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Thread: PSU and Electric Shock

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    Exclamation PSU and Electric Shock

    Hint of the day never ever ever touch the inside of a PSU especiallly when it is on- not even the heat sinks, as you get a mother f*cker of an electric shock !!!!
    The reason i did this was to see if it was over heating, i've got an o'l AT PSU that i am trying to power up to use in a uber ghetto beer cooler. The thing is; it powers up ok- the PSU fans spins up and the fans powered by it spin up... but then it turns it self off?!

    I also have an ol'power supply ripped out of a baby_AT case, it has the mobo connector of a ATX case so we tried powering it up using the guide from http://www.gideontech.com/guides/psu_powerup/ on the assumption that it is a ATX. But to no avail... anyone know how to power-up a baby_AT supply.. preferably without killin myslef (always a bummer).

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    Senior Member Kezzer's Avatar
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    You willeh

    You do realise a PSU (and monitors for reference) can hold enough power to kill you. It's very strongly advised NEVER to touch the inside of the PSU with and bare body parts. I've never known anyone to get killed by it but it's a fact.

    It's probably just faulty, buy a new one.

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    You Sir, are lucky to be alive. I'm no electrician but capacitors in the PSU can store the power for a long time, even when it's off.

    bloody hurts tho don't it.

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    Yeah pretty stupid i know, capaciters are fun things- didn't know that heatsinks carried a current. Learnt my lesson from that one

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    wouldn't have thought a heatsink would carry a current... MAybe you have a loose connection and something is shorting on your heatsink... Maybe.. One thing for sure tho', that was a bloody silly thing to do Mind you I think a fair few people in this forum have done similar so you're not alone matey

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    Senior Member SilentDeath's Avatar
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    The dangerous bit is only the mains-live side of the circuit. that includes the connectors, transformers and the bridge rectifiers.
    large capacitors in psus are used mainly for smoothing of regulated/unregulated dc after its left the transformer (as it will exit as a half sinewave). afaik none of the capaciors in a psu will carry more than 50v apart from ones before the transformer, cant rememeber if there should be any before.
    components using the TO-220 package and most other packages that need heatsinks have the heatsink as either ground or live (with the output, which in a psu wouldnt be more than whatever the component does - shoudnt be any over 50v iirc). psu's are therefore mostly safe bar a few exeptions. the voltage in the parts that hold current is NOT enuf to even shock you. US mains voltage (115v) is considered safe (it might hurt, but shouldnt kill).
    the exeption being that current stored in the capacitors *might* be able to find its way back through the traansformer therefore lower current, but at UK mains voltage - enough to kill.
    you only need 1mA at 200v through your heart to kill you. if you do take apart a psu, ONLY EVER USE ONE HAND and do not touch any conductive objects that give current a path to flow.

    about AT/ATX psu's. atx can be started using pin14(green colour) on the atx connector to ground. thats what the mobo does when you push your power button
    AT psu's are differnt. ive got one here, though i dont know about the differences on the inside, i know it is powered on by a switch at MAINS VOLTAGE. (i have big fat mains cable going to a large switch on this psu)

    Some of that ^ may be wrong. dont blame me if you kill yourself

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    Oooo, a god of the PSU people, cool cheers for the info. Was on UK voltage which was fun... No luck with the atx supply- are there any other supplys that use the ATX style connector? THo the supply could just be dead, nothing to test it on, don't trust it on any of my rigs as its only 190W or something like that.

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    Senior Member SilentDeath's Avatar
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    plug a 12v fan on one of the molex's,
    most psu's like t have something runnign n the 12v line, otherwise they might trip a fuse iirc...
    ive had 3 psus die while testing mobo's, 2 of them worked well a week later, the 3rd probably does but i took the fan out of it to cool my w/c radiator and its not going to get tested without a fan.

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    I always thought a full sine wave came out the secondary of an AC mains transformer. In fact, it does.

    From there, it's smoothed to DC by a bridge rectifier or a couple of single diodes if not so critical, then through a Capacitor or two to fully smoothed DC. And from there through a series of voltage regulators, either IC's or transistor circuits, to define the different rails.

    As fo that heatsink being live, that's unusual, usualy components are isolated from heatsinks with mica washers. Maybe that's what the fault is.

    A DC shock hurts a damn sight more than an AC one, can leave your arm aching for hours afterwards. But worst shock of all is the high (25Kv) line of a TV or monitor, throws you across the room, that one.

    I'm not actually familiar with computer PSU's, but I thought some of them were switched mode (ie, transformer-less) but I really don't know.

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    The heatsink shouldn't have been live, the fets have coverings so they don't short each other out!

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    Live heat-sinks are nothing new. I'm very surprised non of you guys have heard of them before.

    I don't know if its because its cheaper to produce them like this, or if it disperses heat quicker this way, but its like this in a lot of PSU's i have taken apart.

    Oh, and BTW - Ive done the same. When i was only 14 though. The little glass fuse went inside, so i went in with my soldering skills. Fixed the fuse, then gave myself a nice shock, causing me to be looking at the ceiling a few minutes later with people around me shouting and throwing water over me.
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    Quote Originally Posted by |SilentDeath|
    plug a 12v fan on one of the molex's,
    most psu's like t have something runnign n the 12v line, otherwise they might trip a fuse iirc...
    ive had 3 psus die while testing mobo's, 2 of them worked well a week later, the 3rd probably does but i took the fan out of it to cool my w/c radiator and its not going to get tested without a fan.


    hmm, was testing with one fan connected.. i might try it with a few more connected and see what it thinks of that..

    -cheers

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    Quote Originally Posted by Agent
    Oh, and BTW - Ive done the same. When i was only 14 though. The little glass fuse went inside, so i went in with my soldering skills. Fixed the fuse, then gave myself a nice shock, causing me to be looking at the ceiling a few minutes later with people around me shouting and throwing water over me.
    240v is an inconvenience now (joke)

    Electric shocks no fun

    you guys know your electronics don't you.... makes me think i should of apid more attention in analouge electronics lessons

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    I would never touch the inside of a PSU or monitor. Monitor especially since I heard that they can hold a charge of 25,000 volts
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    Senior Member SilentDeath's Avatar
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    floppybootstomp - was a bit tired when i wrote that, thanks for pointing out my mistake
    i admit the insides of psu's do look scary (massive capacitors) but in a pc psu where the intended output is less than 15v, i wouldnt expect there to be any components at any dangerous voltage except the transformers primary and before, the rest should all be safe.... i have only studied/made very simple psus though (most powerfull was 4A 15v) and ATX ones are a lot more complicated in many ways. one thing i didnt think about is the fact they are switching psu's and not linear regulated - that might mean higher voltages inside as switching is a lot more efficent than linear at voltages far from the intended output voltage. swtiching works iirc like a transformer thats variable to get teh output constant, whereas linear jjust burns off al the extra voltage/current as heat...

    the insides of any CRT based screen, microwaves, car ignition (spark plugs are hv?) and iirc the capacitors used for the flashes in cameras are all capable of giving nasty shocks i hear...

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    Senior Member Kezzer's Avatar
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    Quote Originally Posted by |SilentDeath|
    and iirc the capacitors used for the flashes in cameras are all capable of giving nasty shocks i hear...
    lmao, we used to do that in school. Get a camera which has a flash, take it apart, strap the battery to it and tape it up, hit the flash button (or what's left of it) and shove it in someones face. Was a great trick back in school, really friggin' hurt as well

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