Question Removing thermal paste with PSU plugged in but off ?

andrefry

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May 17, 2017
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I’m about to fit a new cooler and was wondering if I need to make sure my PSU is not connected to the plug socket ?

I hear it’s good practise to have the PSU plugged in but switched off so it remains grounded but is this a safe method when removing / adding new thermal paste ? Should I disconnect all power to the mobo but leave the PSU plugged in or is this unnecessary ?

Cheers.
 

Impu7se

Great
Feb 13, 2019
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Just press the pc power switch on the top panel(power on button) for psu to empty all the energy left in it. Then it will be safe
 

InvalidError

Titan
Moderator
The only reason to unplug the PC is so you don't accidentally turn it on while there is no HSF installed. I wouldn't worry about "emptying" the PSU after turning its switch off (unless you plan to remove the CPU for cleaning) as that won't have enough energy to run the system for more than 100ms at most, nowhere near enough for the CPU to warm up by a meaningful amount.
 
Mar 19, 2019
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Jut thought I'd chime in for educational purposes.

The reason Impu7se suggested pressing the power button in for a second or 2 was not due to possibly heating things up momentarily, but to purge the capacitors of stored energy as they keep their charge for a very long time, and of course they can be dangerous to touch.
I say "can" because I've done this and survived.

I am now however somewhat brain-dead as my partner keeps telling me and it must be true cause women are always right.
 
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The reason Impu7se suggested pressing the power button in for a second or 2 was not due to possibly heating things up momentarily, but to purge the capacitors of stored energy as they keep their charge for a very long time, and of course they can be dangerous to touch.
I say "can" because I've done this and survived.
Out of curiosity, how long was the "very long time" the PSU was unplugged before you touched it? Did you actually get a shock that you could hear/see/feel?
 
Mar 19, 2019
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Out of curiosity, how long was the "very long time" the PSU was unplugged before you touched it? Did you actually get a shock that you could hear/see/feel?
I should've clarified. It wasn't a PC at all. it was a Stereo Amp that my dad had. It wasn't working and I thought I would try fix it. Of course I didn't listen to his teachings being an electrician that i should watch out for where I'm poking. The results would be the same PC or not. The Caps on the Stereo may not have been fully charged and therefore giving me a less than full on shock.Sound.... Yup. Could I see it... a little. Painful.... Yup.
 
If the PSU has a power switch on it, yes turning it off (at the PSU) will work.

If the PSU doesn't have a power switch on it, then you want to unplug it. The PSU still sends power to the motherboard even when the computer is off. This is how things like always-on USB power ports stay powered even when the computer is off. How the GPU sends a signal to the monitor telling it to stay asleep, instead of the monitor displaying "no cable detected."

Given how expensive components like the CPU and RAM are, I just unplug every computer regardless of whether or not there's a switch on the PSU. It's just not worth the risk of frying something because you thought the computer was unpowered when it wasn't.

Grounding is not there to protect the computer from static. It's there to protect you from electrocution. Without grounding, if a short in the PSU electrifies something inside the computer with 110V AC power and you touch it, the electricity goes through your body to the ground, killing you. With grounding, the short goes through the ground wire, and blows the fuse or trips the circuit breaker, hopefully before you have a chance to touch anything that's electrified.

The metal chassis of the case is enough to absorb any static charge your body holds (edit: ok, maybe not if you've been playing around with a Van de Graaff generator). So it's good enough to just make sure you touch the metal case (or metal table leg, or some other large metal object) before messing around inside the computer.
 
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InvalidError

Titan
Moderator
The reason Impu7se suggested pressing the power button in for a second or 2 was not due to possibly heating things up momentarily, but to purge the capacitors of stored energy as they keep their charge for a very long time, and of course they can be dangerous to touch.
The only capacitors which may hold a meaningful charge while the PC is "off" are those on the high-voltage side inside the PSU which aren't connected to anything on the output side. All of the PSU's outputs are 12V or less and are safe to lick regardless of how large the capacitors may be as anything under 30V is too low to pass enough current to be harmful short of sticking needles into your body to bypass damp skin resistance.
 
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Mar 19, 2019
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The only capacitors which may hold a meaningful charge while the PC is "off" are those on the high-voltage side inside the PSU which aren't connected to anything on the output side. All of the PSU's outputs are 12V or less and are safe to lick regardless of how large the capacitors may be as anything under 30V is too low to pass enough current to be harmful short of sticking needles into your body to bypass damp skin resistance.
I think the takeaway from this is not to touch anything you don't need to unless you are 100% sure there's no power flow, and like it was said, the only way to be sure is to remove the lead completely.
 

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