PS Railings and Voltages

danyzuko · May 12, 2014

I understand that railings are for regulation of the provided voltage to perspective components or devices and that a molex connector for instance would carry a 5 volt for idle and 12 volt wire for burning on a dvd writer. But it seems that there used to be a -5 volt provision in the PS railing in the past and I'm just curious why a negative number? I mean, I can't wrap my head around the concept of a negative number of volts, or am I missunderstanding something? I guess this goes back to electrical theory but if anyone can explain this that would be great.

Also, I took a quiz that asked about 120w of power 12 amps of current which comes out to 5 Volts. But the answer also says this can travel down either a red or white wire. So that gives me the impression that in general a PS can have 2 or more 5 Volt wires traveling through a red or white wire. This is out of a Comptia book. But I can't find documentation on the internet to support this. All the power supplies I've seen so far only have one 5 volt wire and it's usually red. I just wanted to find a PS product that supports what the book indicates in real life.

Any links are appreciated.

Faa · May 13, 2014

Also, I took a quiz that asked about 120w of power 12 amps of current which comes out to 5 Volts.

10V*12amps=120W

Also molex just has one 12V wire which is yellow and one 5V wire which is red and 2 black ones that are the ground probably for each voltage. 5V is being used for the electronics like PCB and the 12V for the moving parts like whatever the name was again that moves in a hdd. 2.5" HDD's dont use the 12V rail at all except that veliraptor 10K rpm ones or maybe some 7200rpm ones.

SSD's just run off 5V.

danyzuko · May 13, 2014

thanks Master, I understand the math. My concern is, how many 5 Volt wires that typically come out a power supply? Because the Comptia A+ quiz indicated that there are more than one but in the book it says only one 5 volt. Again on the quiz it says 5 volts can be red or white. So does that mean there can be two 5 volt wires, a read and a white coming out of the power supply?

Also I don't understand why there would be a -5 Volt in the railing scheme.