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Computer Power Supply?

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Computer Power Supply?

Unread postAuthor: VH_man » Sat Jan 05, 2008 10:40 pm

I was wondering if you could use a Computer power supply as a power supply unit for hobby electronics.

from what it looks like, it gives out the perfect voltages......

I was wondering if there was anything dangerous about this. My dad says people die from touching the wires that come out and connect to the computer. Im pretty sure this isnt true, ive seen people work with the guts of computers while they are running.

but, has anyone tried this, and is it safe? I WILL NOT be opening up the outer case.
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Unread postAuthor: c19o » Sat Jan 05, 2008 10:53 pm

I used to work on my computer with it running all the time to get the fans running right (problems with speed control). Only thing that can go wrong is you short a wire and it shuts off the comp. I wouldn't touch the insides of the power supply.
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Unread postAuthor: paaiyan » Sat Jan 05, 2008 11:08 pm

Well, it most likely wouldn't kill you unless the current ran across your heart of something. I suppose you could use it to power small electronics, so long as the power is right.
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Unread postAuthor: singularity » Sat Jan 05, 2008 11:34 pm

i use comp power supplies to power almost everything because i can get them for free. i got one thats a workbench psu, another powering a portable air compressor (meant to run off a cigarette lighter), and a last one power an array of leds used as a high intensity light. there has never been one problem with any of them, i have even shorted the positive and negative leads without damage to the PSU (auto shutdown)

most modern power supplys auto shutdown when to many amps are drawn, and at 12v there is no way to kill yourself, or even injure yourself for that matter. have you have you ever seen a "danger low voltage" sign? sure if you open it up you will come across a few high voltage caps but so long as you don't stick a fork in there you'll be fine.

there are some things you might want to be aware of (if your not already) the switch found on the back of a computer PSU will not turn it on, you need to short two pins on the 24 pin connector (green and black i believe), and some power supplies will not turn on unless there is a load on it, so it would be in your best interest to get a high amperage low value resistor just in case.
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Unread postAuthor: ralphd » Sat Jan 05, 2008 11:38 pm

no problem...just watch the amps
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Unread postAuthor: knappengineering » Sat Jan 05, 2008 11:50 pm

@singularity I think that your statement of "at 12v there is no way to kill yourself" is not completely true. It is not the volts that kill people, its the amps. Think of a taser or vandergraph generator, they produce high volts, but do not kill people because the amperage is not above the deadly limit. (I'm not sure the exact value of this limit but almost anything above1 amp is enough to kill someone) Just thought I'd let you know. :wink:
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Unread postAuthor: mark.f » Sat Jan 05, 2008 11:56 pm

I think it's 15 milliamps across the heat, if the mythbusters are anything to go by.

But, also, without the proper voltage, the current won't be able to overcome the resistance of the human body, so voltage matters still.
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Unread postAuthor: VH_man » Sat Jan 05, 2008 11:58 pm

the thing says 20 amp output on the side.....
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Unread postAuthor: knappengineering » Sun Jan 06, 2008 12:00 am

Alright. Thanks for the clarification mark. 15 milliamps across the heart sounds right. ( I was trying to think of what the mythbusters said but couldn't remember) Also, thats true but I guess I was trying to imply that 12v or almost any voltage for that matter could still be deadly if the amperage is high enough.

Edit: I'm pretty sure that 12v and 20 amps is enough to kill someone
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Unread postAuthor: benstern » Sun Jan 06, 2008 12:04 am

20 amps is all the outputs combined at max load.
I just tested and the 12 volt outputs on my hobby computer psu have around 4.5 to 5.3 amps. Totally safe.
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Unread postAuthor: VH_man » Sun Jan 06, 2008 12:07 am

does anyone have any quite reputable sources for these things?

like mabey a datasheet? cuz i have to prove it to my parents before they give me the power cord.......
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Unread postAuthor: benstern » Sun Jan 06, 2008 12:12 am

So it's another case of ignorant parent syndrome, ey?
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Unread postAuthor: VH_man » Sun Jan 06, 2008 12:19 am

it says a different ameprage underneath every voltage

i think its odd, becasue ive seen computer guys stick their fingers in a computer while its on and running and not worry aobut any high amperage/voltage stuff.
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Unread postAuthor: benstern » Sun Jan 06, 2008 12:23 am

What voltages are you gonna use from the power supply?
List the amperage rating on the psu case for that voltage.
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Unread postAuthor: singularity » Sun Jan 06, 2008 1:25 am

@singularity I think that your statement of "at 12v there is no way to kill yourself" is not completely true. It is not the volts that kill people, its the amps. Think of a taser or vandergraph generator, they produce high volts, but do not kill people because the amperage is not above the deadly limit. (I'm not sure the exact value of this limit but almost anything above1 amp is enough to kill someone) Just thought I'd let you know. Wink


ok i know that, have you ever heard of something called ohms law? well i will tell you what it is even if you have.

amps = voltage / resistance (I=V/R)

the human body has incredibly high resistance, from palm to palm the minimum resistance is around 1 mega ohm (one million ohms) so if those values are plugged into the equation....

.00012A = 12V / 1,000,000 Ohms

.000012 amps, thats its .012 milliamps, far less than is required to kill someone or even for them to notice they are being "electrocuted". just to be absolutely positive i went and held one hand on each lead of a 12 v lead acid battery , which are capable of sourcing very high Amp values. as you should be able to tell nothing happened, in fact i could not even feel it (what a surprise) then i went and held the 12v leads on a modded SLI computer power supply, once again nothing happened (an SLI power supply is very powerful)

the amp values posted on the side of most power supplies are the maximum amp outputs, not the amount of amps that are forced into everything that comes into contact with them. this values help in determining if the power supply has enough juice to power graphics cards and CPUs.

as a basic rule yellow wire= 12v+, red wire = 5v+, orange wire (most of the time) = 3.3V+, black wire = 0V (ground)
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