# What t is the difference between the -12v and +12v, -5v and +5v...in a computer power supply?

I am building an AC to DC power supply based on several Ibles and I am not sure I understand what the 2 different voltages are (-12v, +12v; -5v, +5v ....)
Thank You!

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8 years ago
Electrons flow from - to +.

Voltage is potential difference - how much energy is behind each electron trying to get from the cathode (- charge) to the anode (+ charge)

*SIDENOTE*
Conventional Current - a carryover from early electricity (before they knew that it was - electrons moving) thought that there were POSITIVE particles moving through the wires, so 'conventional current' flows from + to -.  This only comes up in circuit diagrams where anything with an arrow on it shows the direction that conventional current flows, even though we know today that the electrons go the other way.
Current always tries to flow to ground.  Ground is generally considered the - side of a circuit, but not always.

Computer power supply:  Electrons flow from - to +...
Ground is Zero volts,
Electrons will try and flow from -12V to ground.
Electrons will try and flow from ground to +12V

If you hook +12 to -12, electrons will flow from -12 to +12 at a potential difference of 24 volts.

If you hook +12 to -5 you get 17 volts, flowing from -5 to +12...
etc etc
+12 to +5 has a difference of only 7 volts, and will flow from +5 to +12. (from the more negative to the more positive.
missplumeau (author) 8 years ago
Thank you Framistan and Frollard,  Now I can get back to building that supply!
Joe, Every power supply ' Ible I have found seems to indicate just that.
Now to get  9 V,   I jump +3v to +12v correct?
8 years ago
Voltage-regulator in this case I say.

L
5 years ago
hi ,.
i need an 10 v supply.
someone suggested me to use an 7805 and connect another 5 volt to its gnd pin my circuit worked but i wanna know how ?
regards
7 years ago
You could, but the 3v line is only rated for less than an amp; even though the 12 volt line could easily source enough current to knock your socks off, the 3v line would explode if you ran too much current to it;

As lemonie suggests, use a voltage regulator.  (google it, or search instructables).  It burns off excess power to ensure proper voltage regulation...or get a 9 volt power supply.
blkhawk8 years ago
The motherboard requires only 5 volts to work. That is why you need an electrostatic discharge wrist band to avoid frying the motherboard. A jolt of electrostatic discharge can be 30 volts, more than enough to destroy any motherboard. This explains why many times people open up their computer cases to work on them only to discover that their computer mysteriously stopped working. If you do not have ESD protection on hand you should touch the chassis of your computer to equalize your electrostatic potential.
The drives in your computer require 12 volts to work. The drives require more power to move all the motors and servos.
The job of the power supply in your computer is to "step down" the 110 V voltage (or 220 V if you live in Europe) to a voltage that your computer can use. Also the power supply "transforms" the alternating current in your home to direct current in order to be used by your computer.
framistan8 years ago
The metal CHASSIS of the computer is GROUND.  but the voltage coming from the MINUS 5volts is just reversed polarity from what you expect.  Just like some very OLD AUTOMOBILES from japan years ago... the CHASSIS of the car was POSITIVE and all the wires going to everything in the car was NEGATIVE WIRES.  It is a completely separate voltage source that  is NEGATIVE with respect to CHASSIS GROUND.  If you built a power source that was POSITIVE 5 volts, you would use a regulator called a "7805" regulator... if it is MINUS 5 volts, you would use a "7905" regulator.  If the regulator is made by SIGNETICS, then the full number would be uA7805 or uA7905.  You can see that the 1st letters of the part number indicate MANUFACTURERS name...   The last two numbers indicate the VOLTAGE. .. etc etc.. Think of it like a car that has 2 twelve volt batteries in it.  One battery has MINUS connected to ground. The Other battery has PLUS connected to ground.  by "ground" i mean chassis.
8 years ago
I think you summed it up pretty well.
Joe Martin8 years ago
I believe that each voltage has a separate negative, hence -12V  or -5V

Someone please correct me if I'm wrong! :)