PSU Charge Dissipation

G

Gary D.

Guest
Excuse me if this is the wrong group…

I have an old home computer PSU and I want to remove the electronics
temporarily while I cut extra metal off the rear circular exhaust port
as it interferes with exhaust air flow. Obviously I would prefer not
to come to any harm!

Can somebody please tell me how many day/weeks it would take for any
stored electrical charge in a computer's power supply unit (PSU), i.e.
in the capacitors, to dissipate enough for it to be okay to touch the
circuit board and yet be safe from any potential electric shock?
 
"Gary D." <ignore@notvalid.net> wrote in message
news:89dcd19iskhu38hrb52nesl19bgoqulmcq@4ax.com...
Excuse me if this is the wrong group.

I have an old home computer PSU and I want to remove the electronics
temporarily while I cut extra metal off the rear circular exhaust port
as it interferes with exhaust air flow. Obviously I would prefer not
to come to any harm!

Can somebody please tell me how many day/weeks it would take for any
stored electrical charge in a computer's power supply unit (PSU), i.e.
in the capacitors, to dissipate enough for it to be okay to touch the
circuit board and yet be safe from any potential electric shock?
Click to expand...
The PSes I've seen have a bleeder resistor, usually about 330k, across
the HV capacitors to remove any charge when the power is removed. This
takes only a few tens of seconds. The PS can work with an open bleeder
resistor, so as a precaution you can measure across the caps with a DMM,
and you should get zero volts. Of course take all the necessary
precautions such as make sure the PS is unplugged, etc.
 
Gary D. wrote:
Excuse me if this is the wrong group.

I have an old home computer PSU and I want to remove the electronics
temporarily while I cut extra metal off the rear circular exhaust port
as it interferes with exhaust air flow. Obviously I would prefer not
to come to any harm!

Can somebody please tell me how many day/weeks it would take for any
stored electrical charge in a computer's power supply unit (PSU), i.e.
in the capacitors, to dissipate enough for it to be okay to touch the
circuit board and yet be safe from any potential electric shock?
Click to expand...
I've done this to several PSUs. The easy way is a strong pair of side
cutters
and snip trough the last set of legs which support the grill. No need to
get
inside the PSU nor remove the PSU from the chassis. Now since this will
leave a big hole with fan blades close by, you ought to get a wire fan
guard to cover the hole. Crank the guard's legs to provide a 1/4"
stand-off
to reduce noise susceptibility and fit them to the fan's mounting screws.

A picture is worth a thousand words - have a look at the 'Recent Build'
article on my website below. Also a bit in there about flatting heatsinks,
too.

To answer the original Q leave it overnight with the mains supply removed,
NOT simply shutdown.

--
Graham W http://www.gcw.org.uk/ PGM-FI page updated, Graphics Tutorial
WIMBORNE http://www.wessex-astro-society.freeserve.co.uk/ Wessex
Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites & Maps
Change 'news' to 'sewn' in my Reply address to avoid my spam filter.
 
On Thu, 14 Jul 2005 13:50:30 +0100, "Graham W"
<graham@his.com.puter.INVALID> wrote:

The easy way is a strong pair of side
cutters
and snip trough the last set of legs which support the grill. No need to
get
inside the PSU nor remove the PSU from the chassis.
Click to expand...
I should have mentioned that I have already done just that.

What I want to do now (which will definitely be awkward/risky with the
contents still in place) is to increase the diameter of the exhaust
aperture because it isn't quite wide enough. Consequently, airflow
hits the aperature edges causing turbulence and hence increased noise.
You'd think that nowadays manufacturers would check something so
obvious, but then it is 6 years old!

As you can see, to achieve the above I really need to empty the box of
the circuit board and components.
 
"Gary D." <ignore@notvalid.net> wrote in message
news:hjcdd193hlqa1vshcmgpqb0fmdskku54rk@4ax.com...
On Thu, 14 Jul 2005 13:50:30 +0100, "Graham W"
graham@his.com.puter.INVALID> wrote:

The easy way is a strong pair of side
cutters
and snip trough the last set of legs which support the grill. No need
to get inside the PSU nor remove the PSU from the chassis.

I should have mentioned that I have already done just that.

What I want to do now (which will definitely be awkward/risky with the
contents still in place) is to increase the diameter of the exhaust
aperture because it isn't quite wide enough. Consequently, airflow
hits the aperature edges causing turbulence and hence increased noise.
You'd think that nowadays manufacturers would check something so
obvious, but then it is 6 years old!

As you can see, to achieve the above I really need to empty the box of
the circuit board and components.
Click to expand...
It's wise to remove the electronics from anything before you do any work
that might make metal shavings. They tend to short out thing when they
fall onto the PC board.
 
Watson A.Name - "Watt Sun, the Dark Remover" wrote:
"Gary D." <ignore@notvalid.net> wrote in message
news:hjcdd193hlqa1vshcmgpqb0fmdskku54rk@4ax.com...
On Thu, 14 Jul 2005 13:50:30 +0100, "Graham W"
graham@his.com.puter.INVALID> wrote:

The easy way is a strong pair of side
cutters
and snip trough the last set of legs which support the grill. No
need to get inside the PSU nor remove the PSU from the chassis.

I should have mentioned that I have already done just that.

What I want to do now (which will definitely be awkward/risky with
the contents still in place) is to increase the diameter of the
exhaust aperture because it isn't quite wide enough. Consequently,
airflow hits the aperature edges causing turbulence and hence
increased noise. You'd think that nowadays manufacturers would check
something so obvious, but then it is 6 years old!

As you can see, to achieve the above I really need to empty the box
of the circuit board and components.

It's wise to remove the electronics from anything before you do any
work that might make metal shavings. They tend to short out thing
when they fall onto the PC board.
Click to expand...
Agreed!. If you need to file edges then it is advisable to remove ALL
components from the metalwork to avoid tiny shards getting into things
like fuse holders, switches etc.. It is possible to shroud the other
electronics stuff if you can attach, temporarily, a fine woven cloth to
prevent the filings migrating to places which could cause trouble
but it would restrict your ability to get enough stroke on a hand file.
Nonetheless, that technique has worked for me.

If you leave the PSU unplugged overnight there won't be any serious
charge in the capacitors and as a safety measure/confidence
booster, you could dicharge any remanent charge with a jump-lead
and a 100kohm resistor connected across the HV caps for a minute
or two after an overnight resting period unpowered.

As an incidental comment, I reduced the fan turbulance on my
GF4-Ti4200 by simply removing the fancy anodised plate which
covered the fin set. The hole in the middle was the offender here
and further checks on temperature rise showed that it was really
only there for decoration and branding!

HTH
--
Graham W http://www.gcw.org.uk/ PGM-FI page updated, Graphics Tutorial
WIMBORNE http://www.wessex-astro-society.freeserve.co.uk/ Wessex
Dorset UK Astro Society's Web pages, Info, Meeting Dates, Sites & Maps
Change 'news' to 'sewn' in my Reply address to avoid my spam filter.
 
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