Archived from groups: alt.comp.periphs.videocards.nvidia (
More info?)
'Kate' wrote, in part:
| Phil : that is fascinating stuff. As capacitors are so small, how can
| they pack such a punch? More importantly, if the monitor is arcing,
| is it a fire risk? I`ll give your suggestion of thumping the monitor
| a try next time the display collapses. If nothing else, it will help
| me to vent my displeasure!
|
| Regarding a 19" monitor, unfortunately my desk is built in and cannot
| be moved. How much deeper than a 17" would it be? If it is only 2",
| it would probably be OK.
_____
#1. The capacitors used in electronic equipment hold a small amout of
energy; the punch comes from the ability of capacitors to release energy
very quickly.
Excessive detail follows -
The basic unit of Capacity is the farad, named after Michael Faraday. It
represents the energy. This is a very large unit, and even the large
capacitors found in desktop computer power supplies are less than 1/10
farad. Such a capacitor, fully charged at, say. 12 volts, could deliver (for
a 100,000 microfarad capacitor) ~ 144 Watt seconds (enough to keep a 155
watt lamp lit for less than a second. Even a three cell flashlight can do
that over 10 seconds. The difference is that the capacitor could deliver
that energy in less than 1/1000 of a second, a flashlight could't deliver
the energy in much less than 10 seconds. Skin resistance will block almost
all of the energy from a 12 volt source, the same voltage delivered to
internal body tissues will cause grave damage.
The capacitor(s) in the high voltage section of a CRT device are charged to
thousands of volts (mainly depending on the CRT size.) The CRT itself also
functions as a capacitor, and since the insulation is a thick piece of
glass, the charge can take a long time to decay. The capacitance is, as I
remember, less than a millionth of a farad. The danger of shock increases
because 20,000 volts is enough to overcome skin resistance and deliver
energy in to body tissues. Also, at that voltage, body capacitance to
ground (no direct connection) is enough to allow current to flow. While it
is doubtfull that a shock from the high voltage capacitor(s) in a CRT
monitor would be directly fatal (I'm still alive), there could be secondary,
and fatal effects.
#2. Thumping the monitor. If thumping brings temporary relief, it may not
be diagnostic. The thumping may have either dislodged dust, or it could
have shaken a component back into contact. You could try using a vacumn
cleaner on your monitor from outside the case, both blowing and suctioning
(with the monitor having been turned off for a few hours.) This might help
if the dust is the problem. Unfortunately, the dust may be slightly greasy,
and hard to dislodge. Another name for thumbing, is 'finding the kick
plate'; the point a which you kick a piece of equipment in order to make it
work properly (originated when computer equipement was the size of
refrigerators.)
#3. The 19" CRT monitor I'm using (Viewsonic A95f) to write this is 19.5
inches deep, and an extra 4 inches is needed for the connecting cable to
avoid stress on the cable (which will eventually destroy the cable.) The
rear is 13.5 inches X 13.5 inches; knocking a hole in the wall to get more
depth might be possible B>) Deeper monitor cases allow longer CRTs (usually
measured by the angle made by the lines from each side of the screen to the
neck.) Longer CRTs have better geometry, use less power, can have faster
refresh rates, and geometry adjustments are easier to make. Very high end
monitors use a microprocessor make relatively automatic adjustments (and
there can be over fifty different adjustments for geometry correction.) But
color and contrast for a CRT are much superior to current LCD screens.
Phil Weldon
"Kate" <@*slamaspam*.demon.co.uk> wrote in message
news:dauru3$67h$1$8300dec7@news.demon.co.uk...
>
> "Phil Weldon" <notdiscosed@example.com> wrote in message
> news:cSyAe.21234$eM6.18239@newsread3.news.atl.earthlink.net...
>> 'Kate' wrote, in part:
>> | There is usually a warning label on the back of monitors
>> | (and TVs) not to investigate inside, but why do they retain power
>> | after they are disconnected? Is there a magneto in there, or
>> | something?
>>
>> _____
>>
>> CRT display devices have at least three particular dangers.
>> The CRT is evacuated, and can implode violently if hit; the front is
>> protected by safety glass, but the sides and particlarly the neck
>> are vulnerable.
>> The CRT phosphors are poisonous.
>> The high voltage system has a capicator stack that holds a relativly
>> low charge, but at a very high voltage, and capable of leaping an
>> inch or more through the air (or in dust, as may be intermittently
>> happening in your monitor.)
>>
>> The low voltage power supply (operating at line voltage, 220 in your
>> case) also has capacitors that can store a charge; though the
>> voltages are lower, the stored charge is higher than for the high
>> voltage system.
>>
>> The shock potential is high, and at the least will be very
>> unpleasant. Even more dangerous is the possibility of involuntarily
>> hitting the CRT in response to the shock and causing an implosion
>> and flying glass.
>>
>> For the above reasons, among others, CRT monitor cases are difficult
>> to open, and have the warning lable outside.
>>
>> If you take the monitor to a shop, keep in mind that intermittent
>> failure of components in the high voltage cirucuit may be the
>> culprit, rather than dust collection. If this is the case, repair
>> may not be economic, compared to replacement (repair could easily
>> cost 50% as much as replacement.
>>
>> Go with the 19", even if you have to move your desk away from a wall
>> to let the rear of the CRT monitor hang over the edge (that's what I
>> do.)
>>
>> A final suggestion; the next time the 'pop' occurs, and the image
>> collapses, bang smartly on the side of the monitor; really. The
>> image may immediately return. Occasional banging may get you
>> through the time necessary for replacement.
>>
>> Phil Weldon
>>
> Firstly, to Steve : it was very kind of you to think of my safety. To
> be frank, I used to be unstoppable when I had a screwdriver in my
> hand, but I have learned to be more cautious. Besides, these days the
> legend "No user-serviceable parts inside" most often appears on the
> back of appliances.
>
> Phil : that is fascinating stuff. As capacitors are so small, how can
> they pack such a punch? More importantly, if the monitor is arcing,
> is it a fire risk? I`ll give your suggestion of thumping the monitor
> a try next time the display collapses. If nothing else, it will help
> me to vent my displeasure!
>
> Regarding a 19" monitor, unfortunately my desk is built in and cannot
> be moved. How much deeper than a 17" would it be? If it is only 2",
> it would probably be OK.
>
> I realise that this thread has moved away from the subject of video
> cards, so I want to thank you all for your patience and for allowing
> me to digress.
>
> Kate
>
>
>
>
Originated in the military. 
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