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I'm looking to learn more about power requirements and such that a Power Supply/Computer work with.

I see this image

diagram

and it has a lot of different pin layouts for various things and such.

For a few projects of mine I'm looking to connect up to the power supply.

I've heard I only need 1 12V+ and 1 ground, but I also need a 5V+ and a ground for another part of the project.

I'm curious if it matters where this is taken from on the whole?

I'm also curious, looking at that 24-pin connector, why the MB needs all of those values? 3.3v, 5v, a bunch of 12 volts....

Does that all get split into individual things or...? I see the VGA connector is 3 12V, and 5 grounds, so I'm curious if individual parts of the GPU require 12V+ or if it requires 36v total, or whatnot?

Essentially I'm just curious how power works, how you know how much voltage, and amps and such you need to power your devices, and all that stuff with connectors and such, to make sure you have the proper power inputs and such?

Any advice is appreciated, thanks a lot!

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  • \$\begingroup\$ All of the voltages are referenced to the ground (black) cables. So you couldn't wire up 3 12V lines to get 36V. \$\endgroup\$ – Tom Carpenter Dec 7 '15 at 17:44
  • \$\begingroup\$ Make sure you have the minimum required load on each of the lines or the PSU won't work correctly. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 7 '15 at 17:48
  • \$\begingroup\$ Thanks guys. So each of the 12V will go to something that needs 12V or..? I'm just trying to figure out how the end connections are. I guess, at a lower level, I'm really interested in what each part does, and how it powers, gets power, etc. I guess this is more basic electronics type stuff though. Also, what's the "min required load?" Thanks! \$\endgroup\$ – XaolingBao Dec 7 '15 at 19:17
  • \$\begingroup\$ There is no VGA connector pictured. There is a PCI-e power connector, which is often used to run a graphics card which you may be mis-naming. \$\endgroup\$ – Connor Wolf Dec 8 '15 at 4:02
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As Passerby says, the main reason why there are several wires for any given voltage is that a computer may require tens of amps, so one wire by itself isn't enough.

The reason for all the voltages is largely historical. For many years, the standard voltage for digital logic circuits was 5V. Many circuits still run at 5V.

However, later chip designs could be made to work at lower voltages, and would run cooler when powered at a lower voltage. So 3.3V was added to the power supply.

+12V is mainly used for powering motorized things, like disk drives and fans. It's also needed for RS232 serial ports. RS232 also requires -12V, so power supplies often have a low-current -12V line as well.

The 5Vsb line is a "standby" power line, to keep some parts of the computer powered on while the computer is in "sleep" mode.

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  • 1
    \$\begingroup\$ Your understanding of PC power systems is somewhat out of date. The voltage of components didn't stop at 3.3V it kept going down. Below 3.3V it starts to get impractical to run supplies all the way from the PSU. Once you start having to have converters on the motherboard it's most efficient to run them off 12V. Therefore modern PCs draw most of their power on the 12V rail. \$\endgroup\$ – Peter Green Dec 8 '15 at 3:55
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An ATX power supply uses multiple wires for the same voltage, to increase its current delivery capacity. And because using multiple if the a smaller gauge of wore can be cheaper than a single higher gauge of wire. The same is done for the VGA connector.

The Layout is chosen to allow easier routing of power on a circuit board, minimizing the need to jump over or bridge traces from one side to another.

No, the voltage needed is not combined for 36 Volts.

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  • \$\begingroup\$ So each of the 12V will go to something that needs 12V or..? I'm just trying to figure out how the end connections are. I guess, at a lower level, I'm really interested in what each part does, and how it powers, gets power, etc. I guess this is more basic electronics type stuff though. Essentially I'm trying to see how I could utilize a PSU for various projects. Currently I need to figure out something for a new project. The info I was given was "I need to know if pc power supply gives 120ac or 12vDC power, cause it must be a dc converter." Idk what they mean... Thanks! \$\endgroup\$ – XaolingBao Dec 7 '15 at 19:17
  • \$\begingroup\$ VGA connectors use multiple grounds to lower the ground impedance, because they carry fairly high-frequency analog signals. It's not quite the same as doing it for current-carrying reasons. On the other hand, there are no VGA connectors pictured. There is a PCI-e power connector, which is generally used to run a graphics card, but that's not a "VGA" connector. \$\endgroup\$ – Connor Wolf Dec 8 '15 at 4:01
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Every wire is essentially a really small resistor, so if you put allot of current (tens of Amps) through it you will get losses in the wires. It is easier to use multiple wires to reduce the combined resistance of the wires than one big one since those are usually less flexible and harder to route in your PC.

The PCIe plug might have extra Ground Pins because a GPU can also receive extra power over the PCIe bus (3.3V and 12V) from the mainboard.

If you just want to power something little then just pick any 12V and GND Pin. If you start to draw serious power from it, say more then 10A then connect to multiple 12V and Ground.

Sidenote: you have to turn on the PSU by connecting the PS_ON pin with ground to get 12V.

For 5V you don't even need to turn on the PSU, you can get standby power from 5V_SB, usually up to 5W.

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