A computer components & hardware forum. HardwareBanter

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » HardwareBanter forum » General Hardware & Peripherals » General
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

Capacitors in PSU are dangerous?



 
 
Thread Tools Display Modes
  #21  
Old April 15th 04, 07:36 PM
Kez
external usenet poster
 
Posts: n/a
Default

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


  #22  
Old April 15th 04, 09:12 PM
kony
external usenet poster
 
Posts: n/a
Default

On Thu, 15 Apr 2004 17:30:32 +0100, Piotr Makley 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.
  #23  
Old April 15th 04, 09:21 PM
kony
external usenet poster
 
Posts: n/a
Default

On Thu, 15 Apr 2004 18:43:06 +0100, Michael Salem
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.
  #24  
Old April 15th 04, 09:24 PM
kony
external usenet poster
 
Posts: n/a
Default

On Thu, 15 Apr 2004 18:38:30 +0100, Tim Auton
tim.auton@uton.[groupSexWithoutTheY] wrote:


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.


It's beyond overkill to advise waiting 24 hours. Even if you didn't know
what you were doing you should've known that there's another very obvious
way an ATX power supply drains besides the bleeder resistors.
  #25  
Old April 15th 04, 09:31 PM
VWWall
external usenet poster
 
Posts: n/a
Default

Piotr Makley wrote:

VWWall 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?


The main enemy of any electronics is heat. 30W additional output
means ~10 W additional heat produced in the PSU. Some marginal
units can't support even their label wattage. It all depends on
how close to the limit you're pushing the supply. Also the PSU
fan takes its input air from within the case. This air will be hotter
due to the 30 additional CPU watts, and without good case ventilation
will raise the internal temperature of the PS still more.

Virg Wall
--

It is vain to do with more
what can be done with fewer.
William of Occam.
  #26  
Old April 15th 04, 09:53 PM
VWWall
external usenet poster
 
Posts: n/a
Default

Kevin Lawton wrote:

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.


Close, but no cigar! There is no "pumping" of the input capacitors. They
are either charged to the peak line voltage from a 240 V line input, or they
are charged with a voltage doubler rectifier circuit from a 120 V line input.
The approximately 340 V DC derived is then switched at a rate of about 50,000
cycles and then transformed down to the desired output voltages. The time
the "switch" is on determines the output voltages, and PWM (pulse width
modulation) is used to regulate these voltages. The main reduction in
size is due to the small core needed for the output transformer. In addition,
the input transformer is eliminated completely.

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 ?


The most frequent reason I have opened a PSU is to replace the fan. Even
with a failed bleeder there is enough leakage current to discharge the
capacitors in less than the "many hours" often quoted. If you're really
worried, let the supply remain un-plugged overnight. Then be very careful
not to cut yourself on the sharp metal edges in most PSUs!

Virg Wall
--

It is vain to do with more
what can be done with fewer.
William of Occam.
  #27  
Old April 15th 04, 10:11 PM
VWWall
external usenet poster
 
Posts: n/a
Default

kony wrote:

On Thu, 15 Apr 2004 18:43:06 +0100, Michael Salem
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.


Also the main switching supply will continue to operate until the capacitors
are down to ~ 250 V, still more drain. With the excption of those PSUs
that have heat sinks at an input voltage potential, it's very hard to
get even your finger tips on the full voltage. A little care is still
advised.

Virg Wall
--

It is vain to do with more
what can be done with fewer.
William of Occam.
  #28  
Old April 15th 04, 10:12 PM
ric
external usenet poster
 
Posts: n/a
Default

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.)
  #29  
Old April 15th 04, 10:24 PM
ric
external usenet poster
 
Posts: n/a
Default

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.
  #30  
Old April 15th 04, 10:50 PM
Franc Zabkar
external usenet poster
 
Posts: n/a
Default

On Thu, 15 Apr 2004 18:43:06 +0100, Michael Salem
put finger to keyboard and composed:

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.


Yes, that's a very common failure mode. These "startup" resistors are
typically of the order of 100K or more. They feed 350V to the base or
gate of the main chopper transistor. If this resistor goes open
circuit, then the PSU doesn't start oscillating, and the main filter
cap doesn't discharge.


- Franc Zabkar
--
Please remove one 's' from my address when replying by email.
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Dangerous upgrades, be aware !!! sigmun General 1 February 5th 04 09:29 PM
Leaking capacitors DFC General 15 January 22nd 04 08:53 PM
So few capacitors on motherboard? larrymoencurly General 32 January 8th 04 10:05 PM
FS: Lots of 500: 50V 22uF Electrolytic Capacitors :: $ 27 SHIPPED Jerry Rakar General 0 December 31st 03 11:45 PM


All times are GMT +1. The time now is 08:45 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 HardwareBanter.
The comments are property of their posters.