Capacitors in PSU are dangerous?

Hello

"Regal" wrote in message
...
I read on some website that the capacitors in a PC's PSU can hold a
charge for long after they have been switched off and that the
charge could be fatal.

Is this really so? Surely that is exaggerating?

I hate to throw another issue into this thread but....everyone seems to have
an opinion so here is another. Voltage will burn you or your equipment,
furniture etc. amps will kill. Voltage can knock you off a platform or in
certain situations kill. Pacemaker comes to mind and certain heart
conditions. Why do I say this OSHA electrical safety course among many
others. I haven't looked at PSU for their specs but I do believe that common
sense will keep you safe be it voltage current/amps or what ever. Yes I know
that amps are related to watts and voltage but that is dependent on
resistance. Here is link to some terms and guidance. To many issues to
address in a limited time but the hazards are more than what will kill you.
Fire, Shock and death being the 3 foremost. The hazards are real the warning
labels are overkill CYA most of the time and common sense is what is needed.
I applaud someone just asking a question to find out. By the way I am
Superintendent of Safety for an Air Force base so I have some knowledge, no
expert but have researched more injuries than I have suffered by several
orders of magnitude. LOL

Something to think about:
Good judgment come from experience, experience well that comes from bad
judgment.
If you don't like that quote here is an original.
Stupid should be painful its up to you if its Motrin or morphine. Ask the
question!

http://www.utsa.edu/compliance/envir.../Sec04-01.html

Chris Stolworthy wrote:
"Regal" wrote in message

I read on some website that the capacitors in a PC's PSU can
hold a charge for long after they have been switched off and
that the charge could be fatal.

Is this really so? Surely that is exaggerating?

No they are serious, I had the Unfortunate experience not too
long ago of puncturing one on accident. Nasty little shock,
let me tell ya. Some nice electrical burns as well.

We used to take a 200V 0.1 uF capacitor and stick the leads into a
110 V a.c. socket. Everyone knows you can't charge a cap from an
a.c. supply, right? Then hand it to someone to hold. Carefully.

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On Fri, 16 Apr 2004 00:11:09 GMT, (Overlord)
wrote:

snip

They also take a finite amount of time to discharge. They can be shorted
in such a way to discharge more quickly, kicking out many times their inputted
voltage in one great electron orgasm.

A rapid discharge will do nothing to raise the voltage, it's the current
you're thinking of.... at most the voltage would be the peak input voltage
(per the input from, position in the circuit, not necessarily the mains AC
voltage).

kony wrote:
On Thu, 15 Apr 2004 14:12:48 -0700, ric wrote:

kony wrote:

This mustn't be relied on. From my own personal experience (with a
monitor): the PSU failed and the monitor died. The cause of the
failure was an open-circuit high-value resistor (10 megohms?).
This allowed a large capacitor to charge with no discharge path
other than leakage. The effect was utterly dead-looking equipment
holding a large charge even when switched off.

One difference would be that an ATX power supply is going to
continue supplying 5VSB, that being another drain.

The +5vsb typically uses a small bias transformer and is unrelated to
the +300vdc buss. To confirm, monitor the +5vsb while you unplug the
AC cord or switch OFF the rear panel switch. The +5vsb goes away
instantly (or as soon as the +5vsb caps discharge.)

It hasn't been unrelated in the units I've traced. They looked
pretty standard... right after rectified there was the voltage
doubler, the large caps everyone is concerned about, with the
bleeder resistors across them, and the power leading to the 5VSB
transformer was directly connected, parallel to the bleeder
resistors. Perhaps I haven't looked closely enough, at enough
different PS 5VSB circuits, but I've not seen anything to suggest any
unit deviated from this.

So, these bleeder resistors, they're constantly beelding off current, even
when the machine is in use yes? Sounds wasteful of power to me.
--
~misfit~

Timothy Daniels wrote:
"Regal" wrote:
I read on some website that the capacitors in a PC's PSU can hold a
charge for long after they have been switched off and that the
charge could be fatal.

Is this really so? Surely that is exaggerating?

A well-designed power supply has bleed resistors across the
big capacitors (usually the filtering electrolytics) and the charge
should be essentially dissipated within seconds, certainly a minute,
of shutdown. As for a "fatal" charge, what's the maximum voltage
used in a PC - 12 volts for the fans? Bottom line - have you ever
seen a "Danger! High Voltage" warning on a PC case?

Dickhead. Who's talking about inside a PC case? We're talking PSU case here.
And yes, I've seen them with warnings on them.

Do try to keep up.
--
~misfit~

Paul Hill wrote:
Niel Humphreys wrote:

"Regal" wrote in message
...

I read on some website that the capacitors in a PC's PSU can hold a
charge for long after they have been switched off and that the
charge could be fatal.

Is this really so? Surely that is exaggerating?


Isn't that CRT monitors not PC PCUs?


Both. They're used in the AC-DC transformer.


And what are the voltages and capacitances?

Hmmm. So it's the power cord plugged into the wall that converts the 120V
wall supply down to 3, 5, and 12V? I wonder why they bother with that great
big box then?

Clint

"Kevin Lawton" wrote in message
...
TFM® wrote:
| ThePunisher wrote:
|| Regal wrote:
||| I read on some website that the capacitors in a PC's PSU can hold a
||| charge for long after they have been switched off and that the
||| charge could be fatal.
|||
||| Is this really so? Surely that is exaggerating?
||
|| You sould check the PSU with a stroboscope before opening it.
|
|
| I ain't opening one. I'm a carpenter dammit, not a sparktrician! If
| a PSU fails, I get another one.
|
| I got to this age by learning to *heed* warnings.

Okay, fair enough. Heed the warning on the PSU label and don't open it up.
The wires inside the PC case only carry 3.3v, 5v and 12v - totally safe to
humans.
Kevin.

On Fri, 16 Apr 2004 02:04:00 GMT, kony wrote:

On Fri, 16 Apr 2004 00:11:09 GMT, (Overlord)
wrote:

snip

They also take a finite amount of time to discharge. They can be shorted
in such a way to discharge more quickly, kicking out many times their inputted
voltage in one great electron orgasm.

A rapid discharge will do nothing to raise the voltage, it's the current
you're thinking of.... at most the voltage would be the peak input voltage
(per the input from, position in the circuit, not necessarily the mains AC
voltage).

mmm... RC time constants... yah, current. What I get for taking a break from doing
taxes. Trying to change gears and didn't make it.... Thinking coulombs (Q/T) and
talking volts. But I really don't want to go back to the taxes.....
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VWWall wrote:

Also the main switching supply will continue to operate until the capacitors
are down to ~ 250 V, still more drain.

Not ATX PSUs. The switching circuitry is inhibited as soon as the
PS_ON signal goes high. If the PSU is under light load, the 300v
buss could still be well over 300vdc.

kony wrote:

The +5vsb typically uses a small bias transformer and is unrelated to
the +300vdc buss. To confirm, monitor the +5vsb while you unplug the
AC cord or switch OFF the rear panel switch. The +5vsb goes away instantly
(or as soon as the +5vsb caps discharge.)

It hasn't been unrelated in the units I've traced. They looked pretty
standard... right after rectified there was the voltage doubler, the
large caps everyone is concerned about, with the bleeder resistors across
them, and the power leading to the 5VSB transformer was directly
connected, parallel to the bleeder resistors.

The primary of the +5vsb bias transformer connected to a 300VDC source?
In parallel with the bleed resistors? Poor transformer. Must have HEAVY
GAUGE windings, or a short life span.