Capacitors in PSU are dangerous?

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 :-)

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

"Rob Morley" wrote in message
t...
| 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.

Crap. 10 mins TOP and lethal? Nope - not after switch off (from shock alone)

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?


Not sure what voltages the caps in a PC PSU would reach, but in general
those caps can be dangerous because, once charged, they can deliver very
high currents for short durations - enough to get some spectacular
fireworks, burn things to ashes etc.

"DCA" wrote:
"Tim Auton" tim.auton@uton.[groupSexWithoutTheY] wrote in message
.. .
[snip]
| 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.
|
Bollox !

You feel free to advise people to do whatever you like, but some
people prefer to advise people of undetermined ability to err on the
side of caution.

Do I wait? No. But then I know what I'm doing.


Tim
--
Love is a travelator.

VWWall wrote:

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.

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.

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!

A common way of taking advantage of the extra-high-tension available in
a television set was to make an arc to earth and use it to light
cigarettes. This was in valve days; don't try it with semiconductor EHT
rectifiers with low impedance, as it'll fry them.

Best wishes,
--
Michael Salem

In article ,
Parish 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.


ROFLMAO :-)


You have to leave the 'scope attached for 24 hours to get a valid
reading ;-)
--
Al Dykes
-----------
adykes at p a n i x . c o m

In article , Martin Slaney
writes
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?


Not sure what voltages the caps in a PC PSU would reach, but in general
those caps can be dangerous because, once charged, they can deliver very
high currents for short durations - enough to get some spectacular
fireworks, burn things to ashes etc.

It would seem to me that the safest thing to do would be to don wellies
and hose the system down with water before touching anything.
--
Roger Hunt