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CPU power usage

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June 3, 2008 2:15:32 PM

This is a question regarding the power usage of a processor that has a fixed frequency and a fixed voltage. If the processor is running at Frequency A and the Voltage is B, will the processor use more power when running at 100% load as opposed to 0% load? If this is the case then the only culprit could be that the current is higher with more load that it is at lower loads. Does this make sense?

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June 3, 2008 2:41:10 PM

shawn924 said:
This is a question regarding the power usage of a processor that has a fixed frequency and a fixed voltage. If the processor is running at Frequency A and the Voltage is B, will the processor use more power when running at 100% load as opposed to 0% load?


Yes, in most cases.

Quote:
If this is the case then the only culprit could be that the current is higher with more load that it is at lower loads. Does this make sense?


Yes. Modern CPUs generally use the majority of their power changing the state of transistors, and will often turn off units in the CPU which aren't being used; when the PC is idle the CPU isn't doing much so there's little power usage from state changes, and most units can be turned off most of the time, so that reduces power consumption further.

Of course, in the real world, when the CPU is idle they'll probably drop the clock speed substantially too (my Athlon X2 spends most of its time at 1GHz and only clocks up to 2.6GHz under heavy load). They may also drop the voltage when idle, since power use is typically proportional to the square of the voltage: a 10% voltage drop saves 20% of the dynamic power consumption.
a c 96 à CPUs
June 3, 2008 2:46:43 PM

Amperage is the flow of electric current.

Watts = Amps x Volts, or ...

P(watts)= I(current) x E(voltage)

As the ""draw"" of current increases at 'fixed' voltage the power consumption (or watts) increases.

This is a crappy example but let's say at idle a cpu pulls 1 amp at 1 volt, or effectively 1 watt. At load it pulls 1.5 amp at 1 volt or 1.5 watt.

This is where PowerNow, SpeedStep, C&Q, etc. can save a bit of power by reducing voltage at an idle state ....

Thank you, Dr. Science :bounce: 
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June 3, 2008 2:50:36 PM

Thanks for the response! Do you have any references that would indicate the fluctuation of current in processors depending on the load?
a c 96 à CPUs
June 3, 2008 2:58:04 PM

shawn924 said:
Thanks for the response! Do you have any references that would indicate the fluctuation of current in processors depending on the load?


I don't have a specific link but you could try to google 'cpu amps idle load' for the specific processor you have in mind ...
!