12v Connector?

[Wag]

I just got a KN72 Delta-L mobo and I noticed it has both a ATX and 12v connector. All the other socket-A mobos I've own had only ATX- is this normal?

My Antec has both connectors- the manual lists the 12v as for the CPU- I should use both, correct? Why do the other mobos only have ATX?

 

[mechBgon]

You're correct, you want to use both. The ATX12V plug provides (surprise) two additional 12V and ground wires to supplement the main ATX cable, which has one 12V wire.

My understanding is that motherboards traditionally would power the voltage regulators from 3.3-volt power. By changing the design to use a 12-volt supply line instead of 3.3-volt, the motherboard manufacturer reduces the amount of amperage that must pass through the sometimes-not-perfect connection between the motherboard and the ATX12V cable. Reducing the amperage also reduces I²R losses (heating in the wires increases with the square of the current I).

So older boards that weren't built to draw heavily on 12V probably would have nothing to gain by having an ATX12V connector, while newer ones that are built for one, will be relying upon its presence. Or that's how I interpret the facts as I know them. Maybe someone with more board-design expertise will speak more about it.

 

[Peter]

Traditionally, CPU power has been regulated off 5V. (Even earlier, when CPUs were still 3.3V, they were powered directly off 3.3V. But these are days long gone.)

Intel, facing the steep power increase in Pentium 4, decided to move the CPU power regulators off the already highly loaded 5V rail onto the 12V rail to balance things better - with the decline of 12V power drawn by storage drives over the two decades we're talking PC, there has been less and less load on 12V, so this was sort of an obvious choice.

The extra connector exists because the ATX plug has very few wires for 12V, and these would be overloaded with the high current drawn by the CPU regulators.

Mainboard designers still were sort of free to choose what to do - CPU power on 5V rail and have an ATX plug only, CPU power on the common 12V supply using the extra plug just for added wires, or, the Intel way, CPU power on a completely separate 12V circuit that exclusively uses the 4-pin plug.