VOGONS


First post, by Cga.8086

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I have seen videos of people just opening the PC Power Supply and start touching things.
It is dangerous because there are 2 big main capacitors that can be loaded. I Think those are 400 volts.

Does anyone know how much time or days do you have to wait for those 2 caps to discharge?
I know that you can take a screwdriver and touch the positive and negative leads on the capacitor and it will spark, but some say that is not good as it can ruin other near components.

there is also a video that teaches how to do it but not with a screwdriver, he uses a soldering iron, and touches the capacitor with the AC Power adapter. each tab to a leg in the capacitor.

anyways i wanted to know how many days you have to wait to take out the pcb without any risk,

Reply 1 of 13, by treeman

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as a kid I opened up a psu, maybe like 5-10 minutes after turning it off. Put the screw driver somewhere between the heatsinks and felt like got hit by Mike Tyson.

Now I always check with a multimeter the voltage on the big caps and heatsinks. Even if its been off for a few hours that kind of memory sticks around

Reply 2 of 13, by TheMobRules

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Usually they have bleeder resistors and the big caps get discharged shortly after cutting off mains power. But sometimes they don't (or the resistor may have failed), so it's always recommended to measure the voltage with a multimeter.

The safest way to discharge is by using a resistor between the legs of the caps, shorting them also works but you may get a nasty spark and could damage some components.

Reply 3 of 13, by RaiderOfLostVoodoo

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Depends on the PSU.

I discharge the PSU by pressing the power button with the power cord pulled. Then I wait a week at least.
My uncle (who is an electrician) said, 1 or 2 days is enough, but if you have the time a few more days won't hurt. Better safe than sorry.

Reply 5 of 13, by vstrakh

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Pierre32 wrote on 2022-04-26, 07:22:

I discharged one yesterday by unplugging it from the wall, then accidentally letting the pins fall across my hand. So there's that option.

You could feel some charge from tiny filtering caps only.
The large ones are behind the rectifier, and under normal conditions they can't push their charge back to plug pins, healty diodes won't allow it.

Reply 7 of 13, by shamino

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Unplug with power on will probably discharge it but don't rely on that. After opening, measure heatsinks and the large high voltage caps with a meter. If voltage is still high, wait or bleed them down with a resistor or short them as preferred. Measure again and after the voltage is low, short the caps as a final precaution just in case you didn't get a valid reading.

There's no reliable period of time you can wait for all PSUs, no matter how long you wait it's still better to measure and/or short the caps. Remember the heatsinks are often live.

Reply 8 of 13, by cyclone3d

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Waiting? What's that?

Only time I've ever waited is when I had already marked a power supply as having specific issues and then putting in my repair pile and not touching it till much later.

I've never been shocked by a PSU but I am also careful to not touch anything that could shock me.

All well made PSUs should automatically drain the caps.

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Reply 9 of 13, by Cuttoon

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That question is a bit like inquiring about the price of a brand new Rolls-Royce. 😉

But yes, just drain those caps.

This shows how he does it:
https://youtu.be/j1hGepWb6nY?t=511

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Reply 10 of 13, by vstrakh

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There's also a "recovery voltage phenomenon".
After the discharge the capacitor gains some voltage back because of "Dielectric absorption": https://en.wikipedia.org/wiki/Dielectric_absorption
Electrolytic caps can recover up to 15% of the initial voltage (~48v when used in 230v grid). Not deadly but might damage your multimeter/scope when used carelessly.
Some guy on youtube recommended to keep those capacitors shorted with the wire with alligator clips for the whole duration of manipulations with PSU.

Reply 11 of 13, by BLockOUT

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wait so when you open a psu
and take out the screws, how to you flip the pcb? to see below the capacitor legs.

what do you touch? because when i do it the only thing i can actualluy grab with my hands is the aluminum heatsink in order to take the pcb out of the box and then flip it.

so touching the heatsink is not safe?

Reply 12 of 13, by Tetrium

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RaiderOfLostVoodoo wrote on 2022-04-26, 06:19:

Depends on the PSU.

I discharge the PSU by pressing the power button with the power cord pulled. Then I wait a week at least.
My uncle (who is an electrician) said, 1 or 2 days is enough, but if you have the time a few more days won't hurt. Better safe than sorry.

This is what I usually end up doing.

Usually I'll label any PSU that I have had powered on or was in a system with the date when it was last used, so I don't open up one that has been live recently by accident.
I'll label any PSU I used and taken out of a system, even labeling what system it was in.
I usually got plenty spare PSUs, so I'm not in a rush to test a PSU right away.

Definitely better safe than sorry 🙂

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Reply 13 of 13, by zapbuzz

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Hi my appologies I didn't read replies.

each cap has 2 pins. if you have insulated aligator clips joined by wire you can securely short them safely but you only would need to discharge the big ones. But only if you can't wait 10 to 20 minutes. you can use a resistor by ohms law to slowly discharge but if the tops of the caps are square and not bulging you probably better off learning how to use a multimeter capacitor test instead to check health individually it doesn't require their removal usually.