Can anyone explain to me what -5V and -12V is, and what it is that separates this from ground? And where can I get -12 V from the PSU? I am just asking out of curiosity.
-5 volts and -12 volts are just that Their voltage level is actualy below the ground level. They are not used for running anything, but for signal reference. Mainly analog, and RS232 communications. Both exist on the ATX connector; Pinout here: http://xtronics.com/reference/atx_pinout.htm But both do NOT handle much current. Usualy not more than 100mA or so. You PSU should list how many AMPS it can sink on -5 and -12 volts.
So does it matter which one I use?? Because i feed LCDs +5v and use -12v sometimes on the molex connector?? Thanks. God Bless & Kind Regards, Richard
It's +12v, +5v and two 0v wires on the 4-pin drive molex connectors. The transformer secondary winding in your ATX psu is centre-tapped with the centre taken to ground (0v). The winding ends give +12v and -12v, and other taps about midway each side of the middle give +5v and -5v. But it's much thinner wire on the negative ends so the current is very limited.
The output of your power supply will give 12v 5v 3.3v All with a reasonable ammount of power, and -5v -12v at low power (100mw etc) The -5 and -12v are only available on the main lead to the motherboard, the yellow and red leads on a molex are for +12v and +5v with the two black leads being 0v or -ve. I have only seen the -5v lead used once by someone fitting a fly lead from their motherboard to overclock a fan, by using -5v and +12v the were feeding it 17v, but this is not at all recommended.
Aha, now I understand what you were asking. Yes both black wires are 0v / -ve, the all go back to the same ground point in the power supply. I don't know why +12 and +5 have their own -ve return, but they are both ok to tap for 0v.
if u short 12v to 0 v then the wires get a tad warm (if yer psu dosent shut off) if u shorted 5 and 12 to one 0 line it would get really warm and start smoking (probably) also lots of things have to 0 Vs for safety reasons uk plug socket for example . . . i think also it helps if u put the floppy drive power connector in wrong cos u short 5 or 12 v to ground and yer psu will shut off (well mine does) and u dont fry yer floppy drive or live drive EDIT there are some other reasons too but ill let others explain them ;-)
I think the molex connectors are rated at around 5A max. So if the 5v and 12v were both up to the limit, there'd be 10A going back to the psu. Just 1 black wire to carry that would be too thick to go in the molex fitting. And not as flexible as the two thinner wires if they made a bigger molex ground pin. All the wires of the same colour join up in the supply, it's just safety (and flexibility) in numbers.
And just to confuse the issue ... The +12V and +5V rails each have their own ground wires to reduce interaction and hence stability. e.g. if the 5V current fluctuates heavily, (which it can do), if it shared the same ground wire as the 12V, then the 12V rail may also fluctuate due to the heavy ground current causing a volt drop across the ends of the ground wire. This is caused by the cable having resistance. (Another good reason for not having excessivly long PSU cables since the longer the cable, the higher its resistance. The higher the resistance the greater the volt drop). By having separate ground wires which all connect back to a single electrical connection, called a star point, this interaction is minimised since this star point is the single, (or should be), reference ground used to regulate all the voltage output rails.
