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Old October 19, 2011, 07:42 AM
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Default A few questions about Power Supplies

I'm not sure if I'll be able to get the answers that I'm looking for as they are partially design type questions, but I thought I'd ask anyways.

I was wondering why computer power supply manufacturers use multiple wires to deliver the same voltage level to a device. My own theory is that they do this to allow them to spread the current draw across multiple wires, which allows them to decrease the size of them, rather then using one large cable rated for 30Amps+.

Example of this would be the pci-e power connector, where there are 3 separate 12V connections.

Building ontop of that assumption/question leads me into multi-rail power supplies. In the case of this, do power supply manufacturers ever use multi-rails in the same connector (the 3 12V in the same pci-e power connector) or do they run the multiple rail in separate connectors (2 pci-e power connectors to a gpu would have 1 rail in one pci-e power connector and a second rail in the other pci-e power connector).

Thanks for any insight people may be able to provide as if my theoretical views are correct, my device may have become simplified, which means should be cheaper to build and able to sell it at a lower price point.
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Old October 19, 2011, 08:42 AM
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Honestly I don't know the answer either, but here's my best guess.

a) The wire gauge MAY have originally (or still be) part of specifications for ATX (or other) PSU or
b) power usage was lower on computers therefor small gauge wires were all that were required to supply the components. As power usage grew they just kept adding wires and modifying old connectors to support the higher draw vice starting from scratch and having incompatibilities between old and new.

That said the bottom line is you require a certain gauge of wire to support the draw going through it (or multiple smaller wires) and thick wires would be harder to work with and would be incompatible (to thick to fit) in current connectors. They definitely could run single thicker gauge wire, but it would involve redesigning everything at this point.

As for the rail question I have no idea and will leave that to someone with more knowledge on the subject.
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Old October 19, 2011, 08:56 AM
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Taking a 12v rail as an example, I thought that there were separate wires and connectors to provide power to different parts of the computer component connected to the psu. ie. the 12v rail could be used by the gpu chip for one connector, and the other 12v rail could be used by other components of the video card. I just figured it was a way to help distribute power to a device basically.

Also, some wires in a connector are not used for voltage (ie. the power on wire for the mobo atx connector).
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Old October 19, 2011, 09:18 AM
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There are multiple wires because of the current limits for the wire size. Read up on the ATX spec to discover the actual limit, I don't recall it.

As for your multi-rail question, there are never different rails in the same connector. It's up to the installer to pick which connector and rail to use to which card/device. For example, if I had a GPU that required 25A on the 12V and I had a 2-rail PSU with 4 PCI-Ex6 connectors capable of 20A per rail, I have to make sure that I'm connecting one PCI-Ex6 from each rail, and not both from the same rail. This will either overload the PSU, or cause the card to be unstable.
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Old October 19, 2011, 09:39 AM
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Yeah, well the ATX Specs aren't exactly follow percisely, though they are limited by what the wire gauge will allow them to run current through (as well as the connectors themselves whom are rated at lower max current). I wanted to play the better safe then sorrying question game as to not damage a few systems even though theoretically all they are doing is load splitting (but without seeing actual datasheets for the power connectors and how the power is distributed to the pc part that's using it I was just speculating).

It's also good to hear about the multi-rail, it should mean my reduced number of current measuring method should serve me fine.
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Old October 19, 2011, 03:55 PM
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You need enough copper to carry the current draw for the distance. Multiple thin cables are easier to route than one thick cable. But I think the real reason is compatibility of manufacturing process. A thicker gauge cable needs a thicker connector, thicker crimp machine, different tooling in general. It's easier and cheaper to stick with all the same tooling but just add a few more pins.

When you tie two independent rails of a PS together, bad things can happen. The PS is trying to regulate the voltage of the output of each rail, but due to tolerances etc the exact output voltage may differ by tiny bit (say 12v vs 11.95v = 0.05v difference between two 12V rails). The 12V rail is trying to drag up the 11.95V rail to 12V by sourcing current. The 11.95V rail will try to drag down the 12V rail by sourcing current. It's like an isometric exercise. The power supply fights itself. Worst case is self-destruction. Obviously that's not happening, so they're including something in the design of multi-rail units to account for this. The passive method is series resistance on each rail, so when the rails fight they fight against the resistance instead of each other. Wasteful and gets hot though. Active methods would be better, and is probably what they do. But I'd still not tie rails together unless it was the only way I could make it go. Btw, the same concepts come into play when bridging or paralleling power audio amps, or any time you tie two current sources together.

A video card has a DC-DC power supply onboard to convert the 12V to a lower voltage. Usually at least one for the GPU and one for the RAM. They don't include a DC-DC converter for each 12V line in; the 12V lines are tied together first into a common bus before going into the converter(s). So to put two different rails on the same PCIe power cable would pretty much guarantee they'll be tied together eventually, so might as well tie them inside the PS unit instead and call it a single rail right from the start.

Big fat high-current 12V cables and connectors are common in the Car Audio world.
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Old October 20, 2011, 04:55 AM
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They could do that, but I guess its the fact they'd rather use smaller more pliable cables and run multiples rather the one larger cable.
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