Every psu I've opened has already had such resistors fitted, they equalise the voltage across the two big electrolytic caps for the voltage doubler bit to be accurate IIRC, the bleeding safety function is secondary.

Value is a few hundred k, so they still take several seconds to bleed big caps, and, of course, you should take precautions as above in case there's a fault with them.

Biggest danger is thinking that because the computer is switched off, the power supply is dead. ATX supplies are always on until the mains is switched off.

ditto. Always keep your (left) hand in your pocket, don't forgot also to avoid having somebody going around you and wear (isolating) shoes

An electrician always works with one hand behind his back. That way, if he gets zapped, it won't go through his heart.

Also, I've been zapped by the capicators they have in flash cameras, and a live electrical outlet.

Being a tube-amp modifier-type-of-guy, I'm always very careful around electricity. One thing I've found is that rubber surgical gloves can help in this regard - you can still do delicate work with them, but they are non-conductive.

I've also replaced *many* fans in power supplies back when I was a computer/network consultant. Usually, the fan bearings would start to go out and it was slightly more economical for the end user to replace the fan with a nice Panaflow than buy a whole new power supply. Never got shocked.

-Derek

Thought I should link to this page:
http://www.eio.com/repairfaq/REPAIR/F_captest.html
detailing the PROPER way to drain caps (hint: you DON'T use a screwdriver )

OK, I couldn't resist adding to this ancient thread:

I have been shocked many many times by 120/60Hz. I used to change outlet recepticles as a kid in my house live (you can't see if the lights go off...). It always scared me, but never maimed me. No scars either.

HOWEVER, such a shock can easily be fatal. I have read very recently that a young boy (~10 years old) got shocked and killed by his computer case because the recepticle wasn't properly grounded and the PSU had a fault.

I have also shorted out a standard house wire and it has literally arc welded a huge hole in a 36 inch bolt cutter (my sister's boyfriend cut the wrong wire) in < 1 second. .

I believe it all has to do with your personal resistance. I now use gloves when doing electrical work -- more because it also protects against the metal and oils on the metals.

I know this posting is a few yrs old already but I wanted to put a reply for anyone who will stumble upon it and need the answer to this.

The actual way to drain a capacitor is to short the component, in other words you would just touch the leads together with a screwdriver or use an alligator cable and clamp an alligator tip on each component lead. This shorts out the component and drains it of any stored charge. I think this would happen quickly but I would check each cap after with a volt meter to make shure. I was always told not to even open a PSU (It's one thing to change a fan) because they are pretty cheap and easy to replace. Actually I would not try to work on any computer part to component level (can you Imagine trying to find a bad part on a Mobo, could take weeks) because these things were designed as FRU's or Field Replaceable Units and its better to replace the part.

GL

Voltage hurts and amps kill. Think about it. I've been hit w/ a stun gun, 40,000 volts. But hey, not much current. Meanwhile, 1 amp in the right place can kill you.

I think unplugging the power cord and pressing the power button a few times is the best way.

Also, Sometimes if you really want to test if something is "hot", use the back of your hand, not your front. If you grab a hot wire, your hand will constrict and grasp the wire, further shocking you. If you hit it with you back of your hand, you will naturally jerk away.

Dear all,

I have been reading these older messages about power supplies taht started off with how to make sure it's drained. Some insightfull comments, even the ones where screwdrievrs tend to get arc-ed.

Keep in mind that (I saw one comment on this) that the heatsinks within an open SMPS (the technology used to create power supplies) (Switched Mode Power Supply) are connected to the MOSFET's (transistors) and carry a voltage that is much higher then the power plug outlet voltage.

The way the volume (HxWxB) of the power supply is kept down is by decreasing the transformer size. If you ever looked into a pwoer supply you might have noticed a lack of big transformers. When you think of it, a 20A 3.3V regular transformer is huge, so how come the transformer of the power supplies are smaller?

The technological secret can be found in an increase in the frequency of the unit. When the transformer is used at 50Hz (60Hz) the volume of the magnetic component (the older iron plates or newer round cores) needs to be large to accomodate for a "slow" swing on the frequency. If the frequency however is increased the volume of the transformer can be reduced.

Right at the beginning of the transformation process within an SMPS unit (before the transformer), the regular power is fed into a diode bridge set to create a dual 240V(120V) sinus swing. That voltage is often flattened by capacitors (hence the good advice to turn or leave on the primary power switch, as for some models this does drain the capacitors (when not connected to the board)). Then the magic starts. This voltage is fed into a high frequency "chopper" usualy made up of a MOSFET (they tend to switch the fastest with the lowest amount of heat dissipation), but this process does create a higher (into the 1.000 Volts!) voltage there caused by capacity and coil effects. Hence the heatsink can (and will) give you a kick you have never felt before.

Sofar the technological explanation of this.

As you might have geused by now, I designed an SMPS from scratch some years ago. When I first turned my model on, the MOSFET went into the seiling with a pop, only to come down a few seconds later. It was caused by my then lack of knowledge of how to wire (winding) a transformer for such higher frequencies. The inherent capacity in the transformer kicked the MOSFET into greener pastures. I also burnt my nose on the later working model when smelling that all to familiar smell of burning components. In trying to locate the spot my nose tipped the heatsink of another transistor ... found it though. Looked funny for a few days with a small horizontal red stripe on my nose.

Some good advice?
- Do NOT EVER touch any components inside a still connected system. The warnings are there for a purpose.
- Use the one handed advice given before if you should venture further at your own risk after ignoring the first.
- Turn your one hand upward (fingertops facing you), touching with the BACK of your hand if you should ever decide to disregards the previous two at your own risk. (If after all previous warnings you do eventually run into those Amps, you muscles will contract. Geuss what happens ... your finger might pull away from the source).

Be well all, read the warnings, follow them.

Regards,
J

i do two things, in this order.

Unplug the power cable, then hit the power switch on the front of the PC. It will briefly power on and then immediately back off again.

Then once i have the PSU open, i take a screwdriver with an insulated handle and short the large capacitor lead to ground. This ensures ever last bit of charge is discharged.

I use a lightbulb to discharge capacitors. works great and it slowly discharges the caps to a safe level. sizing the lightbulb to the circuit your working on. for example a car tail light bulb works well for 12V sources. I use a 120V 60 watt bulb for working on audio power amps and a 240V 60 watt bulb for working on voltages over 160V but under 300V and high wattage high voltage resistors for anything over that.

NEVER use a screwdriver to discharge caps. they can store a LOT of energy and I have seen caps explode in peoples faces. screw drivers arc and discharge hot slag right into the eyes etc etc etc.

So stun gun cant kill us? I'm afraid when my brother showed me his stun gun.

And that's why they are called NON LETHAL WEAPONS in law enforcement agencies across the globe.