Discharging a PSU

[MetaDFF]

As we know, a PSU even when unplugged can carry sufficient charge in the caps to seriously hurt somebody if they accidentally touch the terminals.

Is there a safe way to discharge the PSU before working on the insides? I guess it's possible to leave it unplugged for several days to let the stored charge dissipate, but there is no guarantee that the charge is gone (although the longer you leave it unplugged the less likely there will be any residual charge).

Based on Wikihow they suggest attaching a 10 Ohm power resistor between the black and red wires to discharge the caps. This method seems to make sense since you are adding a load across the caps to dissipate the stored energy. Has anybody tried this method of discharging their PSU before?

 

[Rubycon]

A few days? A PSU will be discharged within minutes. If you turn off the system and leave the ATX power plugged into the motherboard most of the residual power will be gone in a few seconds of power off.

Large electrolytics are fairly leaky on their own accord. When they are in a circuit the circuit resistance will pull them down rather quickly. By the time you disconnect the wires and get the PSU out of the case, remove the screws holding the cover, etc. it's a good bet that you're safe.

 

[tynopik]

Originally posted by: Rubycon
A few days? A PSU will be discharged within minutes.

mmm, I wouldn't count on that. I know microwave capacitors definitely will hold a charge for days.

 

[Rubycon]

Originally posted by: tynopik

Originally posted by: Rubycon
A few days? A PSU will be discharged within minutes.

mmm, I wouldn't count on that. I know microwave capacitors definitely will hold a charge for days.

Yes but he is asking about a PC Switch Mode Power Supply not a microwave oven. MOT's are AC devices - completely different cookie here.

 

[tynopik]

well i suppose i should guess from your username that you know something about capacitors

 

[MetaDFF]

Originally posted by: Rubycon
A few days? A PSU will be discharged within minutes. If you turn off the system and leave the ATX power plugged into the motherboard most of the residual power will be gone in a few seconds of power off.

Large electrolytics are fairly leaky on their own accord. When they are in a circuit the circuit resistance will pull them down rather quickly. By the time you disconnect the wires and get the PSU out of the case, remove the screws holding the cover, etc. it's a good bet that you're safe.

I do notice that once I flip the power supply switch off (with the cord still plugged into the wall) the LED on the motherboard does go off after a few seconds, which seems to indicate that the PSU is discharged.

If the PSU does indeed discharge so quickly, then what is the deal with people saying that opening their PSU is dangerous after it is unplugged?

Sorry for being skeptical

 

[Rubycon]

Originally posted by: MetaDFF
I do notice that once I flip the power supply switch off (with the cord still plugged into the wall) the LED on the motherboard does go off after a few seconds, which seems to indicate that the PSU is discharged.

If the PSU does indeed discharge so quickly, then what is the deal with people saying that opening their PSU is dangerous after it is unplugged?

Sorry for being skeptical

It's something that goes back a long ways. Ask any TV repairman. The CRT can hold a charge for *months*. And a good one that will jolt you hard. Most people working on those (CRT's) that got shocked did not get injured from the shock itself but secondary injuries from pulling their hands back so hard that something got in the way and ripped skin, etc.

 

[Modelworks]

Originally posted by: Rubycon


It's something that goes back a long ways. Ask any TV repairman. The CRT can hold a charge for *months*. And a good one that will jolt you hard. Most people working on those (CRT's) that got shocked did not get injured from the shock itself but secondary injuries from pulling their hands back so hard that something got in the way and ripped skin, etc.

As someone who used to work at a warranty repair center for GE/RCA I can attest to this.
It never happened to me, but had a co-worker who cut himself pretty badly.
He jerked back when his hand came across the flyback output and sliced his arm on the crt neck.

We worked with rubber mats on the floor and isolation transformers, but with voltages that high, it still hurts.