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What does the -12v and -5v on a PSU do?

post #1 of 6
Thread Starter 
Hi could someone tell me what the (minus) -12v and -5v power lines do? Are they important?
    
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post #2 of 6
They don't DO anything, just know that the lower it is the better, most PSUs have -3.3, -5, and -12 rails of under 1W and that is considered good, I would not worry about it at all.
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post #3 of 6
Thread Starter 
Feew! thats good then, on speed fan,

-12v is -6.93v
-5v is 0.58v

Mad, well the comp seems ok
    
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post #4 of 6
I wouldn't worry about it too much. I don't trust SpeedFan, mainly because my voltages on my Digital Multimeter and what SpeedFan reported were way off! Anyway, used MBM5 and set some compensation for the readings, and it works great now.
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post #5 of 6
All I can think of is that if its a negative voltage, then it would be a ground that is limited to carrying 3.3, 5 or 12 volts. So in that case the current would be flowing from the motherboard to the PSU. Why? I have no clue.
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post #6 of 6
The negative voltages were primarily used in older, Legacy systems, also, in certain "Hibernate" modes.
The "modern" PSU does not, in fact, "create" negative voltage, but inside circuits, there will be some areas, that a negative V is found.
(example)
Say you want a fan at about half speed-- you can resitance down the input voltage, with a thermistor or rheostat, or you can set up a voltage offset connection from 12 to 5 volts instead of 12 to 0 (ground) and get 7 volts through fan and lower speed and quieter fan. Internals of computer are still analog\\digital blends overall.
(or)
To replace a one with a zero in a RAM bit inside the chip, you have to pull down the voltage. To do so fast, you use inverted voltages internal to circuit. BUT, it is polarity inversion and inverters are not longer needed EXPLICITLY as visible things in PSUs.
Ideally, the negative Voltage, should be equal to, or less that Positive Voltage.

Using resistance, builds more heat than simply offsetting the V, so in a PSU, where heat is already present, it makes no sense to add more, where it's non-productive.
Hope this gives you some insight.
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