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ESD Inquiry

Discussion in 'Electronic Repair' started by JJD, Dec 29, 2003.

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  1. JJD

    JJD Guest

    I have several questions about the 'safe way' to open up and fiddle
    around with a computer.
    My computer user's manual lists these steps:
    1. Turn computer off.
    2. Wear grounding wrist strap and attach to metal chassis.
    3. Touch bare metal surface on back of computer.
    4. Unplug the computer's power cord.

    I understand that any static charge on me would be pulled to ground in
    step 2 but do I then just touch the bare metal momentarily or do I
    have to hold on to it throughout (step 3), and if I do would that not
    defeat the purpose of step 2?

    Also when I unplug the computer's power cord, if say I am shifting
    over a floor or whatever, and build up some static charge, it will
    discharge into the metal chassis, but then what? The metal chassis
    will then be charged, what "bank" will hold the chrage before it is
    pulled to ground when I plug the cord back in? And what happens if I
    do not plug it back in for some time after taking off the wrist strap?
    Will the chassis stay charged, or bleed off how?

    Also I heard that I can leave the power cord plugged in but turn off
    the machine using a switch in the back, isolating any circuits within,
    some PSU do not have this switch though. Anyway please answer the long
    questions above.

    tia
     
  2. JURB6006

    JURB6006 Guest

    Yes unplug the power cord.

    Also do not plug the power in unless the AGP card is screwed down, and on newer
    PCs also that the processor fan is attached and connected.

    As far as the static, you don't have to be completely discharged, but you must
    be at the same charge as the PC. That's the reason for touching the metal
    chassis. You can be at 100 volts, as long as the PC is within a couple of volts
    of your body you're OK. It's a <u>difference</u> of voltage that is your enemy.

    JURB
     
  3. Sunny

    Sunny Guest

    Step 3 should not be neccessary in addition to step 2, but it can't
    hurt. A momentary touch is all that's required.
    Exactly, it's a potential problem - so I disagree with step 4.
    An ATX PSU sends standby power to the motherboard when it's switched off
    at the front panel but still connected via the power cord (assuming the
    back panel switch, if present, is on). You should not plug or unplug
    components in the computer while standby power is applied as this can
    cause damage.

    My recommendation in place of step 4 is:

    - If PSU has a back panel switch, turn it off and leave power cord
    connected.
    - If PSU does not have a back panel switch, replace power cord with one
    that has had the active and neutral pins cut off, leaving just the
    ground pin. An alternative is to attach your wrist strap to a ground
    other than the computer chassis, disconnect the power cord, and repeat
    step 3 every few minutes while working.

    Using this method means standby power is off but chassis remains grounded.

    Sunny
     
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