Quote:
Originally posted by SysCrusher
I figured I get something wrong. Does heat effect electrical conductivity? Know that I got it way off topic. I stand corrected though and I didn't look at that way.
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Yes. Copper has a TCR (Temperature Coefficient of Resistance) of about 0.4%/C.
So if you raise the temperature of a copper wire by 10C the resistance of the wire will increase 4%.
Quote:
Originally posted by Cathar
Now this is something that has often played across my mind.
Electrical resistance rises as a conductor/semi-conductor gets warmer. Electrical resistance is the main source of heat from CPU's.
If a more efficient water-block (or heatsink, or whatever) keeps a CPU cooler, then will that CPU dissipate less heat due to the lower electrical resistances due to it being cooler?
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I don't know the TCR of CMOS transistors and in any case I suspect it varies greatly with the doping of the particular IC. (A quick google didn't help.) I did find a datasheet for an N-channel MOSFET that indicated the TCR for the transistor was 1%/C but I have no idea how representative that is.
The TCR of copper will get you a 2% lower resistance in the copper interconnects if block 'W' gives you a 5C lower temp than block 'X'. Not a big difference. Also the copper resistances will be small compared to the semiconductor resistances, so I think the copper resistance can be ruled out as a significant factor. The semiconductor resistance may well be much more significant.
Another area where better cooling may have an impact is transistor leakage. As the transistors used in CPU's get smaller and smaller, they tend to leak more current even when they are shut off. Leakage in MOSFET's generally increases with temperature, so a cooler processor is probably consuming less power due to lower leakage current. Leakage tends to be very nonlinear with temperature though, so I don't have any prediction of how significant this may be.
It would be easy enough to test this. Leakage current is idle current. Just connect the proper supply across the VCC and GND pins of a sacrificial processor, ground the clock input, and measure the current draw as you vary the processor temperature. (If one had a CPU socket off a MOBO, it wouldn't even be necessary to sacrifice a processor.) This may be listed in the manufacturer's data. I've never looked.