Capacitors in PSU are dangerous?

Regal

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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?
 
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"Regal" <stua_NOTTHISBITsmith@yahoo.co.uk> wrote in message
news:Xns94CCAAEC4846D628D1@208.42.66.156...
> 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.
 
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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.
 
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Timothy Daniels <TDaniels@NoSpamDot.com> 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.
 
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Some switched mode power supplies start by rectifying the mains voltage to
350+volts dc and storing this on a capacitor.That is enough to kill you in
certain circumstances.All capacitors from main powered equipment should be
treated with respect.
Ken Reynolds
 
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"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*
 
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Timothy Daniels 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?

That's a maximum *external* voltage. The OP asked about *internal*
voltages.
The 300vdc buss charges to the peak value of the AC input sine wave (times
2 in the U.S.) or 132 * 1.414 * 2 = 373vdc max (typically 330-340 volts.)
That's enough to give a nasty jolt. The amount of this charge left over
time depends on, of course, the size of the capacitors and the bleed
resistors (if any) in parallel with them.
 
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"Regal" <stua_NOTTHISBITsmith@yahoo.co.uk> wrote in message
news:Xns94CCAAEC4846D628D1@208.42.66.156...
> 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?
--

Niel Humphreys
 
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Niel Humphreys wrote:
> "Regal" <stua_NOTTHISBITsmith@yahoo.co.uk> wrote in message
> news:Xns94CCAAEC4846D628D1@208.42.66.156...
>
>>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.

--
Paul
 

ThePunisher

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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
 
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In article <c5mfs7$3etsj$1@ID-206197.news.uni-berlin.de>,
Parish <me@privacy.net> 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
 
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In article <Xns94CCAAEC4846D628D1@208.42.66.156>, "Regal"
stua_NOTTHISBITsmith@yahoo.co.uk 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.
 
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VWWall <vwall@DEADearthlink.net> 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?
>
> 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!


Thinkingof power supplies ...

if a faster processor needing extra power was installed (say, it
needs an extra 30 W) then would that noticeably reduce the life of
the power supply?
 
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"ThePunisher" <thepunisher@ntlworld.com> 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.
 
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"Tim Auton" <tim.auton@uton.[groupSexWithoutTheY]> wrote in message
news:itet705nu64c644sobf9pa1antd4usia90@4ax.com...
> "ThePunisher" <thepunisher@ntlworld.com> 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
 
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"Timothy Daniels" <TDaniels@NoSpamDot.com> 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.
 
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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 :)
 
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"Tim Auton" <tim.auton@uton.[groupSexWithoutTheY]> wrote in message
news:11ft705ehj55au54lu1gd0ob90frda1af3@4ax.com...
| "Timothy Daniels" <TDaniels@NoSpamDot.com> 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 !
 
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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.
 
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"DCA" <MAPSdca860@MAPSntlworld.com> wrote:
>"Tim Auton" <tim.auton@uton.[groupSexWithoutTheY]> wrote in message
>news:11ft705ehj55au54lu1gd0ob90frda1af3@4ax.com...
[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.
 
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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
 
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In article <407EC5E0.7020407@dsl.pipex.com>, Martin Slaney
<slazNIET_SPAM@dsl.pipex.com> 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
 
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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.

tee-hee
 
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On Thu, 15 Apr 2004 17:30:32 +0100, Piotr Makley <pmakley@mail.com> wrote:


>Thinkingof power supplies ...
>
>if a faster processor needing extra power was installed (say, it
>needs an extra 30 W) then would that noticeably reduce the life of
>the power supply?

Generally no, it would be a progressively shorter lifespan but even so
there are other factors that could decrease lifespan or cause failure
before that became significant.

On the other hand, if the power supply can't maintain the additional load
it may cause a lot of additional ripple which is harder on the
motherboard.

Rather than looking at the incremental wattage increase a determination
should be made of the total system power usage... not necessarily an exact
figure but in the ballpark, then using a quality name-brand unit so at
least there's some assurance it's capable of wattage stamped on it's
label.
 
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On Thu, 15 Apr 2004 18:43:06 +0100, Michael Salem <a$-b$1@ms3.org.uk>
wrote:

>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.

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