# max capacitance of 1 cu ft box?

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

1. ### [email protected]Guest

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
calculations.

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

E-mail:
Nick_Levinson
Domain:
yahoo.com

2. ### John FieldsGuest

On 13 Aug 2005 12:18:50 -0700,
""
<>
wrote:

---
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. ### JeffMGuest

Nick_Levinson

and using a username that is a reasonable length?

4. ### Tom BiasiGuest

<>
wrote in message
Nick,
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.
I agree with JeffM.
Regards,
Tom

5. ### ImpmonGuest

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 EldredGuest

<[email protected]qrstuvwxyza
bcdefghijk.com> 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.
Bob

7. ### Tom MacIntyreGuest

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.

Tom

8. ### Tom MacIntyreGuest

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.

Tom

9. ### NickGuest

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.
Thanx.

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
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

E-mail:
Nick_Levinson
Domain:
yahoo.com