PSU

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I all,

...so here i am listening to my A+interactive cd, and im cleaning out one of my jolly old download machines. striping every thing apart and giving a ole good clean...

I just stripped the PSU and screw driving the dirt out of the insides and vacuuming it away, and then all of a sudden.... "The two main components that should be replaced rather than serviced are Monitor and PSU, as they still can hold electricity for a very long time!" or something like that

i slowly remove my screwdriver, and start to replace the PSU together, luckily no shock, so my question is to ya all'

Have you ever been shocked by repairing any of the component on your mobo???

 

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yikes! easy with the scary bit! yeah unless you know how dont you dare open PSU's and monitors.. oh and add Universal Power Supplys to that list! i dont think motherboard can shock you unless you got a short somewhere.. just dont go sticking screwdrivers into holes! the one thing that did happen to me was a blown capacitor burnt my face as i was looking in! thankfully no war wound to prove it!

 

Well, no. A mobo isn't going be holding a charge much over 5 volts and that isn't going to be felt even if you do get shocked by it.

I'm just curious as to how you made the jump to the mobo from the psu and monitor. Both of them hold much higher voltages with a CRT having internal voltages in the 10's of 1000's of volts, and a psu being dangerous because of voltages in the 100's, but never, never open the case on a powered up psu.

A psu is mainly dangerous for completely different reasons than a monitor. They can both kill you, but opening a psu that is hooked to the power grid(a power outlet) is really dangerous because of how they work. Never, never open the case of a power supply until it has been disconnected from the power source for 15 minutes or so. Even then have a reliable volt meter to test it with to see if it's still holding a charge. The capacitors have bleed resistors hooked across them to drain the charge, but if that resistor fails the capacitor may hold a charge for quite a few hours.

A monitor can hold a lethal charge for days, weeks, or even months.

 

They can even weld a screwdriver to the terminals on the HV transformer when shorted... (thats is it hasnt blown it apart first!!) It can make a nice bang tho

As an electronics tech i've had a few shocks with PSUs but wouldnt work on one while its still plugged in an switch on unless its absolutly nesscerary! There arent really any serviciable parts in Switch mode psus these days anyway.

And i have seen a mobo catch fire once... when a PCi card was plugged in while there was power to the board!!

 

things blowing up again? hmmm maybe I should stay from this thread!! the others will be laughing at me again if I say anything...

 

Power supplies can indeed hold hefty charges if they are faulty. As Freddy said, they are designed to discharge through a bleed resistor but if a fault exists this *discharge* may not happen. I actually got quite a jolt once upon a time from a Philips G8 TV PSU. I had replaced the faulty one with a new one and was putting the faulty one into my car to take it back to the workshop. Without thinking I touched the mains dropper resistor. Bad thing to do, nearly smashed the thing through the car window, it gave me a mighty jolt and burnt two small holes into my thumb. The smell of burning flesh was not nice either. I was 500 yards away from any power source at the time

This is why in computers the PSU is in a cage.

CRT Monitors have similar characteristics to televisions. HT DC voltages are supplied by a PSU which is not in a cage. EHT (Extra High Tension) is supplied to the anode of the CRT. The conductive coating on the outside of the CRT acts as a capacitor along with the coating on the inside. The glass is the dielectric. The EHT is generated through a process of back EMF from the LOPTX. If you didn't understand that, do not go in there, unless you are on a death mission

 

Cheers, i have to clean my other machine out, i am constinly hearing a wizzing noise, probably the fan is sticking, so im going to clean that out and see of i can swag the stuff around, but nevertheless, would the electric side come up in my A+ hardware exam?

 

LOL - been there done that...only I have an open and unuseable PSU sitting on a passenger seat when driving and I have plonked tools on top and got a jolt for my troubles.

I know someone who set their car boot smoldering by having an old CRT monitor with a loose cover rolling around in the boot.... fortunately they carry a fire extinguisher.

The best jolting I have seen someone get is with servicing CNC machine tools and the bigger capacitors [coke can sized btw] they have to regulate voltage for the controllers. This apprentice was told to clean out the cabinet without touching these capacitors.....there was a howl of pain and we hauled him out from under the bench where the jolt had thrown him and ended up taking him to hospital [just in case]

The nearest I came personally to a frying is many years back [apprenticeship] where a sparkie forgot to disconnect 3 phase power [440 Volt, 85 Amp] prior to me poking the wires back down a hole in the wall with a screwdriver...the short blew the end of a large screwdriver to pieces but fortunately I was only holding the the insulated handle. [the sparkie was sacked for that effort]

The trouble with high voltage/low amps electrocution from monitors and PSU's is that you can permanently damage the nerves around your heart if it happens enough, and apparently is a reasoneable common type of injury.

Therefore I will no longer go into a PSU or Monitor - more trouble than they are worth looking at the price value!!

 

The closest i even been electricuted is when i touched the HD or HT lead on my car, but that was not funny, but this is; the power cable to my comp was had a deep cut in there, and my mate aka network administrator was going to shove another hdd in my comp, he goes to pull out the power cord and i just see him fly on the floor, i burst out into laughter, i mean i was hysterical , cuz all i see a fat blob bounce on to the floor, 5 secs later he swearing at me and say you fking w*ker, i got electricuted, my response? oh s*t i forget to mention the power cord, my bad!

since that day forward, he double checks everything before he comes near my computer or anything i own!

BadBoyz for life, almost premature!

 

I didn't know that. Oh crap

 

Oh dear I have to admit that there is something very funny about watching someone else get a shock. It stops being funny however, when it gets scary

 

Well when I was on the tools it was a standing order from the owner of the business - skylarking with electricity mean that you would be sacked..so if you took apart someone's power drill as a prank ...

In a couple of other places I worked...a machine with broken wiring would have a shop stewards ban on it until the wiring was fixed.

 

But he is getting a bit pee'd off cuz the poor soul always gets shocked when im around, actually when im not around aswell, e.g. he was chipping my ps2 and ring ring, i call, when he looks at the person calling, he gets shocked and blows my port one on my ps2... there is at least 2 more, but can not remember them at the mo...

for guidance, he turns off his phone when dealing with anything electrical just in case i call... he he he

thecheekmaster
T-H-ED-E-V-I-L

 

I know as an apprentice we used to gently clip the terminals of a Megger to someone who was sleeping then just crank it up... they soon wake up (for anyone who dont know a megger is normally used to test for insulation problems.. it can deliver upto 500vac...)
And while doing electronics we regualrly played catch the charged Cap!! I prefered to drop it rather than ger a belt as i've had holes blown in my steel rule by charged capacitors...

And i've even had a hard drive go flash bang smoke on me once while fiddling inside the case.. didnt get a belt from it but sliced me hand on the edge of the case as i pulled away quickly!! funny thing is turns out the hard drive carried on working too.

 

I remember helping someone wire the radio batteries in a militay Landrover he was using a metal adjustable touched the metal side of the bty tray causing the bty to blow up and him losing the sight in both eyes with bad acid burns to his face.

 

The safety problem with switched power supplies, other than their internal capacitors, when they are operated with their enclosure open is the rapidly changing magnetic field that they generate. All you need to create an electrical charge is a wire, or any kind of electrical conductor, and relative movement between the magnetic field and the electrical conductor. That means a screwdriver, or any other metallic, ungrounded, object placed in the rapidly reversing magnetic field generated by a psu with the enclosure open is capable of becoming a highly charged electrical conductor. They will also disrupt devices such as pacemakers and the like.

A switched psu can be a very dangerous object.

 

So the morale of the story is ?

Do not attempt under no circumstances to open and attempt to repair/clean a PSU.

Curiousity killed the cat, PSU can kill U.

 

Yes, except I would say *any* rather than *no* just to avoid the double negative confusion

 

I remember once, a good few years back, I bought a 500watt power supply in a huge ATX case.

I didn't have a motherboard to chuck in it, but was p**ing about with it anyway. Id plugged the power supply into the mains, and left it on wandered off and forgot about it. When I came back, i noticed a little notchy thing on it saying 240v /110v, or something like that. Without thinking, I got a pen, chucked it in the notch and changed it from 240v to 110v.

BANG

A bright light smacked my face as the power supply went bang. For weeks my eyes felt as though I had something big in them whenever i closed them.

 

Hmmm, US mains 110AC, UK mains 240AC were you trying to create the second big bang theory of creation?