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What rail does an Athlon XP-M 3000+ (2.2GHz) draw power from?

Posted on 2006-10-27
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Last Modified: 2010-04-25
Greetings,

I'd like to know from which rail an Athlon XP-M 3000+ (2.2GHz) draws its power from on a mainboard with 20-pin ATX connection to the power supply unit.

I've read that since the mainboard only uses a 20-pin ATX it would be drawing power from the +5V, on the other hand I've also read that Athlon XPs are completely on the +12V rail.

Thanks,

m
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Question by:Moother
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Callandor earned 300 total points
ID: 17821302
The +12v rail is the more important voltage to check when buying power supplies, because it provides power for the cpu and peripherals.  In the old AT days, the +5v rail was the dominant one, but nowadays, an extra 4-wire connector with +12v is connected to P4 class motherboards, because cpu draw has exceeded the ability of the motherboard connector to provide all that is needed.

See http://en.wikipedia.org/wiki/Power_supply_rail
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by:garycase
ID: 17821382
The actual voltages on the CPU are quite low (~ 1.5v VCore), but are derived from the voltage reduction and regulation circuits immediately beside the CPU socket.   Typically these are "fed" by the 12v rail, since power distribution is more efficient at higher voltages.   On some AMD motherboards (without an auxiliary 4-pin 12v plug) the ATX power supply connection is very close to the CPU, but I'd still expect the CPU voltage regulators to be fed by the 12v rail  ... although I suppose it would be possible to regulate down from the 5v rail.   I'm fairly sure the answer is 12v ... but a simple way to confirm this is to use a multimeter and carefully measure the input voltages to the voltage regulators just beside the CPU socket.
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by:garycase
garycase earned 200 total points
ID: 17821396
... as Callandor noted (and I inferred) the most important rail in modern power supplies is the 12v capacity.   Note that many of the higher wattage supplies now have 2, 3, or even 4 separate 12v rails to ensure there is adequate current capacity for all of the modern demands (multi-core CPU's;  high power GPU's; etc.).
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by:Moother
ID: 17832791
After researching a bit more I found out that unless the motherboard makes use of the 24-pin ATX connector, an additional 4-pin connector or an additioanl 8-pin connector from the power supply it is extremely likely that the CPU/s on that mainboard is/are powered through the 5V rail, which is exactly my case.
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by:Callandor
Callandor earned 300 total points
ID: 17839811
It was my contention that recent power supplies with the extra square 4-wire connector for P4's use the +12v rail.  24-pin motherboard connectors are later models and do the same.
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