Capacitors in PSU are dangerous?

ric wrote in message ...
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.)

I checked some +5Vstandby circuits because I wanted to know why the
one in my Raidmax/Powmax had a much smaller transformer than the
others, and I found that they didn't use separate capacitors but were
powered from the same big ones that ran the main PSU. Most even ran
the +5Vstandby transformer at exactly the same frequency, although
Enermax used 2x and Raidmax/Powmax used 4x. The one exception may have
been a super-cheap 250W (PC Power & Cooling shows it as an example of
a bad PSU) where the +5Vstandby circuit was held on a completely
separate circuit board, but I don't really remember.

"Timothy Daniels" wrote in message ...

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

I have a 350W Enermax that was included in an Enermax CS-1251-B case,
and I couldn't find any bleed resistors across those capacitors. I
didn't even see empty spaces where they could have been installed, but
every other PSU I've checked, even some really cheap ones, had those
bleeders. Are bleeders also necessary to help keep the voltages
across the two big capacitors equal, as the capacitors are rarely
rated for even 20% tolerance?

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

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.

I'm assuming the worst case - ie component failure,
where the only discharge of the caps is self-discharge.
In that case though, 24 hours may not be enough. Hmmm,
I think we need some experimental data.

In February 2003, I charged up a 6,800uF capacitor to 18V. Fourteen
months later, it still measures 12.44V. So I don't think that 24
hours would be enough for an ATX PSU's 1,000uF capacitors to
self-discharge completely in the worst case.

larrymoencurly wrote:

.... snip ...

In February 2003, I charged up a 6,800uF capacitor to 18V.
Fourteen months later, it still measures 12.44V. So I don't
think that 24 hours would be enough for an ATX PSU's 1,000uF
capacitors to self-discharge completely in the worst case.

That sounds like an outstanding quality cap. I suspect something
else is going on, such as no longer being polarized and thus
having much smaller capacity. If you measure the delta E at some
known current drain you could find out.

--
A: Because it fouls the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing on usenet and in e-mail?

Parish wrote:
| Kevin Lawton wrote:
|
|| Clint wrote:
||| 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?
||
|| Eh ? What ? Where did you get THAT from ?
|
| I think he misread "The wires inside the PC case only carry 3.3v, 5v
| and 12v..." in your previous message as "...PSU case..."

Yes, I think he probably did.
Kevin.

"Kevin Lawton" wrote in
:

... The wires inside the PC case only carry 3.3v, 5v and
12v

pedantry
That applies to ATX-style models. AT-style models have a mains power
lead inside the case, for the front panel power switch.
/pedantry

On Thu, 15 Apr 2004 17:44:44 +0100, Tim Auton
tim.auton@uton.[groupSexWithoutTheY] 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.

Eh?


Tim

I think he means a Stereoscope
..... hmm or maybe a stethoscope......
......... Oh a sillyscope....

On Thu, 15 Apr 2004 16:17:25 +0000, 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.

Further you can generally get them instantly discharged if you are going
to work on them by yanking the power cord, while they are on. They keep
trying to work, and the charge in the capacitor(s)on the line side of the
PSU rapidly discharge.

JAB.

--
Jonathan A. Buzzard Email: jonathan (at) buzzard.me.uk
Northumberland, United Kingdom. Tel: +44 1661-832195

Frank le Spikkin wrote:
| "Kevin Lawton" wrote in
| :
|
|| ... The wires inside the PC case only carry 3.3v, 5v and
|| 12v
|
| pedantry
| That applies to ATX-style models. AT-style models have a mains power
| lead inside the case, for the front panel power switch.
| /pedantry

Well that's true - - - for many of them.
But then the original IBM PC / AT / XT family had the power switch
protruding out of the side of the PSU and through a hole in the side of the
PC case.
Or the PS/1 which was powered from a PSU in the monitor case.
Or the later PS/2 which had the mains switch integrated in the PSU and
operated by a long push-rod from the front panel.
Not to mention the many Compaq models which had their own 'solution'.
I could go on and on and on . . . . but I wouldn't like to do that :-)
Kevin.

CBFalconer wrote in message ...
larrymoencurly wrote:

In February 2003, I charged up a 6,800uF capacitor to 18V.
Fourteen months later, it still measures 12.44V. So I don't
think that 24 hours would be enough for an ATX PSU's 1,000uF
capacitors to self-discharge completely in the worst case.

That sounds like an outstanding quality cap. I suspect something
else is going on, such as no longer being polarized and thus
having much smaller capacity. If you measure the delta E at some
known current drain you could find out.

It's a blue Sprague, marked 81D, 8939L 3301, 105 Celcius, ripped out
from a trashed circuit board (probably for a burglar or fire alarm).