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max capacitance of 1 cu ft box?

Discussion in 'Electronic Basics' started by [email protected], Aug 13, 2005.

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  1. If I open an electrical thing (such as a monitor but perhaps anything)
    that was recently unplugged from a 120VAC power source (with normal
    tolerance 110-130VAC) and having no battery to worry about, I have to
    beware of a stored electrical charge due to capacitance, either
    intentionally designed using capacitors or simply as a function of
    conductors being separated by insulation.

    I don't like using the discharge procedure, not having tried it and
    usually not being in a rush. Therefore, I prefer to wait until
    capacitance is low enough to be negligible or zero before opening the
    case and working inside.

    Apple Computer told me there's 10,000 volts inside a Macintosh SE,
    which is powered from a 120VAC line. (The SE is a computer with a
    monitor in the same box.) I assume their box is not alone.

    How long should I wait after unplugging? Assume a 1-cubic-foot box with
    any internal design, so I can multiply/divide for the actual cubic
    footage of the thing I'm thinking of opening.

    Years ago I looked in an electrical book (title forgotten) and found a
    formula that suggested an hour per cubic foot would suffice. In
    reality, of course, the box usually is mostly air, but I assumed the
    box was solidly electronic for the calculation, so the hour seemed
    plenty. Then someone challenged me on that and said a few days was more
    like it. In another book, I saw too many variables to support good

    Assume the most dangerous combination of dimensions, plate area, number
    of capacitors, plate separation, separator material, etc. The estimated
    time for capacitance to drop to negligibly close to zero doesn't have
    to be precise, just safe. Can you suggest a reasonable time or formula?

    Thank you.

    -- Nick

  2. John Fields

    John Fields Guest

    On 13 Aug 2005 12:18:50 -0700,

    No, because there's no real way to tell, other than by measuring it,
    which comes with its own set of hazards.

    The best thing to do is to exercise the discharge procedure, be
    safe, and be done with it.
  3. JeffM

    JeffM Guest


    Have youi considered getting your head out of your ass
    and using a username that is a reasonable length?
  4. Tom Biasi

    Tom Biasi Guest

    wrote in message
    The volume of the box has nothing to do with the dangers you describe.
    Large electrolytics with high working voltages are to be respected. Most
    modern equipment has bleeder resistors to discharge the caps.
    The static charge on the CRT can "surprise" you if you come in contact with
    the anode, after all it is a big "capacitor."
    If you are not that certain about procedures, educate yourself before
    opening the box.
    BTW: I don't know the position of your head with relation to your anus but
    I agree with JeffM.
  5. Impmon

    Impmon Guest

    Weeks. The CRT itself acts like a capacitor and could store charge
    for several days or so. It is safer to get a long screwdriver with
    good insulated handle, heavy gauge wire, and 10 Mohm resistor all in
    series. Clamp the cable ro any ground (metal chasis of monitor
    usually works), and then work the screw driver under the suction cap
    on the CRT until it makes contact with the metal clamp inside.

    You should do the same with each caps just to be safe. Even a $10
    disposable camera with flash bulb carries a nasty kick if you touch
    the caps with your bare hand.

    There is no foolproof system for determing the remaining charge of any
    CRT. I've been zapped by charged CRT and trust me you don't want to
    get shocked while carrying the CRT, they make a terrible mess when you
    drop them in reflex. :) Never needed medical attention yet but better
    safe than sorry.

    PS really long email address. Try something shorter like .
  6. Bob Eldred

    Bob Eldred Guest

    <[email protected]qrstuvwxyza> wrote in message
    What is this about? The capacitance of the surface area of a volume such as
    a one cubic foot sphere or cube? Or the the capacitance internal to an
    electronic device such as the Mac SE? In the first case we are talking about
    100pF or there abouts and the charge is like that one gets when shuffling
    across a carpet and draws a small spark. The second is capacitance found in
    the power supply circuits of the device and may be many microfarads and in
    some cases could blow the end off of a screw driver if shorted.

    If you have to ask this question, you have no business opening an electronic
    device! There are warnings on the device about hazardous voltages inside and
    qualified techs getting into it. Heed the warnings! A tech will discharge
    hazardous caps to work on the circuits, but of course, he knows how to
    identify them and what to do. If you don't, keep out. To answer you
    question, some caps can hold there charge for a week or more so be careful.
  7. I've only done a few hundred PC monitors over the years, but
    typically, unlike TV's, they are discharged by the time you get the
    cover off, but..YMMV.

  8. That is, the CRT is discharged. All bets are off for capacitors. CP01
    (I think) in Thomson TX 85/86/825/826 were nasty ones. :)

  9. Nick

    Nick Guest

    John's and Impmon's answers are likely the best. I'll probably just
    have to cautiously learn discharging and trust it won't get colorful.

    Tom Biasi and Bob, the dangers are precisely why I asked, and being
    knowledgeable before undertaking a procedure is precisely why I asked.
    I already agree with both principles. Having to ask a question is a
    qualification, not a disqualification; the necessity for asking comes
    from the nature of electrical circuits, and getting the information in
    advance is exactly what should be done when anything dangerous is
    undertaken. The lectures are better suited to those who don't ask. The
    issue to worry about is lack of judgment: the person who doesn't know
    what to ask when or when to turn back is the one who shouldn't do the
    work. I apply my expertise to many difficult problems and have refused
    various assignments when I saw fit to do so.

    Tom Biasi, the size of the box proves little, but is a basis for
    inferring the possibility of there being more capacitance or less of
    it, all else equal, depending on intent, design, etc., since capacitors
    supporting more capacitance tend to be bigger. There's no practical way
    of knowing whether there's a bleeder resistor inside and whether it
    hasn't been dislodged or broken by past damage until it's opened, and
    I'm not sure I'd always recognize it anyway; the design isn't binding
    on used equipment.

    Bob, internal capacitance is my concern, because that's the one that
    can be deadly. Carpet static and doorknob shocks are no worse than
    annoying or surprising to healthy adult humans, although the shocks can
    damage equipment.

    Tom MacIntyre, I'm wondering if I was lucky when (based on one book) I
    waited an hour before opening a Mac SE, which I did many times, never
    with a shock from being too early. The SE is roughly a cubic foot. You
    might be lucky, too, based on other comments. Scott Mueller, author of
    Upgrading and Repairing PCs, suggests weeks as a possibility. On the
    other hand, it seems questionable why a designer would want more than
    an hour of usable capacitance, or more than a few minutes, for that
    matter, but maybe designs are only for minimum durations, not long
    ones, leaving us to allow for long durations, or discharge manually.

    Jeff and Tom Biasi, the username was an error. When I registered with
    the newsgroup, I changed the default (the e-mail address as nickname)
    to Nick, then detoured through a link and came back to the registration
    page, and didn't check that the fields hadn't changed. (If it hadn't
    changed, Google or the newsgroup server goofed, but I'll assume I
    erred.) I changed it a moment after posting my message and seeing the
    ridiculous heading, but my subsequent change had no effect on that
    post. Maybe it will on this one.

    Impmon, the e-mail address is real, although I prefer getting e-mail at
    my Yahoo address given in the message. I had intended a strategy of
    spam-resistance, but it doesn't seem to work, since I get more at Yahoo
    than at abcde.... My guess is someone out there manually reads posts
    for spamproofed addresses and figures out the real forms, then markets
    the list.

    Thanx, all.

    -- Nick

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