005
    #7
  8. It does.
    What I was taking offence to, (if you read the original post) was the
    suggestion that there would be a crowbar on an _overcurrent_ situation.
    This as you point out, would be self aggravating, pointless, and even
    potentially dangerous, being much simpler just to shut off the drive.
    Crowbars are still used on switch mode supplies, being applied across the
    capacitors on the incoming rail, as one of the few ways to get rid of the
    excess energy stored here, in the event of a potentially dangerous output
    situation, which has not been controlled by the normal regulation. These
    are required to meet the specifications in certain countries, especially
    for office/industrial applications, and are common on current generation
    supplies.

    Best Wishes
     
    Roger Hamlett, Aug 28, 2005
    #8
  9. Lady Margaret Thatcher

    Flash Guest

    I'm sorry, but I wouldn't want to throw a crowbar into my PS. It might
    let the smoke out!

    Did that once with a 286 computer. The fuse blew and I replaced it
    with a "slow blow" fuse. The mushroom cloud was impressive. Git 'er
    done!!

    Cheers...
     
    Flash, Aug 31, 2005
    #9
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