Capacitors in PSU are dangerous?

Overlord wrote:

Under the right? circumstances yes.
Your UK voltage is like 230volts? That's the rms value, like an average
effective value. The peak value would be 325 volts then.
Also, the capacitors take a finite amount of time to charge up in a circuit.
They also take a finite amount of time to discharge. They can be shorted
in such a way to discharge more quickly, kicking out many times their inputted
voltage in one great electron orgasm.

Many times their charging *current*? Yes.
Many times their charging *voltage*? No.

You have a camera?
You push the button half way and see the little blinking light; it's charging a capacitor.
The light goes steady; the capacitor is charged.
You push the button the rest of the way in and it discharges in a smaller time constant
to flash the flash bulb. Those itty bitty batteries in the camera can't flash it on their output
voltage on their own. The discharge is many times the voltage that the battery slowly put
into the capacitor. If those little 1.5volt (or whatever) batteries can charge that capacitor
to discharge and flash that flashbulb, think what a nice flash you can get from discharging
the capacitor that you charged with the 230volt line current, properly or improperly
handled....

Nice story, but technically flawed. A capacitor will only charge up to
the peak voltage applied across it. I think you will find that cameras
use voltage multipliers to achieve their flash voltage, like a small
fly back circuit or something. [I've never taken one apart, so I don't
know exactly.]

~misfit~ wrote:

kony wrote:

On Thu, 15 Apr 2004 14:12:48 -0700, ric wrote:


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.)

It hasn't been unrelated in the units I've traced. They looked
pretty standard... right after rectified there was the voltage
doubler, the large caps everyone is concerned about, with the
bleeder resistors across them, and the power leading to the 5VSB
transformer was directly connected, parallel to the bleeder
resistors. Perhaps I haven't looked closely enough, at enough
different PS 5VSB circuits, but I've not seen anything to suggest any
unit deviated from this.


So, these bleeder resistors, they're constantly beelding off current, even
when the machine is in use yes? Sounds wasteful of power to me.

Typical values are 220,000 ohms across 470 mfd capacitor charged to
about 170 V. This is about 1/8 watt lost in the bleeder resistor.
Incidently this is a time constant, (RC), of about 100 seconds. The
voltage will drop to 17 V in 2RC or 3 minutes 20 seconds, assuming
no other leakage paths.

Virg Wall
--

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

ric wrote:

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.)

It hasn't been unrelated in the units I've traced. They looked pretty
standard... right after rectified there was the voltage doubler, the
large caps everyone is concerned about, with the bleeder resistors across
them, and the power leading to the 5VSB transformer was directly
connected, parallel to the bleeder resistors.


The primary of the +5vsb bias transformer connected to a 300VDC source?
In parallel with the bleed resistors? Poor transformer. Must have HEAVY
GAUGE windings, or a short life span.

Note kony said "power leading to" transformer. I agree.

See:

http://www.pavouk.comp.cz/hw/en_atxps.html

for an example.

Virg Wall
--

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

VWWall wrote:

It hasn't been unrelated in the units I've traced. They looked pretty
standard... right after rectified there was the voltage doubler, the
large caps everyone is concerned about, with the bleeder resistors across
them, and the power leading to the 5VSB transformer was directly
connected, parallel to the bleeder resistors.


The primary of the +5vsb bias transformer connected to a 300VDC source?
In parallel with the bleed resistors? Poor transformer. Must have HEAVY
GAUGE windings, or a short life span.

Note kony said "power leading to" transformer. I agree.

The circuit at your given URL uses a switching circuit for its +5vsb
output.
The PSUs that I have worked with used a linear circuit for the +5vsb, hence
my use of the term "bias transformer." For the 5-10w of +5vsb, I assumed
most *were* linear. Evidently not.

TFM® wrote:

Niel Humphreys wrote:

"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?

Isn't that CRT monitors not PC PCUs?



It's a capacitor, dude. It'll hold a charge.

I've heard of folks working on electric motors casually tossing a charged
capacitor to an innocent victim. Knocks the **** out of 'em.

TFM®


I had that done to me, charged up on a spark plug testing machine, wakes
you up :-)


--
But why is the Rum gone?

Regal schreef:

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, capacitors can hold a charge for a long time. But in most casses
this is a result of a defect in the PSU, in most casses the
bleederresistor, like mensioned before. When you want to open a computer
PSU, one that works correct, do the following

1 Turn off the computer
2 Disconnect te powerplug
3 Turn on the computer with the powerplug disconnected

Stap 3 will unload any loaded cap. The same goes with other apparatus
that works normaly. If an apparatus is defective than the cap can stil
be charged. I've seen, and experionced, cases where the cap was loaded
for over a month and still zapped the hell out of me. If it is fatal, i
dont know but it hurts like hell, it can burn your fingers, and isn't
very pleasant.

If you ever feel the urge to open an electrical apparatus, think about
what your doing, think again, and pray to your god that the
bleederresistor isn't defective.

Succes
Chris

On Fri, 16 Apr 2004 14:28:43 +1200, "~misfit~"
put finger to keyboard and composed:

kony wrote:
On Thu, 15 Apr 2004 14:12:48 -0700, ric wrote:

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.)

It hasn't been unrelated in the units I've traced. They looked
pretty standard... right after rectified there was the voltage
doubler, the large caps everyone is concerned about, with the
bleeder resistors across them, and the power leading to the 5VSB
transformer was directly connected, parallel to the bleeder
resistors. Perhaps I haven't looked closely enough, at enough
different PS 5VSB circuits, but I've not seen anything to suggest any
unit deviated from this.

So, these bleeder resistors, they're constantly beelding off current, even
when the machine is in use yes? Sounds wasteful of power to me.

No, these "bleeder" resistors are of the order of 220K and the caps
are of the order of 470uF. The time constant (RxC) is approx 100 secs,
and the dissipated power is about 300mW. I believe the resistors
perform some kind of "equalisation" function in dual voltage supplies
(110V/240V) that have two filter caps in series. Single cap supplies,
OTOH, have no such resistor.


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

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 ?
The mains lead carries 120v or 240v depending on where in the world you are.
The SEALED (with screws) PSU converts this down to 3.3v, 5v and 12v.
The high voltages are all INSIDE the PSU and are inaccessible unless you
ignore the warning on the label and open it up (or push a long metal thing
through the cooling vents).
The loose wires inside the PC case, which you can get at, only carry
harmless low voltages.
Kevin.

| "Kevin Lawton" wrote in message
| ...
|| TFM® 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.
|||
|||
||| I ain't opening one. I'm a carpenter dammit, not a sparktrician!
||| If
||| a PSU fails, I get another one.
|||
||| I got to this age by learning to *heed* warnings.
||
|| Okay, fair enough. Heed the warning on the PSU label and don't open
|| it up. The wires inside the PC case only carry 3.3v, 5v and 12v -
|| totally safe to humans.
|| Kevin.

On Thu, 15 Apr 2004 16:48:08 +0100, Regal
put finger to keyboard and
composed:

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?

There are 200V or 400V caps on the primary side of the SMPS. Any
competent tech will discharge these caps before servicing the unit. I
have a load consisting of a 100W 240V lamp which I use for this
purpose.


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

TFM® 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.



I ain't opening one. I'm a carpenter dammit, not a sparktrician! If a PSU
fails, I get another one.

I got to this age by learning to *heed* warnings.

TFM®


I open them all the time, to check fuses and remove/replace fans etc...

--
-Luke-
If cars had advanced at the same rate as Micr0$oft technology, they'd be
flying by now.
But who wants a car that crashes 8 times a day?
Registered Linux User #345134