Calculate heat dissipation from processor

[no183]

Hey guys. i am on a little project here, may i know hot to calculate heat dissipation from processor and HDD ? is it using P=I^2R will do ? if so what is my R and I?

 

[berkeman]

Hey guys. i am on a little project here, may i know hot to calculate heat dissipation from processor and HDD ? is it using P=I^2R will do ? if so what is my R and I?

The better way is to look at the power supply current and voltage for the parts you are interested in. Get their power dissipated from their input I and V.

 

[no183]

hmm, let say a processor, while it has the state of load and idle, how do i calculate it?

 

[berkeman]

hmm, let say a processor, while it has the state of load and idle, how do i calculate it?

Look it up in the datasheet for the processor.

 

[no183]

hmm, where do i need to get that btw?

 

[russ_watters]

From the manufacturers website - it can't be calculated from scratch.

 

[no183]

http://processorfinder.intel.com/details.aspx?sSpec=SLA9U
my example as stated in the link, so is 65W?

 

[berkeman]

http://processorfinder.intel.com/details.aspx?sSpec=SLA9U
my example as stated in the link, so is 65W?

Wow, that's a lot of power. But yes, that does appear to be the maximum allowed power dissipation. Lordy.

 

[russ_watters]

Yeah, Berke - that's the reason PC processors have basically stopped getting faster. People aren't going to buy a pc with a water cooled chiller attached to it.

 

[klipper76]

Based on my time working as an engineer working for http://www.coolitsystems.com/" , here's what I've learned:

This is/ can be actually really difficult to determine depending on what your reasons for wanting to know what the power of a proc. is.

If you don't need a terribly accurate guess, say within 15% going by the thermal design power (TDP) of the processor will probably be fine. Note, this number will only be valid when the processor is loaded at 100%, even then the actual power will be a little bit lower than the TDP.

If, however, you need a more accurate power or you have overclocked the processor, or it isn't running at 100% then you need to get crafty.

You can get a more accurate measure of the power of the processor by probing the current going into the EPS rail of the motherboard. There will be a number of yellow and black wires going from the power supply to the motherboard near the processor. You need to cut the mesh sleeve of the wires and use a clamp meter on the yellow wire, make sure you get all of them and none of the black wires (this is the reason for cutting the sleeve). This supplies 12V to the switching power supply that steps to voltage down to about 1.3V for use by the proc.

Of course, this number won't be the actual power going into the processor, you need to account for inefficiencies in the aforementioned switching power supply, say 85%-90%. This will get you to within about 5% of the actual power dissipation.

If you need to get any more accurate than this you would need to probe the current of every single ground pin on the processor. This is not a practical solution.

Depending on the processor expect power level from about 50W for a low power desktop processor all the way up to 220W for a high end desktop processor.

 

[no183]

looks like i found the correct person, is like this, i want to do a finite element analysis on heat transfer at the motherboard. the heat dissipated involve processor, N/S Bridge heat sink, RAM and VGA card, other negligible.. any idea?

 

[klipper76]

That's a stinky problem.

The only thing you can do is take the TDP values for each of the parts independently. Unfortunately, there isn't really a good way to measure the actual power of each. You can however measure the power of every element of the motherboard together. To do this, cut the sleeve on the large motherboard power connector and use a clamp meter on all the red, yellow and orange wires in the bundle. The red ones are 5V, yellow is 12V and orange is 3.3V. You have to measure the current of each rail individually to calculate the total power.

The VGA can be done in a similar fashion as the processor if it is a medium to high power card, there will be a cable going to the end of the card furthest from the I/O (it might also be on the side near the end). Cut the sleeve on that and probe the yellow wires. Again, they will be 12V. For a low end card (that doesn't have this cable) all you can know is that it will be less than 75W (the power limit for a PCI-E bus)

Sorry I can't offer anything more than that though.