On 30 Sep 2006 12:40:39 -0700, q_q_anonymous@yahoo.co.uk
wrote:

>
>q_q_anonymous@yahoo.co.uk wrote:
>> kony wrote:
>> > On Fri, 29 Sep 2006 13:51:52 +1000, "Rod Speed"
>> > <rod.speed.aaa@gmail.com> wrote:
>> >
>> >
>> > >> Even when unplugged, you could still have
>> > >> the charge in the capacitors. (second risk)
>> > >
>> > >Nope, they have bleed resistors across them.
>> > >
>> >
>> >
>> > ... and the PSU contiues to runs the 5VSB and control
>> > circuit after unplugged, which would drain off most of the
>> > power even if the resistors weren't there.
>>
>
>I'll check how long my ATX(so, with 5VSB) takes to discharge. with a
>multimeter on 5VSB.
>
>According to this guy quoted below, 1000 seconds (16.6 minutes). Though
>it seems he didn't take into account the 5VSB+control circuit that you
>mention.

First, understand the effect of a bleeder resistor- it is a
fixed resistance such that the lower the caps are bled, the
lower their voltage is, the slower the remaining voltage is
bled off.

We don't have to care if the caps are drained down to 0.0V,
not even 10V, only that they are drained down to a safe
level. There can be some disagreement about how high a
voltage is safe, but we can be quite conservative and just
pick a very low voltage, perhaps 25V (which right now, I can
touch between two fingers without even the slightest
sensation from it.



>He also suggested that some may not have the restistors. And perhaps
>some may have different rated resistors

These are certianly possible, but rather than drift around
in various details, we can instead focus on the primary
issue, whether ANY ATX PSU retains a dangerous charge after
having been unplugged for a few minutes. So far, nobody has
ever found one AFAIK.

Most are drained to a safe level in under 30 seconds. Some
in half that or less, particularly if it was still hooked up
to the system when unplugged instead of isolated with no
5VSB load.



>
>
>http://forums.silentpcreview.com/vie...&view=previous
>larrymoencurly
>"There are bleeder resistors across the big high voltage filter
>capacitors. If the bleeders are 200K ohms and the filters are 1000 uF,
>then they should be almost completely discharged in 200K * 1000 uF * 5
>= 1000 seconds. "

"Completely discharged" will take an infinitely long time if
only considering these isolated variables because of the
issue presented above, that the further the voltage drops,
the slower the discharge rate. A half-life figure might be
more useful, but we still don't need to be concerned because
we can in fact measure these things, and I have several
times because I repair ATX PSU. I've even posted specific
measurements of one I'd disassembled and hooked up expressly
for the purpose of timing the discharge (not in this thread,
was months or maybe even a couple years ago (time flies!)).


>http://forums.silentpcreview.com/viewtopic.php?t=6123
>larrymoencurly
>"
>A 6800 uF capacitor (about ten times the size of those in ATX PSUs)that
>I charged to 18V several months ago is currently at 13.4V, so I
>wouldn't rely upon leaving the PSU unplugged for several weeks to drain
>off the voltage.

It is true that a cap can hold a charge for a very long
time, but we are not considering a disconnected cap, we are
considering a whole ATX PSU. If one wants to argue that
perhaps these load resistors were to blow out and that would
be a problem, then it would only be reasonable to wonder
about any and all other potential problem in life,
ultimately cocluding that use of electrically powered
products in general are not safe, we shouldn't use ANY of
them.

Show one ATX PSU that retains a dangerous voltage level
after being unplugged for 5 minutes. There has to be a
first one if it's even remotely realistic to consider.