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-12v leads to GND? Answered

first off, i apologize for asking so many questions.

google couldnt tell me a single thing about what i wanted to know, which was how the -12v is created and why a computer needs it.
so i turned to a broken power supply to see is the -12v connects to a IC of some sort, which i would assume creates the strange voltage, confirmed by a datasheet.

i completely traced the traces with a multimeter and at one point a capacitor went to ground close to where the -12v connection is on the board, after an inductor of some sort. the trace continued, and lead to a pin on the transformer, much to my disappointment. BUT that same trace was directly connected to ground, so now i am confused, does the inductor type piece createthe -12v? there was a diode as well, but i thought nothing of it or its place on the board.. i have a feeling i should have, shouldnt i?

fed up and confused by the power supply, i turned to my motherboard. where the -12v leads to a smd capacitor and then traces from the capacitor, to ground, and nowhere else as far as i can tell with a multimeter and a dark blue motherboard.

then i bring up the picoPSU, how do they make the -12v, if it is even made? is there a chip responsible? or a inductor like i found in the power supply?

-5v has been omitted form power supplies, so im assuming that the same can be done by -12v, right?

and wouldnt you get a -v type measurement if you put the + test lead on the - terminal and vice versa?


The -12V is created by using a center-tapped transformer coil, which has three connections: one at each end and one in the middle. 24V is produced across the entire coil, 12V across each half. You can think of the three connections as either 0V, 12V, and 24V, or -12V, 0V, and +12V, or even -24V, -12V, 0V if you prefer; the difference is just in which of the three wires you connect as the ground. (Actually the transformer puts out slightly higher voltages which are regulated down, "but the idea's the important thing.")

As to what it's used for: Some circuits do want BOTH positive and negative voltage supplies; the reasons get into a level of detail I really don't feel like addressing here. Suffice it to say, having both + and - 12V coming out of the power supply WAS needed originally when PC power supplies were being designed, MAY still be needed by at least some adapter cards, and is part of the spec for a standard PC power supply module and PC bus so you're going to get it whether your machine currently needs it or not.

-5V wasn't needed by any of the circuits, so power supplies weren't designed to provide it. +5V was needed by the standard small- and medium-scale ICs of the time, so it was designed in.

Getting much beyond this level of detail is going to require going into the details of PC circuitry, and as I said I'm just not up for that right now. But I think it's enough to answer the questions you've asked: + and - 12V are there because PCs needed them, and are produced by connecting the center tap of a 24V coil to ground.

you are using "need" as past tense, do you mean that newer motherboards wont need the -12v line?

i have the motherboard from a packard bell (circa 1999-2000) that i was using to experiment, and i cut the -12v line and it was beeping, i reconnected it and restarted the pc and it worked fine.

im not sure if my experiment was relevant, but in the case of using a, perhaps, xbox power brick rated 12v @ 16.5A is there any humanly capable way to get -12v from that?

or does a DDR ram motherboard need -12v? (or does that depend on the manufacturer)

im asking this because im trying again at a small form factor PC, inside of a xbox 360, and i want to not only keep it looking as original as possible, but i dont want to go out and buy anything special.

I honestly haven't looked recently, so I simply don't know. And it's possible the answer could vary from motherboard to motherboard. Contact the motherboard's manufacturer and ask them?

If all you have is a +12V supply: Ugly. To get -12V as well, you would have to first step the 12V up to 24V and then split it as described above. That means using a DC-to-DC converter. AND it has to be able to provide enough current to meet the requirements on both lines, plus the requirements for the 5V (which, presumably, you'd regulate down from 12V). Strikes me as more trouble than it's worth, though I can see the aesthetic appeal.

Laptops certainly manage to work off a single supply. But they're designed from the ground up for that... and the power bricks generally provide something on the order of 16 to 20 volts unfiltered pulsed-DC at about 5 amps, which suggests that the laptop is doing additional power conditioning internally and might be getting more than one voltage out of it.

thanks, i got lots of the info i needed now.

but i still wonder how picoPSU's make the -12v, with just a 12v input?

Simple approach: Generate +24V using a DC to DC converter, then rename things with a -12V offiset:: +12V becomes 0V, 0V becomes -12V, and 24V becomes +12V. Derive +5V from the difference between the upper two, so it's +5V relative to the new 0V reference.

There may be better solutions, but that would certainly work. And it would avoid the inefficiencies of going from 12VDC to 120VAC and then plugging a "normal" power supply into that.

so let me see if ive got this right...

instead of a 12v supply use a 24v supply.

+12v is now 24v
GND is +12v
-12v is GND?

then just take a 5v from the actual 12v line and then step that down again to get the 3.3v?

i think i understand, but with 24v being supplied to the 12v line, wouldnt that damage the motherboard?

Voltages are RELATIVE, not absolute. +12V is 12V above ground. Whatever ground is.

The only time this could become an issue would be if you were hooking up to another circuit which had a different definition of ground relative to some _shared_ potential. Which doesn't occur since the PC's being powered in isolation.

so your saying so long as i find a 24v power source with high enough amperage, i can theoretically make a PC compatible power supply?

Yes. Should just be a matter of designing and using the right regulators.

And apparently at least some laptops' power supplies work happily from 20V, since that's what comes out of the power brick for mine. But that's a machine explicitly designed for that specific power supply and vice versa, rather than being constrained to the ISA standards.

all right! well that is good news for me, now its just a matter of finding a high amperage 24v source.

i have a stereo that has a transformer that outputs 24v, but im guessing that even though its just the transformer the amperage is kind of low, isnt it? or does all the extra circuitry define the wattage of a power supply?

my laptop uses 19v, and i have a old powerbook 190 that uses 24, but i think 90W is a bit low for running a standard PC motherboard.

if i can use the transformer from the stereo that would be great, since it would be a big learning experience and it would be omething cool to do.

just one question about the voltages, so if +12v is now 24v, and GND is +12v, and you said that +12v would have to be 12v higher than whatever the ground voltage is, does that mean that my 5v line would technically have to be 17v? and what about prephirals like the cd drive, hard drive and usb devices?

my cd drive and hard drive use only 5v, and they have rather low power requirements, but when searching for a suitable supply, i should keep in mind the amps required for the components, right?

and if thats the case, i would need to look about the cpu, cd drive, hard drive, but i dont know about the motherboard.

5V is 5V above ground, 17V about -12V.

Wattage of a power supply is a combination of what the transformer can provide and what the voltage regulation circuit can provide. Whatever's lowest gets cooked first if you push it too hard.

wait a second, you said voltage is relative, so that means that if i were to use something like a 7805 with 12v being ground and 24v being +12v, it would create the 17v right? because its stepping the 12v down to whatever 5v above ground is, right?

please tell me i got that right..

(or is simply unable to deliver the voltage you're requesting.)

so, my 5v line would be 17v? and the 3.3v line would be... 15.3v, what would i use to get those voltages?

I still remember way back when dynamic memory needed -12V


It's all relative, -12V is relative to 0V, but your multimeter will read +12V if you connect it the other way around. Similarly -12V to +12V will read 24V.


well out of curiosity i had gona and tested the -12v line both ways, with the red lead on -12v and the black lead on GND, it read -12v, but the other way around it read 0v.. i am confused..

Yeah, you're confused. Either you had the test leads connected incorrectly, or you're using an analog meter which doesn't read below 0.