On 9/1/2022 5:05 PM, Skybuck Flying wrote:
> Say PC user want to clean power supply internally, decides to do it the next morning, so the PC not used for a night. (Power supply is not turned off, and stays plugged into wall socket which is powered).
>
> Would the PC power supply still contain a high/dangerous charge or would it be low charge/power/risk ? :)
>
> Bye,
> Skybuck.
>

Standard "technician practice" is to assume the bleeder resistors
failed open circuit, and the unit is still charged.

There are a *lot* of joules of energy stored in the main cap.
If that arcs over, the noise will be so loud, it'll knock out
your hearing for ten minutes. NEVER stick a screwdriver across
the main cap in an ATX supply.

The +5VSB board, sucks on the main capacitor, and after
around 30 seconds, most of the charge is gone. On an Asus
motherboard, the green LED (+5VSB powered) goes out at the
60 second mark. But the +5VSB SMPS will shut down before
the mains cap is completely discharged.

*******

This is an old old diagram, but the concepts involved are still valid.

http://www.pavouk.org/hw/en_atxps.html

Resistor R2 and R3 in the top row, are the bleeder resistors.
They're the ones that drain the main cap C5 and C6 to zero. If the bleeder
resistors fail (burn out, go open circuit, may be no visible damage),
then the main capacitor remains dangerous.

DO NOT stick a screwdriver across capacitor C5 or C6. If you do,
the arc effect is strong enough, to take a chip out of the
screwdriver blade, as well as wreck your hearing from the noise.

So really, everything dangerous tends to be in the upper left of
the diagram, and that's where you should focus your attention.

When I changed out the fan in an ATX PSU, you would not believe
how timid and respectful I was, while working in there. I swore an
oath that high voltage was never going to get me again, after
one day as a kid, I was thrown across the basement floor by HV and
passed out :-/ But it was a lesson WELL LEARNED. I lived up
to my oath. And I have worked on HV since my little lesson about
HV, so it's not like I don't take chances. I've worked up to 15kV
and still have my laser power supply from university (I built a
laser in Experimental Physics and did a demo for some first-years
-- it was the crappiest laser you can build!).

Paul