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shouldnt these voltages double?

W

wayne

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Aug 4, 2003
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i have a question about voltages

lets say you do a 7v fan mod by wiring 12v to 5v as ground... then that means 12v - 5v = 7v for the fan
but on the molex connector, its 5v, -5v(ground), 12v, -12(ground) so lets say a fan runs at 7v... then you decide to hook up another fan to run "12v" with input of 12v and output to -12v (ground) then doesnt it mean
12v (supply) - -12v (ground) = 12v + 12v = 24volts for the fan?

is that even possible or am i confusing myself with differnt voltages for grounds?... are these grounds separated and labeled by differnt voltages? i posted in another thread that it does but someone said it doesnt but i remember in reviews, that teach how to change the molex connectors, say that when you put the new molex on, its important to remember to NOT mix up the wrong grounds because they accept different voltages

thanks
 
nope, you'll never (without a stepup transformer) get more than 12 V out of a molex.

When you wire a 12V line to a 5V, the 12 V line is at, of course, 12 Volts above the ground voltage of 0. When you hook that to the 5V, it is a drop of 7 (like you said), but the 5V is still at 5V above ground. The potential is always going to be either 0 or above, never negative. So, that means at most, you can have a drop of 12V to 0V. in summary, the 5 and 12V rails are defined relative to the ground (arbitrarily 0 V). This means after hooking a 12V fan to a molex (12V to ground) it can be viewed as either a drop from 12 to 0, or if you wanted to be weird, a drop from 0 to -12V. No matter what, you'll never get more than 12 V.

the reviews that say the ground wires are seperate voltages are lying. outright. ever looked inside a PSU? if so, then you may have noticed that every single black wire is attached to the same lead to the PSU. its this little block of black wires in an array that is grounded. Same with all the other molex wires: they all come from the same block (so they are at the same voltage).
 
Ground is 0V. Remember that. All the other voltages(+12, +5, +3.3, etc.) are a potential between itself and ground, which is 0V. There is no -5V or -12V in the drive connectors. I have no idea where that even came from. Ground is ground and ground doesn't "accept voltages". Ground is 0V. All ground wires are at the same potential. You don't have a separate ground for +12V and +5V because all grounds are the same. They are physically soldered to the same pad inside the power supply.
 
Hey wayne, I'm noticing you asking tons of related questions in different posts.

Instead of starting new posts, why not reply to the ones you;ve started?

I am discussing this whole -12V thing in another thread that you are participating in. Please reply there.
 
Originally posted by SarverSystems
Hey wayne, I'm noticing you asking tons of related questions in different posts.

Instead of starting new posts, why not reply to the ones you;ve started?

I am discussing this whole -12V thing in another thread that you are participating in. Please reply there.
Click to expand...
i want to keep other peoples thread based on their questions, not pollute it with my problems... so i thought its much easier and better to ask separate questions so we dont get confused on several topics in the same thread
 
Get a multimeter and test it out for yourself. You'll never see a voltage difference higher than 12v(well, not *much* higher, not 24v anyway). Touch the two terminals to any two molex pins, and you won't see more than 12v. You'll see 12(12v->ground), 5(5v->ground), 7(12v->5v), and 0(ground->ground).
 
I believe -12v and -5v can be found in the 20-pin ATX connector. They usually aren't provided much power, since they're only used on the mobo, so I wouldn't bother trying to hook them up to fans.
 
Ground is 0 V, period--not -5 V, not -12 V--0 V. On the 4-pin disk drive power connector, there are two voltage lines present: +5 V and +12 V. The other two pins on the connector, the grounds, are at 0 V effectively. In other words, there is no potential difference between grounds. Like distance, voltage is a difference between two things. Voltage is a difference in potential--ask your physics teacher if you don't understand this.

On the 20-pin ATX power connnector, there are -5 and -12 V lines. These aren't grounds; rather, they are akin to the +5 and +12 V lines of the disk drive power connectors, except the potential difference is inverted. Connect a -5 V line to the +12 V line and you'll get a potential difference of 17 V across the two lines. Connect the -12 V line to the +12 V line and you'll get 24 V. I don't recommend it though. Most fans intended for computers run at a nominal 12 V and shouldn't be given higher voltages. Also, reverse current flow on the -5 or -12 V lines can be detrimental to the reliability of your power supply, putting all of your components at risk.
 
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