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PC Power Supply Discussion Troubleshooting and discussion of computer power supplies

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  #11  
Old 11-25-2009
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Quote:
Originally Posted by blivit4 View Post
By that logic the CPU power would also be derived from 3.3v, but it isn't.

DC-DC converters aren't resistor divider networks, or basic 3 lead regulators that dissipate a lot of heat.
Power is still power, so as voltage goes up the current goes down for a given number of watts.
The lower the difference between input/output, the higher the efficiency of DC/DC conversion.
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  #12  
Old 11-25-2009
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If you don't use many HDDs you don't have to care about those two rails.
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  #13  
Old 11-25-2009
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Quote:
Originally Posted by blivit4 View Post
By that logic the CPU power would also be derived from 3.3v, but it isn't.

DC-DC converters aren't resistor divider networks, or basic 3 lead regulators that dissipate a lot of heat.
Power is still power, so as voltage goes up the current goes down for a given number of watts.
No, cpu gets the majority of it's power from the 4/8pin cpu power connector, memory gets it's power from the 20(+4) connector.

I'm pretty sure the cpu and memory have different VRM's. At least on my old ECS RS482-M754 it did when a MOSFET for the ram exploded when attempting a overclock on the ram. CPU was stock 2.2ghz.
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  #14  
Old 11-25-2009
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Yes but you can toast CPUs with an integrated memory controller with too high memory voltages.
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  #15  
Old 11-25-2009
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Quote:
Originally Posted by 370forlife View Post
I'm pretty sure the cpu and memory have different VRM's. At least on my old ECS RS482-M754 it did when a MOSFET for the ram exploded when attempting a overclock on the ram. CPU was stock 2.2ghz.
Of course the CPU and RAM have their own VRMs. They run on completely different (and independent) voltages.
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  #16  
Old 11-25-2009
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Quote:
Originally Posted by 370forlife View Post
No, cpu gets the majority of it's power from the 4/8pin cpu power connector, memory gets it's power from the 20(+4) connector.
The CPU gets its power from the VRMs. The CPU VRMs get their 12v from the 4/8 pin power connector. The memory VRMs get their power from the 20/24 pin connector. It doesn't matter which connector is used if it's all 12v. The fact that something blew on the memory VRM could have no effect on the CPU VRMs even if they both use 12v.

The most probable reason for using 12v to derive the CPU voltage is that fewer pins/lighter gauge wire is needed to go between the PS and the MB. If 5v were used, the the current on the cables would be more than double. This matters because there is resistance in the wires between the PS and the MB and on the traces on the MB running from the connector to the VRMs. The result is that the voltage drop due to resistance would more than double.

For example, assume .01 ohm of resistance and the CPU is using 120w.

From W = V x I, I = W/V. Therefore: 120/12=10A
From V=I x R, 10 x .01 = 0.1V drop

With 5V, the voltage drop due to conductor resistance is 0.024V.

A 0.01V drop is 0.08% of 12v, but a 0.024 V drop on 5v is 0.48%.

Whatever efficiencies are gained by keeping the input and output voltages closer are counterbalanced by the higher cost of heavier gauge wire/larger traces on the MB.
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  #17  
Old 11-27-2009
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Using +12V line for power hungry circuit can also reduce the loss across PSU cables due to I2R. When powered from higher voltage, it requires lower current.
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