Capacitors in PSU are dangerous?

"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.

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.

--
ThePunisher

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?

The capacitors in the input circuit of a PC PSU are charged to
~320 V DC. This can, indeed, be lethal, but they are shunted by
bleeder resistors which will reduce the voltage to a safe value
in a minute or two. By the time you get the unit out of the computer
case and remove it's cover, the voltage is probably safe. To be sure,
wait five minutes after unplugging the PSU before touching anything inside.

A PC monitor uses high voltage, (up to 25,000 V), on the CRT. The tube
glass envelope is used as a capacitor, and can hold a charge for some
time. Because the energy content is quite low, contact with this
very high voltage is usually not deadly, but may result in serious
injury from muscle reaction. Don't remove the housing from a CRT monitor
unless you understand how to safely discharge this voltage!

Virg Wall
--

It is vain to do with more
what can be done with fewer.
William of Occam.

"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?

*TimDaniels*

In article , "Regal"
says...
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?

Yes it is so, no it's not exaggerating. Having said that, I'm still
here so you probably won't kill yourself if you're reasonably careful
and vaguely clueful.

"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.

Eh?


Tim
--
Love is a travelator.

"Tim Auton" tim.auton@uton.[groupSexWithoutTheY] wrote in message
...
"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.

Eh?


Tim
--
Love is a travelator.

The stroboscope is effective only as long as the PSU is spinning or
vibrating rapidly. Or if you are hallucinating that it is...
--
John McGaw
[Knoxville, TN, USA]
http://johnmcgaw.com

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?

You do sometimes see that warning on the PSU !
A switched-mode power supply works by 'pumping up' capacitors to a high
voltage with sudden bursts of energy and then regulating the output down to
the required level, switching the current off and on as needed. Draw more
current and it uses bigger bursts of energy in each 'pump'. That's why it is
so efficient in size terms. If you were using a linear power supply to
supply the currents used inside a modern PC it would be far bigger and
dissipate a lot more heat.
Under normal circumstances, the bleed resistors should do their job - but
you wouldn't open up the PSU under normal circumstances would you ? Under
fault conditions - what if a bleed resistor has failed ?
Kevin.

"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?

The maximum voltage inside a PSU is the mains of course!

They should have bleeder resistors, but that doesn't mean they all do.
When giving advice to people of undetermined ability and experience on
usenet I think it's safest to assume the PSU is badly designed and
will potentially hold dangerous voltages for some time after it's
unplugged.

If someone is sufficiently lacking in knowledge to have to ask the
question they should assume that *every* part of the inside of their
PSU is lethal for 24 hours after it's unplugged.


Tim
--
Love is a travelator.

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.


ROFLMAO :-)