+12v rail question

dandragonrage said:
CPU should be using +5 in an XP
Click to expand...

depends on mobo implementation
if the mobo has a +12V Auxillary connector (typically 4 pin) its powered off the +12V


http://www.theinquirer.net/?article=11279
When I initially asked about this on a mailing list, Solaris x86 advocate Al Hopper told me I was drowning in my tea, and that it was "all very simple". I love the simplicity of Unix people.

He explained that the later P4 CPUs take their power from a 12 Volt feed and, using the onboard voltage regulators, generate the high current, low-voltages they need to operate (anywhere from ~ 1.6V to around 2.7V). So the first requirement is a PSU that has plenty of power available from the 12V supply. Since the older ATX compatible PSUs didn't supply much current from the 12V section you have to ensure that your new PS delivers enough current (or power in Watts) from the 12V section. That's why using your old P3 ATX PS is a big "no-no".

He then described the history of the post-P3 power supply mess:

The earlier Athlon motherboard manufacturers decided to solve the 12v problem by using an additional 4-pin square connector to get the extra 12V those CPUs required. However many older PSUs didn't provide the 12V 4-pin square connector.

The motherboard makers then wised up and decided that there was nothing magical about a square 4-pin connector, so they put a normal hard disk type socket on the motherboard and provided the 12v power via a standard hard disk (4-pin inline) connector. Problem solved - you may now use your older PSUs provided they supply sufficient 12V current (many did not).

Further confusion came from PSU manufacturers not specifying the capabilities of the PSU in a way that allowed the end user to verify it's 12V power output rating.

In the meantime the ATX spec was saying "use the new 6-pin" inline connector - and very few motherboard makers implemented it.

Just to be sure, some motherboard makers, implemented *both* the hard disk style 4-pin inline connector and the square 4-pin connector. They said "use either or both in any combination you like".

The spec then evolved to the 24-pin main connector. Again, most motherboard manufacturers did not wish to make their customers mad by mandating that they replace their power supplies. This might cause their customers to avoid motherboard upgrades. Some used a "special" 24-pin connector with the extra 4-pin connection blocked off, or colored so that the user could plugin a 20-pin plug into the correct end of the 24-pin socket on the motherboard. Many others simply ignored the 24-pin requirement in the specs. Again - problem solved - use your older PSU.
Click to expand...

of course that mess is now sorted out with the standard 24 + 4 aux (ATX12V v2.01)
 
All of your drive motors, fans, lights, etc.

But in the grand scheme of things, your typical Athlon XP system (which doesn't often have a 2x2 12V connector so you know CPU voltage is regulated off of the 5V rail) still has WAY more of a load on the 5V rail than it does on the 12V.

Drives, fans, lights, etc. are probably only pulling about 8A tops at any given time.
 
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