M
mike3
On Feb 16, 9:57 pm, (e-mail address removed) (Gordon Burditt) wrote:
<snip>
Thank you for the good answer.
<snip>
Thank you for the good answer.
M
mike3
How come a monitor has so much more juice in it than a desktop's
PSU? Is it because of that electron gun?
D
DonC
mike3 said:
Sort of. It's because of the extremely high voltage necessary to thrust
electrons into the phosphor coating the screen. That's why it's called an
"electron gun."
P
Paul
mike3 said:
As you'd guess, "free" and "accurate" don't always go hand
in hand. I've worked with a number of people, who in their
daily lives, feel that they should not share their knowledge,
unless someone is paying them. And certainly, there are people
who do repair work during their working day, and don't want
to continue to do repair work, by answering questions on USENET
in the evening.
Inside the computer, there are a lot of items which are
modular, replaceable, and operate at a low voltage. The
existence of such, allows the average home user, to
make changes, or do simple maintenance. For example,
adding a hard drive to the computer, or adding a
stick of RAM.
Anything involving the AC power line, should be treated with
more care. A number of devices in the household, do store a
considerable amount of energy. They also have protection
features, to protect against the most common failure
conditions. Opening up such an item, is not really smart,
if you don't know anything about what is inside them.
I can give you an example of something that happened to me.
At work, we had a number of microwave ovens, located so that
employees could heat up their lunch, at lunch time. The
microwave ovens were not cheap ones - they were the so-called
"commercial grade" ones, the same kind you used to see next to
sandwich vending machines, years ago. They still have roughly
the same power rating as a home machine, but are more solidly
built (fewer plastic bits that can break off, and the door
could take more abuse).
Now, there were a few individuals at work, who used to make
buttered popcorn, virtually every day, in the afternoon.
When buttered popcorn cooks, steam is released, and the steam
contains water, butter, and *salt*. Over time, this mixture
coats parts inside the microwave.
OK, so it is the evening and I'm working late. I take something
up to the microwave, push the button. BAM! The most loud explosion
you can imagine happens. Just like a firecracker going off right
next to you. I couldn't hear for about ten minutes after it
happened, that is how loud the sound was.
Now, what happened there ? Inside the microwave, there is a
transformer, a high voltage diode, and what could be an oil
filled high voltage capacitor. The capacitor is rated for
5000V. It holds a significant amount of energy. If the path
between the (+) and (-) terminals happens to conduct (such
as if there was a coating of salt, grease, and moisture
on the PCB), the cap can arc over. What I heard, was the sound
of a lot of energy being discharged at one instant.
The funny part of all this, is the microwave wasn't damaged at
all. It still operated afterwards (but not by me!). I still
phoned maintenance the next day and had it taken away, so it
could be properly (and safely) cleaned by someone qualified to
do it. The 5000V in that capacitor could easily kill someone, if
connected to them by accident, so shouldn't be cleaned by an
amateur.
So while you might assume your microwave oven is a benign beast,
it isn't necessarily so, as I got to learn first hand.
The ATX power supply has a main capacitor in it as well. If you
do the calculation, it holds a significant amount of energy.
It has a couple features, which help drain the energy. The capacitor
has a bleeder resistor across it, which could take a minute or
two to drain all the juice. The +5VSB standby rail of the supply,
and its associated circuit, also tend to drain the cap, and on
my current computer, that takes about 30 seconds, for the loading
of the +5VSB rail on the motherboard, to take the stored energy
out of the main cap. But now, consider a theoretical scenario.
The bleeder resistor is broken. There is no indicator, to say it
is working or not. The power supply doesn't monitor it. Now,
further say, that the +5VSB rail is not drawing any energy.
(Say the +5VSB circuit is broken somehow as well.) Now, the energy
in the main capacitor is still there. Not a problem, unless the
amateur working on it, happens to touch the terminals.
C5 and C6 are high voltage. R2 and R3 are bleeder resistors.
"Second power supply", bottom left, is the +5VSB circuit, and
draws energy from the main caps.
http://www.pavouk.org/hw/en_atxps.html (ATX schematic)
Professionals make no assumptions about the state of any
capacitors inside a powered device. If there is a possibility
they contain significant energy, then a discharge method may
be applied, to make them safe. Due to the noise issue, you
don't just jam an insulated screwdriver in there - depending
on the thing being discharged, you'd want to use a resistor of
some sort, to discharge the energy a bit slower. Note that capacitors
don't drain all their energy, when discharged just the once.
Some capacitors can show a residual charge (still enough to
kick you on your ass), even after being discharged. They
may require repeated attempts, or even leaving the discharge
device connected to the capacitor, while working on it.
Now, people who work with this stuff every day, probably
don't do a lot of risk analysis, and think through the
possible range of things that could happen. For example,
I worked on a project involving high voltage, and my
first reaction to my boss, is I wanted no part of the
project. I found out, from another company we were working
with, that their star designer "got shocks all the time in
the lab". That is not where I wanted to be (as careless
as that guy)
I've only been knocked on my ass just the once by high
voltage. I was in my basement, as a kid, and playing with
an ignition coil. One minute, I'm standing next to the
work bench. The next, I'm on the floor, a few feet from
the bench, and don't really remember how I got there. In
all the high voltage stuff I've done since then, I've never
had that happen again. But I do think through, all the
things that could happen, before I reach for the
screwdriver.
HTH,
Paul
W
w_tom
Voltage in a TV is something between 10,000 and 20,000 volts.
Computer power supply voltages as much as 350 volts. Both are
dangerous. However one can open and fix their TV - but not the
computer power supply? Nonsense. Some here have posted myths based
in fear.
We routinely opened TVs to find failed vacuum tubes. I was doing it
even at the age of 12. So what makes it dangerous? Ignorance. We
did not blindly fix things as so many now do with TVs. For example,
so called 'computer experts' typically don't even know how electricity
works. They may also claim opening a power supply is a legal
violation.
The danger inside a TV, a computer, and also a microwave oven is
eliminated by a discharge resistor. But sometimes that resistor
fails. Then the power supply is a major and unexpected threat. The
informed simply discharge capacitors everytime after completely
removing power cord from the wall using a screw driver. No
exceptions. Then no risk.
Furthermore, when working on any part of a computer, always
completely disconnect that power cord.
You are encouraged to learn how power supplies work. But also
encouraged to take routine precautions - always. Learn the dangers
before sticking a hand inside.
Find a capacitor that did not discharge, short it with a screw
driver, then appreciate why that charge is so dangerous. It's one
thing to be told. Experience (the discharge) puts knowledge into a
whole new perspective.
What makes that voltage so dangerous? A discharge circuit that
passes through the heart. Informed and cautious techs may also keep
one hand in a pocket. Then dangerous voltages will not discharge arm
to arm - through the heart. Electrical tape around tool handles is
another routine precaution. Drier skin conducts less electricity. If
that 'much less' 300 volts inside a power supply does discharge
through you, then better appreciate why we do these routine
precautions.
Most dangerous are microwave ovens. Voltages inside TVs are almost
as dangerous. Computer power supplies are trivial by comparison.
It's not the power as much as voltage that creates danger.
Appreciate that computer techs need not even have electrical
knowledge to obtain A+ Certification. Their answers to your questions
are more often based in myths. This post intended as a supplement to
multiple replies from DonC.
R
richard
Tell me about it. I was changing tv tubes frequently as a kid myself.
After seeing how a few repair guys did it.
Changing a tube is akin to changing a light bulb. Pull out the old
one, put in new one.
A friend of mine got a job with Wang. He went around checking the
machines and all he did was, pull out the bad card, and replace it
with a new one. Then he went on to another job and told me that when
he has to open a laser operated device, everybody in the room has to
leave.
I know the old school of electronics. I don't know a damn thing about
this new microchip technology. But like you say, the so called techies
of today, don't know jack squat about electronics. What they know is
theory and how to interpret the data presented to them.
Ask an A+ guy how to build a radio, he can't tell you.
W
w_tom
When a tech could not explain why the computer would run diagnostics
all night; crash only when he was not there. Superstition then took
hold. He would loudly announce he was leaving for coffee, stomp out
of the room, then sneak back to spy on that machine behind a doorway.
And still that computer would crash only when he left the building.
Things always occur for good science reasons. Somebody foolishly
grounded that computer to an elevator. Computer only crashed when the
tech used that elevator to get coffee. A problem solved when one with
basic electrical knowledge could then follow the evidence.
A+ Certified computer tech need not know how electricity works to be
declared 'computer literate'. Shotgunning then becomes the solution.
Unlike replacing a light bulb, one first needed respect a TV's
dangerous parts. To understand some basic electrical principles. Some
principles that are routinely unknown by A+ Certified computer techs.
Explains why some here post fear of insides to a power supply.
M
mike3
Well, If I was going to do this, I'd want to really know how to do
it right, ie. I'd want accuracy, and if that costs a lot of money,
then
I'd like to know how to be able to get it. Can most people get the
required money if they work enough? And can one do it with a
minimum of starting capital?
That's what I'd think.
So then how can you learn about that first?
Was it as loud as shooting off a rifle without any hearing protection?
Why do you need to be getting paid in order to handle the
capacitor properly? (You said shouldnt be done by an "amateur".
I suppose you mean someone who isn't experienced with this
type of thing.)
So then it takes a bit of paranoia as well. I guess this is one
of those things where the more paranoid one is, the better.
S
Shadow36
..
A+ Certified computer tech need not know how electricity works to be
declared 'computer literate'. Shotgunning then becomes the solution.
Unlike replacing a light bulb, one first needed respect a TV's
dangerous parts. To understand some basic electrical principles. Some
principles that are routinely unknown by A+ Certified computer techs.
Explains why some here post fear of insides to a power supply.
A+ Certified computer tech need not know how electricity works to be
declared 'computer literate'. Shotgunning then becomes the solution.
Unlike replacing a light bulb, one first needed respect a TV's
dangerous parts. To understand some basic electrical principles. Some
principles that are routinely unknown by A+ Certified computer techs.
Explains why some here post fear of insides to a power supply.
P
ProfGene
How many people do you know who are in prison for removing the tag frommike3 said:
a mattress?
D
DevilsPGD
I'd guess none, since that's not actually illegal.