ESD

Discussion in 'A+ Certification' started by soup, Nov 7, 2009.

  1. soup

    soup Guest

    In the GTS book it says you MUST unplug the Computer and clip your
    anti-static band to it, to balance out the voltage in you and on the
    computer. Generic advice seems to be to leave the computer plugged in
    but switched off to ground the voltage in you and in the computer I am
    quite willing to go with "computer must be unplugged and voltages
    balanced" for the sake of any test. I was just wondering if they say
    computer must be unplugged as Americans usually don't have switched
    sockets. Then you rely on the switch working to shield you from 240
    volts, but any voltage between you and the computer can be leaked away.
     
    soup, Nov 7, 2009
    #1
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  2. ***IF*** the computer has a true master power switch, leaving it plugged
    in but turned off (MASTER switch (usually on the back of the power
    supply) is better, because the power cord grounds the chassis (and you,
    if you are using a wrist strap). However, many computers (especially
    models made by the large OEMs) do not have a master power switch, and
    the only way to truly turn them COMPLETELY off is to unplug them.

    The issue is not the AC power line voltage (in the US, typically 115-120
    volts), but the 5 volts standby, and sometimes 3.3 volts, that is still
    present when a modern computer is "shut down".


    soup wrote:
    > In the GTS book it says you MUST unplug the Computer and clip your
    > anti-static band to it, to balance out the voltage in you and on the
    > computer. Generic advice seems to be to leave the computer plugged in
    > but switched off to ground the voltage in you and in the computer I am
    > quite willing to go with "computer must be unplugged and voltages
    > balanced" for the sake of any test. I was just wondering if they say
    > computer must be unplugged as Americans usually don't have switched
    > sockets. Then you rely on the switch working to shield you from 240
    > volts, but any voltage between you and the computer can be leaked away.
     
    Barry Watzman, Nov 8, 2009
    #2
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  3. soup

    soup Guest

    Barry Watzman wrote:
    > ***IF*** the computer has a true master power switch, leaving it plugged
    > in but turned off (MASTER switch (usually on the back of the power
    > supply) is better,
    > The issue is not the AC power line voltage (in the US, typically 115-120
    > volts), but the 5 volts standby, and sometimes 3.3 volts, that is still
    > present when a modern computer is "shut down".


    I am not talking about the switch on the computer but the socket on the
    wall usually in the UK they are switched but in the US they are usually
    unswitched .

    I would never leave the power cord plugged in to an unswitched socket as
    standby voltages can still be present even if the computer's power
    supply is switched off.
     
    soup, Nov 14, 2009
    #3
  4. Re: "I would never leave the power cord plugged in to an unswitched
    socket as standby voltages can still be present even if the computer's
    power supply is switched off."

    Not if the computer is turned off with a switch on the REAR of the power
    supply itself. That switch is the same as unplugging the computer.


    soup wrote:
    > Barry Watzman wrote:
    >> ***IF*** the computer has a true master power switch, leaving it
    >> plugged in but turned off (MASTER switch (usually on the back of the
    >> power supply) is better, The issue is not the AC power line voltage
    >> (in the US, typically 115-120 volts), but the 5 volts standby, and
    >> sometimes 3.3 volts, that is still present when a modern computer is
    >> "shut down".

    >
    > I am not talking about the switch on the computer but the socket on the
    > wall usually in the UK they are switched but in the US they are usually
    > unswitched .
    >
    > I would never leave the power cord plugged in to an unswitched socket as
    > standby voltages can still be present even if the computer's power
    > supply is switched off.
     
    Barry Watzman, Nov 14, 2009
    #4
  5. soup

    soup Guest

    Barry Watzman wrote:

    > Not if the computer is turned off with a switch on the REAR of the power
    > supply itself. That switch is the same as unplugging the computer.


    I must have misunderstood what switching the computer off meant (i.e.
    the switch on the computer rather than the switch on the power supply)
    The piece I had read was:-

    •An ATX power supply provides soft power . This is a condition where
    the motherboard always has power even when the computer is turned off.

    I must have mistaken that *computer is turned off* to mean the switch on
    the power supply rather than the switch on the computer.


    The issue of the switches being faulty still would worry[1] me . Is
    this balancing the charge (ie you at 100 volts the chassis of the
    computer you are working at also at 100 volts(note, not real figures)),
    rather than earthing the entire system, theory a goer?

    [1] 'Worry' is maybe too strong a word 'thoughtful off' too weak
    probably something in between (can you tell I am no linguist?).
     
    soup, Nov 15, 2009
    #5
  6. soup

    Bill Eitner Guest

    soup wrote:
    > Barry Watzman wrote:
    >
    >> Not if the computer is turned off with a switch on the REAR of the
    >> power supply itself. That switch is the same as unplugging the computer.


    Except that the ground is still there (the chassis is
    still connected to a zero volt reference--generally
    a ground rod or cold water pipe somewhere).

    > I must have misunderstood what switching the computer off meant (i.e.
    > the switch on the computer rather than the switch on the power supply)
    > The piece I had read was:-
    >
    > •An ATX power supply provides soft power . This is a condition where
    > the motherboard always has power even when the computer is turned off.
    >
    > I must have mistaken that *computer is turned off* to mean the switch on
    > the power supply rather than the switch on the computer.
    >
    >
    > The issue of the switches being faulty still would worry[1] me .


    There's often an LED on the motherboard to indicate
    standby power is still present and that you shouldn't
    be working inside the case. If there's no LED or you
    suspect that it or the power supply switch is faulty, the
    routine should be to either check for the presence of
    the standby voltage (+5 VDC between the 20/24 pin connector
    purple wire and ground) or unplug the AC cable at the
    power supply and connect a clip lead between the grounded
    connector on the AC cable and a bare metal point on the
    case/chassis. That same point or area is where you should
    also connect your wrist strap (if you believe in using
    one--which is another debate). This is a good procedure
    in situations where the power supply doesn't have a switch
    (assuming you believe that connecting the chassis to the
    reference is necessary--more on that below).

    > Is
    > this balancing the charge (ie you at 100 volts the chassis of the
    > computer you are working at also at 100 volts(note, not real figures)),
    > rather than earthing the entire system, theory a goer?


    The point is to establish and maintain a voltage
    reference. The common way to do that is to start
    with a patch of ground (or Earth) near the building.
    That's done through contact between a piece of metal
    with a relatively large surface area and the ground
    itself. That contact area is said to be at zero volts
    DC (zero frequency). Since it has no frequency the
    extra considerations that apply to AC don't apply.
    To put it simply, this zero volt reference can be
    transported with only the resistance of the conductor
    to raise it above zero. So, if we use heavy conductors
    relative to the expected amount of current flow--which
    should be zero unless there's a fault which should
    only be momentary as ground faults trip protection
    circuits that quickly stop the flow of current, we'll
    have a close to zero volt reference anywhere along any
    of the conductors in the circuit.

    When it comes to computer/electronic servicing the
    zero volt reference is a convenience more than it really
    is a necessity. For safety and charge balancing purposes
    something has to be the reference. It doesn't have to
    be a patch of ground near the building, however that's
    what has been settled on.

    > [1] 'Worry' is maybe too strong a word 'thoughtful off' too weak
    > probably something in between (can you tell I am no linguist?).


    What's more confusing than the reference when considering
    ESD is the purpose of the megohm resistors in the wrist
    straps and other ESD devices. The idea is the resistors
    limit the current so that the person wearing them doesn't
    get shocked if they contact a source of high voltage
    relative to the reference (which is supposed to be zero).
    The problem is that allows the person to be at a higher
    potential than everything else that is connected directly
    to the reference (like the computer). So, using the
    common ESD devices is of no value when it comes to
    preventing ESD damage to computer/electronic components.
    Further, if the person touches the chassis or anything
    else that is connected to the reference while in contact
    with a source of high voltage they will still be shocked.

    When you really think about it logically the point really
    is simply charge balancing between you and the chassis
    of the computer you're working on. At the A+ level where
    all that is done is removing and replacing block-level
    devices the technician can simply unplug the AC cable at
    the power supply and physically touch the bare metal
    chassis to balance the charge between himself and the
    chassis. Handle devices by the places that connect to the
    chassis (in the case of processors and other semiconductors,
    handle them by their edges and don't touch any of the
    electrical contacts) and touch the chassis occasionally
    to balance any static charge built up by walking around
    and handling other objects. That's really all there is
    to the ESD thing at the A+ level.
    --
     
    Bill Eitner, Nov 15, 2009
    #6
  7. The ground still being there is BENEFICIAL. You WANT the ground to be
    there. The reason you wear a wrist strap is so that YOU are at ground
    potential. For this to be as effective as possible, the computer should
    also be at ground potential, which is best achieved if it, too, is grounded.

    Re: "The issue of the switches being faulty still would worry[1] me"

    and

    "If there's no LED or you suspect that it or the power supply switch is
    faulty, the routine should be to either check for the presence of the
    standby voltage (+5 VDC between the 20/24 pin connector purple wire and
    ground) or ....."

    Look, if you are worried that the computer might still be active, just
    push the power switch (power button)(on the computer), as if you were
    trying to turn it on. If it doesn't turn on ..... then you know that
    the power really is killed. No need to start measuring things, although
    the motherboard standby power warning light is also a perfectly good
    indicator of the power supply status, except that not all motherboards
    have such an LED.
     
    Barry Watzman, Nov 17, 2009
    #7
  8. soup

    Bill Eitner Guest

    Barry Watzman wrote:
    > The ground still being there is BENEFICIAL.


    Why?

    > You WANT the ground to be
    > there.


    Why?

    > The reason you wear a wrist strap is so that YOU are at ground
    > potential.


    No, you're not--you're at a megohm above ground.
    That's a MILLION ohms above ground. You're at
    every potential EXCEPT GROUND.

    > For this to be as effective as possible, the computer should
    > also be at ground potential, which is best achieved if it, too, is
    > grounded.


    For what to be as effective as possible?

    You're babbling.

    > Re: "The issue of the switches being faulty still would worry[1] me"


    It wouldn't worry me.

    > and
    >
    > "If there's no LED or you suspect that it or the power supply switch is
    > faulty, the routine should be to either check for the presence of the
    > standby voltage (+5 VDC between the 20/24 pin connector purple wire and
    > ground) or ....."


    You edited my quote.
    That's bad form.

    > Look, if you are worried that the computer might still be active, just
    > push the power switch (power button)(on the computer), as if you were
    > trying to turn it on. If it doesn't turn on ..... then you know that
    > the power really is killed. No need to start measuring things, although
    > the motherboard standby power warning light is also a perfectly good
    > indicator of the power supply status, except that not all motherboards
    > have such an LED.


    You suck.
    --
     
    Bill Eitner, Nov 18, 2009
    #8
  9. soup

    jeremy Guest

    > in but turned off (MASTER switch (usually on the back of the power
    > supply) is better,
    > The issue is not the AC power line voltage (in the US, typically 115-120
    > volts), but the 5 volts standby, and sometimes 3.3 volts, that is still
    > present when a modern computer is "shut down".


    I am not talking about the switch on the computer but the socket on
    the
    wall usually in the UK they are switched but in the US they are
    usually
    unswitched .

    I would never leave the power cord plugged in to an unswitched socket
    as
    standby voltages can still be present even if the computer's power
    supply is switched off.
     
    jeremy, Nov 27, 2009
    #9
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