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Power Supply questions

post #1 of 11
Thread Starter 
Hi

In preparation for my first overclock, I've been researching how power supply units work to better understand what I'm doing. Some of this will probably sound stupid- I'm very new to this, sorry!

So far I've discerned that a PSU basically converts AC current from an electrical outlet into DC current for use in the computer. The power is then divided into three "rails." The 3.3V rail, the 5V rail and the 12V rail.

But I'm confused as to what the rails are, and what their purpose is. Are the rails the separate wrapped bunches of cables coming out of the PSU? There are certainly more than three of them. Which rail does the 24pin motherboard connector use? What about the 8pin supplemental CPU connector?

Some of the cables have more than one connector on them, for instance the cable I'm using to power my HDD and Optical Drive has multiple connectors going down the length of the cable. I've plugged one into each drive. Is it a better idea to use a separate cable for each drive? Or will they each receive the correct amount of power regardless?

Thanks!
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post #2 of 11
The "rails" are just wires that carry different voltage. Different devices use different voltages. Your RAM is powered by the +3.3V rail. Hard drives are powered by the +5V and +12V. The motherboard may be powered by either the +12V or +5V or, usually, both. The CPU and your graphics cards are powered by the +12V and make up the bulk of the system's power consumption.

There's also the +5V standby (+5VSB) which provides power to your system when it's off or in standby mode, and powers your USB devices. There's also the -5V and -12V. Neither are used today; -5V was removed from the ATX12V spec in 2003 (so any PSU that still has it is obsolete) and the -12V will probably be removed from the spec in the next couple years.



Cables are bundles of several wires that carry various different voltages, and sometimes do special functions. Generally:

Black - Ground (needed to complete a circuit)
Yellow - +12V
Red - +5V
Orange - +3.3V
Purple - +5VSB
Green - PS_ON (power supply on)
Gray - PW_GD (power good)
post #3 of 11
Thread Starter 
Ohh, thanks! +rep. So does using two devices on one bunch of cables affect the amount of power each device is getting? Or are the bunches composed of sets of each wire for every connector? (i.e if a bunch of cables for devices that need one 3.3V wire and one 5V wire has two connectors, are there two 3.3V and two 5V wires inside- one for each connector?)

Another question, my BIOS has a little graphic with the three rails on it, like this:



What is this showing exactly? Why are the values for all three rails higher than their namesake? (i.e 5V is showing 5.120V)
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post #4 of 11
Quote:
Originally Posted by Rokabud View Post
Ohh, thanks! +rep. So does using two devices on one bunch of cables affect the amount of power each device is getting? Or are the bunches composed of sets of each wire for every connector? (i.e if a bunch of cables for devices that need one 3.3V wire and one 5V wire has two connectors, are there two 3.3V and two 5V wires inside- one for each connector?)

Another question, my BIOS has a little graphic with the three rails on it, like this:



What is this showing exactly? Why are the values for all three rails higher than their namesake? (i.e 5V is showing 5.120V)
Im gonna go out on a limb here,

I'm guessing the "rails" are over volted like that to compensate for the resistant of the lines/connections.

keep in mine this is my car audio side coming out but this is more so basic principle of wiring then anything.
    
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post #5 of 11
Strange,it says you have an i5 2400.
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post #6 of 11
Thread Starter 
Quote:
Originally Posted by djsi38t View Post
Strange,it says you have an i5 2400.
google image. My BIOS looks similar
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post #7 of 11
Software and BIOS monitoring tools are usually inaccurate. If you want to check the voltage your PSU is delivering, use a digital multimeter. A basic one can be had for $4-$20.

As for chaining multiple components off one cable, if it wasn't fine to do they wouldn't put multiple connectors on one cable.
post #8 of 11
Thread Starter 
Quote:
Originally Posted by Phaedrus2129 View Post
Software and BIOS monitoring tools are usually inaccurate. If you want to check the voltage your PSU is delivering, use a digital multimeter. A basic one can be had for $4-$20.

As for chaining multiple components off one cable, if it wasn't fine to do they wouldn't put multiple connectors on one cable.
Ahh, ok.

Thank you all for the help, I understand PSUs much more now, I think. +rep!
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Rokas' PC
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post #9 of 11
Quote:
Originally Posted by Rokabud View Post
Ohh, thanks! +rep. So does using two devices on one bunch of cables affect the amount of power each device is getting? Or are the bunches composed of sets of each wire for every connector? (i.e if a bunch of cables for devices that need one 3.3V wire and one 5V wire has two connectors, are there two 3.3V and two 5V wires inside- one for each connector?)

Another question, my BIOS has a little graphic with the three rails on it, like this:



What is this showing exactly? Why are the values for all three rails higher than their namesake? (i.e 5V is showing 5.120V)
Electricity works like this:

Here's an example,

If you are browsing the internet, your computer will not need very much power to do this, so therefore it does not draw any more power than it needs to.

If you are playing an intense 3D game, say Crysis, and your computer is working hard, it will draw much more power than if you were just browsing the internet.

In electronics, the load (your computer), only draws as much power as it needs. If it only need 100W, it only draws 100W. If it needs 800W, it draws 800W.

I'm not sure exactly what you are saying...But just some theory.
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post #10 of 11
Thread Starter 
Quote:
Originally Posted by TheLaw View Post
Electricity works like this:

Here's an example,

If you are browsing the internet, your computer will not need very much power to do this, so therefore it does not draw any more power than it needs to.

If you are playing an intense 3D game, say Crysis, and your computer is working hard, it will draw much more power than if you were just browsing the internet.

In electronics, the load (your computer), only draws as much power as it needs. If it only need 100W, it only draws 100W. If it needs 800W, it draws 800W.

I'm not sure exactly what you are saying...But just some theory.
Yes I think I understand watts/ current. Amps and volts is where things get fuzzy.

Correct me if I'm wrong, but I understand it like this (I think I read this analogy on wikipedia,)

If electricity were water flowing through plumbing pipes, the amps would be the rate the water was moving and the volts would be the water pressure. I was confused when I read that analogy because I figured that electricity would travel at the speed of light. I'm hoping that's not an incredibly naive assumption =S

Also in the analogy, the wattage/current is analogized to the diameter of the pipe carrying it. But this doesn't make since, dice I've also heard that wattage is a product of the amps and volts (amps * volts= watts, correct?)

Also, it seems to me that in the case of amps and volts in that analogy, the voltage (water pressure) would have a direct result on the amps (rate of flow). But nothing like that is mentioned, so im assuming they're totally independent.

And, in the context of computers/ overclocking, I haven't heard amps mentioned at all. Just voltage and wattage. Why is this?
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Rokas' PC
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Hard DriveOSMonitorKeyboard
1TB WD 6GB/s Windows 7 Home Premium x64 24" 2ms latency 1920x1080 Logitech G15 
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