Please share any/all methods that you know for fully discharging a power supply, thus making it safe to open up and work on the internals.

I'm hoping there are some known sure-fire methods for doing this, that are both simple and non-technical enough for everybody to follow.

I'll start (gulp):

Personally, I've been advised that keeping the supply switch in the ON position, but with the supply unplugged from the wall, and attached to one of these ATX power supply testers overnight, will insure that the supply is fully discharged by morning. Is it true? (I haven't been shocked yet...)

Next please?

i have been shocked, not from a charged capacitor but from a live heatsink during test run with open casing. i wanted to see how hot the heatsinks gets, i was sitting on a thin carpet at the moment, the shock went up to my elbow. was feeling kinda funny, like if someone grabbed your arm and shook it ferociously. nothing serious happened, i'm not really keen on trying that again though.

the safest way to discharge a psu would be to leave it unplugged for some time. the capacitors will loose their load over time but i have no idea how long it will take for the capacitors to fully discharge, days? weeks?

when working on the psu make sure you are not grounded, wear shoes with rubber soles and make sure you are on a thick carpet.

Wrote this in some other thread:

1. Disconnect the power supply from the wall outlet. If the PSU has a switch, leave it on.
2. Hit the power button on the computer a few times.

This will drain the capacitors for you. The fans will spin briefly at the first power button push. I usually push it four or five times to make sure the capacitors are completely drained.

I guess the idea is to discharge any remaining charge to ground. Seems reasonable... (unless you run into anything that has a huge charge, in which case the resulting spark could scare you witless. )

My 2 cents -

I've only ever been zapped when working on a PSU that was plugged into AC and/or running. I often open up a PSU seconds after unplugging from AC. My rule of thumb is simple: never touch anything metallic that I can't touch from the outside of the case. Usually, all you ever really want to do is yank at a thermistor or swap the fan... These things can be done without touching anything else or with tools that have insulated handles.

Sorry no guarantees.

Please be careful, Mike. I'm sure we'd all rather read your articles and posts rather than obituary

BTW, if anyone is looking for some references to this subject in some of the older threads around here...
What is the easiest way to discharge a power supply?
Oh, and [url=http://forums.silentpcreview.com/viewtopic.php?t=3361&view=previous]Please help me avoid getting shocked to death!
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Nice to see and compare.

A 6800 uF capacitor (about ten times the size of those in ATX PSUs)that I charged to 18V several months ago is currently at 13.4V, so I wouldn't rely upon leaving the PSU unplugged for several weeks to drain off the voltage.

Some of my PSUs make a noise just before the LED on the mobo goes dim, and it's never taken more than twenty seconds. If for some reason this doesn't work, then PSUs usually have or are supposed to have an approx. 200K ohm resistor across each big high voltage capacitors, and if this capacitor is 1,000 uF, then the resistor should discharge it in 200K ohms X 1,000uF X 5 = 1,000 seconds, or 16.7 minutes. Unless a resistor or something else around one of those capacitors looks burnt or ruptured, then the resistors have probably done their job. But you have to know what those resistors look like.

One time I was shocked pretty good with 110 volts, hurt like hec but hey I am still kicking.

Mike your a wild man! No really mike is right, just be very careful and pay attention, I swapped my psu fan in less than 15 minutes.

Here's how I do it:

1) Unplug PSU from 120VAC
2) Try to turn PSU on
3) Wait until fans in PSU stop, and LED's anywhere die
4) Open PSU

I've never had problems.

Are you sure that will drain the capacitors? I'm thinking that it might only drain them to the potential below which none of the pc components can be driven - lowest is probably 1.5v for LED's. So maybe the potential doesn't drop much below 1.5v? If so then there would still be approx 1/10th of the original stored charge - still a significant amount of charge.

I make like mike and open when I want, avoiding touching anything metallic or just instinct and common sense. What I like to do though is turn the switch on the psu OFF while the comp is still running (so usually in bios screen) and then hit the power button a couple times to try to get some more out. Works for me. Often opened the psu only minutes after comp was on, never shocked, but once shocked when touching the heatsink WHILE psu was on...wanted to see how hot it was...stooOOo00OOoooopid! Didn't hurt much, tingled my finger for few seconds.

-Ken

Aye, another vote for the "dont touch anything you normally couldn't" crowd. Recently I've been moving power supplies from one computer to another, changing fans, changing fan voltages. Heck, half my PSUs around here have the cover off them all the time. Only touch the case, and use insulated tools, not a problem.

a one handed electrician is a living electrician - remember if you use two hands and *whoops* eh, complete a circut the path of curent flow is (as i have been told) through your chest (and heart) - so if you dont want to have electricity passing through your torso JUST USE ONE HAND and the *whoops* goes not much further than your wrist... maybe a nasty shock and burn, but no electricity through the heart.... at least thats a theory im willing to believe....

Yep. Unplug the power cord from the PSU and turn on the PC to discharge the caps. Works like a charm. I have opened up over a dozen PSU's and replaced their fans over the past few years with not even the slightest shock. Unless you're stupid enough to touch the capacitor terminals, PSU's are generally safe to work with.

I spent a while working in a computer repair shop working on 15-year-old dot matrix printers. Those beasts had caps the size of 12oz coke cans inside them. IF you want to make sure you don't shock yourself, the way we always did it was to take a very well-insulated screwdriver (plastic handled ones, nothing special), flip over the circuit board, and touch the two capacitor leads at the same time with the screwdriver. It might make a small 'pop' noise, but you won't shock yourself. That's how we did it, and that's also how the guys in CRT did it...by the way, don't EVER open up a CRT monitor if you don't know what you're doing. The CRT tube stores about 50-60,000 volts even when it's turned off.

damn it... i wish i was read this thread earlier today before i opened up my power supply. i have the "seagateIV/psu" coil whine problem...but i didn't know it was this problem exactly yet. so i opened up my computer case and psu while the computer ON

...stopped all the fans by hand, and the whine still came from the psu. so i tried to touch some stuff.....

freaking A!...i touched a capacitor and it zapped me...my whole arm was shaking...i guess i'd better becareful and NEVER do that again. damn..its was freaking and I'm glad i'm alright. Lession learned. Also learned that u can control the 12V rail cuz it has a built in pot in side the psu =D. I wonder if lowering it/raising it will help the coil whine?

Best piece of advice in the thread.

I was into valve/tube tech for a while, and this was the top piece of advice.

Also, if you must poke around, use a chopstick. It's not conductive (unless you're a smartass with metal chopsticks) and it's quite good for poking things with.

Yeah, going along with some other posts.
The path through the body has much to do with the shock danger. A current passing from finger to elbow through the arm may produce only a painful shock, but that same current passing from hand to foot or through the chest from hand to hand may well be fatal. Therefore, the practice of using only one hand (keeping one hand behind your back) while working on high-voltage circuits is a good safety habit. Even better would be to disconnect all sources of power from the equipment you are about to repair. Do not rely on insulated tool handles, rubber-soled shoes, etc., to protect you.

Be really carefull with what you think is an insulated tool especially when dealing with stored charge. Also make sure everything is dry, its common sence but sweaty hands lowers the skins resistance from arround 500,000ohms to as little as 1000

Caps can bite and bite hard.

BTW - You Americans are lucky you only have to deal with 110V, here its 240V and it hurts...alot.

One last peice of knowlege via google. - The frequency of the AC has a lot to do with the effect on the human body. Unfortunately, 60 cycles is in the most harmful range. At this frequency, as little as 25 volts can kill. On the other hand, people have withstood 40,000 volts at a frequency of a million cycles/sec or so without fatal effects.

actually it is very crap that the americans are stuck with 110V (as crap as their inches and pounds)
why: to get the same power out of a device they need to draw more than twice as much current (P=VI). it is a high current which is the killer, and the cause of electrical fires also (which is why residential buildings in US all have to be defaced with those uuugly fire stairs down the side)
be thankful if you have 230/240!

i've had the old "240V boot" and they really aren't that bad.. i'm with MikeC just be careful it's not worth freaking out about

True, wim, very true.
Ive never really understood why theyve kept the Imperical mesurements after all these years. 240V isnt so bad. Unless it is high current 240V and then it stings, oh that and being 6 feet in the air on a metal ladder while being stupid and young, you quickly learn alot of lessons.

On PSUs though i think its wise to be carefull, not to freak out. it can be done perfectally safely, but you dont really want to screw it up. Either that or Cover yourself in salty water, put cuts all over your skin and lick all the capacitor contacts ( really its the best way to be safe ) oh and just to be even safer, swallow a sword while doing all of the above with unpinned grenades in every pocket.

Careful with the "only 110 Volts" thought. 110V is less likely to throw you onto the floor, but it can still kill you dead. I found that out once; when doing some wiring, not knowing that the circuit breakers had been re-closed, I grabbed a 110V wire . I physically could not let go of the wire; I had a friend around to smack me with a board, and break the circuit. If I'd been alone, I may or may not have been able to break away from it.

I've been grabbed by electricity, and thrown by electricity, and I'll take thrown any day, thanks. I got a couple of little burns from being thrown, but at least it was over quick.

So, uh, don't be stupid and play with things that are connected to house current.

wooOOoooOOOO! he's back from the dead

people conduct electricity at different rates as well, my threashold of sensation is 2 9v batteries clipped in series on dry skin, im about 45% more sensitive than most people. i can step barefoot on a live extension cord and feel that 60HZ buzz- not like "ouch" but like "what the heck is that" i kill watches and clocks constantly, and i will never go near the bermuda triangle again, but thats a little OT for SPCR.

Shorting the green (PWR_ON) wire with a black (GND) wire on the ATX header will force the power supply to try and turn itself on. A paper clip does the job nicely. If the supply is unplugged the caps will drain to safe levels quickly. This, to me, is a better bet than the push-the-button-over-and-over approach since it takes the motherboard logic out of the equation.

Internal leakage in caps varies widely by composition and construction. I'd guess that in this cost conscious world PSU suppliers aren't spending a lot on caps, so they'll drain themselves fairly quickly.

Funny one-arm-electrician story: The college that was kind enough to give me a diploma at one time specialized in power transmission (fairly recently getting a lot of power across the country was a new problem). An old professor there told me once that when they were working inside the [Faraday] cages they would work with their left hand in their right pocket. That way if they got zapped, and at those voltages it would jump out at you, it would hit their arm and run down their leg rather than go through their heart & other vitals. Then he showed me the scars on his left arm. Yeikes!

I had to register just so I could relate my experience, which could have been fatal, but is now very funny. I was working at Bob's Big Boy here in CA back in the 70's. I was a cook, and during cleanup one night I stupidly picked up a 220 volt commercial toaster! NO!, I didn't pick it up from the outside, and YES!, It was plugged in! hahaha thumb in one toaster slot, and fingers in the other. HAHAHAHA Knocked me back about three feet against the fridge! Man, what a shock! WHAM! HAHAHA I'm still here so can laugh about it now, but the moral here is, if you are tired or young, be very carefull! hahahehehe Take care all.

Tuberocity, welcome to the forums!

Damn, that must've hurt! I hope you learned something from it... Good thing you're still here to tell us about it.

ancient thread, but one other suggestion, applies more to big wires than PSUs but either way...
if you need to touch something and can't be bothered to figure out if it's live, just use the back of your hand (instead of touching it and involuntarily grabbing it if you get shocked), then you'll have a hard time being shocked for more than a fraction of a second.

The simplest solutions are often the best. Good tip Straker!

If your PSU is designed like mine with caps in the neighborhood of 200v, you can add bleeder resistors across the caps. A good value is 47K ohm. Going less than 33K ohm will probably cause the resistor to overheat (go a lot lower and you'll get fireworks) or the PSU may malfunction. After soldering the 47K ohm resistor across each large capacitor, fire up the PSU and turn it off. Use a multimeter to monitor the discharge curve. Personally, it causes me no discomfort to touch 40VDC, at this point I can handle the PSU, your milage may vary. Take note of how long the PSU takes to reach this safe voltage so you get a good idea of when you can begin to handle it. Remember that caps of higher capacitance take longer to discharge so you should measure those.

I take no responsibility for what you do, the above is provided for entertainment purposes only.

I use a 25watt lightbulb in a holder and two leads to discharge the caps.
lightly contacting the two stripped-ends of the lead to the capacitor terminals
for a few secs makes the lightbulb glow briefly and discharges the caps .
Then i test the circuit "dead for sure" with a multimeter before i start touching
it with my hands . upto now this has worked .