[Gllug] OT: Multiple rails in in ATX PSUs

general_email at technicalbloke.com general_email at technicalbloke.com
Thu Oct 13 02:05:39 UTC 2011


I wonder if I understand this correctly...

All new ATX power supplies must have at least two 12v rails. This is so
the motherboard may have it's own 12v power separate from the other
components in a system. There are a number of PSUs on the market which
have three or even four 12v rails. However... the ATX power connector
only has one 12v terminal and this terminal must provide for all the 12v
needs of the CPU, motherboard, memory, CPU fan and PCI bus. That is, by
my rough calculations, the lions share of all the power a modern system
might consume, anywhere between 90w and 280w under full load. The other
components, assuming no powerful graphics card is being used would
struggle to draw 50w.

Doesn't this mean then that the power required of the two rails rail is
almost always massivley unbalanced? I notice the amps available via each
12v rail are normally about the same on most PSUs. If over 75% of the
power has to go via the motherboard's ATX connector why would PSUs be
designed like that? It would mean a PSU that could deliver 300w into the
12v rails can only shunt 150w through the far greedier motherboard
connector and thus would be inadequate to power a system that wants say
only 175w for CPU, mobo, memory, fan & PCI cards and 25w for drives and
case fan - despite the PSU being rated for 100w more than that overall.

Is that how it works or am I missing something?


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