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Connecting 12v & 5v

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I've read that you can use the 12 & 5 volt lines on a psu to run fans @ 7v. I tried this and the pus would not ever turn on. Is there a trick?

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yes, where the the psu normaly connects to the mther board u need to connect any green adn black wire togather to give it a jump start , or u can do as i did and just cut off the mobo connecter unsolder all other wires that arent being used and run a switch

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That isn't what he is refering too.... You can run fans at 7v by using the 12v and 5v lines on the molex connector...



the hot or red wire from the fan goes to 12v or yellow on molex... the ground wire from the fan goes to the red or 5v line....

if you do it backwards the fan won't spin.

hope that helps

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[quote name='dangerousfigure' date='May 24 2004, 12:39 PM']connect  any green adn black wire togather to give it a jump start[/quote]
I did that. It works with out anything else connected to the psu, but not with the 12 & 5 corssed.

Below is a scamatic of what I want.

Connect the 12 & 5, run a wire from their to a load and then connect the load to a ground.

thx dangerous

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Hmmm are you trying the 7v trick with the PSU alone... it 'may' not work like that.....

when you are doing the 7v trick you are actually feeding power back into the ground......
it works because there are always other things on the system to counter act that...but if you try it alone..it may not..

or am I not understanding what you are doing??


[img]http://www.silentpcreview.com/files/images/su_1257.jpg[/img]

This is from the silentpcreview.com website:
[quote]Caution! There are people, who will say "You can't feed current back into your power-supply" and they are right!

If you plug something between the 12V and 5V lead, you have to be absolutely sure, that no current is fed back into the PSU. Which means, that you need to have other loads on that 5V lead, which suck out more current than you feed to that lead from the 12V lead.

For example: Your fan uses 100mA at 7V. You take 100mA from the 12V lead and feed them into the 5V lead. If you have a different fan (or whatsoever), that sucks out 200mA from the 5V lead and feeds them to ground, you're fine. 100mA are sucked out of the 5V lead, 100mA come from the 12V lead and 200mA go to ground.

Since usual fans only use 1...3W, you are usually perfectly safe with using the 7V trick there. There a lots of devices in typical computers, which draw much more power out. Peltiers or things with power consumptions running up to dozens of watts, require careful calculation of what goes out of the 5V lead and what's going in there from the 12V lead.[/quote]

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[quote name='ijagwalaafq' date='May 24 2004, 03:29 PM']I tried this and the pus would not ever turn on. Is there a trick?[/quote]
You meant PSU, right? I know you did, but I still laughed for some stupid reason.

7V? An alternate would be to run a voltage divider from a 12V source instead. Still some math involved, but at least you won't have any problems with current feedback.

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another alternative would be to buy an el-cheapo 4-way fan controller for like $5...

that's what i did, hey, call me lazy... call me cautious... i dunno, most i've done is a custom led mod (with resistors!) for my Tt ducting mod

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7V is taking the difference of the two voltages 12V and 5V. (+12 - +5)
In a circuit that has two opposing voltages, current will flow in the polarity of the stronger power source with the voltage, in this case, 7V, since the flow is 12V with 5V suppressed by the opposing but weaker power source. Edited by a rabbit

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Ya, I understand that, but then I would hook thoes two up to a ground. Electricty follows the path of least ristance (lowest opposing voltage) right? Which in this case would be the ground. See the scamatic 6 posts ago.

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Sorry, ijaqwalaafg, my last post was for exeter_acres, who did not understand it. But for you, current will feed back into the 5V power source to ground unless an additional circuit draws an equal or greater amount of current causing it to go through that device, rather than through your PSU, to ground, in which feeding current back into you PSU could destroy it. Edited by a rabbit

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