Where does the power go?

Clopedion

Junior Member
Jan 23, 2002
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My computer has a tendency to warm up my room, and I looked up the data on power consumed by the CPU to find out how much underclocking would change that. It surprised me, then, that the power used by the CPU is about 50 W. Since my power supply provides about 300 W, how is that typically distributed among computer components?
 

Evadman

Administrator Emeritus<br>Elite Member
Feb 18, 2001
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Just because it provides 300 does not mean that all 300 are used. Also, that 300 is output power, not input. Some is lost in the conversion process, which is why the PWS has heatsinks and a fan.

The CPU is just the largest single user of wattage. It's not 50, more like 80 or so ( depending on OC'ing and make ) :)
 

Mark R

Diamond Member
Oct 9, 1999
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For a low-end computer (e.g. basic celeron), the distribution is something like this:

CPU: 15 W
HD: 5 W
CD: 5 W
Graphics: 5 W
Mobo: 10 W
RAM: 5 W
Other periphs (e.g. kb/mouse/modem/sound): 5 W

PSU: 20 W

Monitor: 50 W

Remember, that a rating of 300 W on a PSU, does not mean how much power it takes, rather the maximum amount of power that it can supply. A PSU takes a small amount of power for itself, roughly dependent on how much power is needed from it.

For a higher end PC, e.g. fast Athlon with fast graphics, etc.

CPU: 20 - 70 W (depending on CPU load)
HD: 10 W
Graphics: 15 W
Mobo: 10 W
RAM: 5 W
Other periphs (e.g. kb/mouse/modem/sound): 5 W

PSU: 45 W

The other thing to note about fast CPUs, is that their power consumption is significantly higher when they are working, than if they are just sitting idle - this means that your PC is using more electricity and producing more heat, when running quake, or seti@home, then if it is just sitting at a blank windows desktop.

Monitor: 100 W
 

Jerboy

Banned
Oct 27, 2001
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All the energy your PC sucks in turns into heat and is dessipated into your room.
 

RSMemphis

Golden Member
Oct 6, 2001
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Mainly to make sure you have enough power on the 3.3 V line.

Which is why Dell can give you an underpowered power supply - they have enough juice on the 3.3 line, sometimes feeding an extra one onto the mobo.
 

454Casull

Banned
Feb 19, 2002
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<< All the energy your PC sucks in turns into heat and is dessipated into your room. >>


Some is converted into kinetic energy.
 

Elledan

Banned
Jul 24, 2000
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<< Heat is kinetic energy. >>


kinetic energy - energy which a body posses by virtue of being in motion. Compare with potential energy.
 

alpineranger

Senior member
Feb 3, 2001
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Temperature is a measure of the average kinetic energy of a sample of particles. Therefore heat is kinetic energy. In this case, we're talking about the kinetic energy of the molecules that comprise air.
 

Evadman

Administrator Emeritus<br>Elite Member
Feb 18, 2001
30,990
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<< Some is converted into kinetic energy. >>

Forgive me, but I have never seen a computer fly around once powered on :) Your talking about molecular kinetic energy, not unit :)

<edit>
Man, I love physics.
 

interchange

Diamond Member
Oct 10, 1999
8,031
2,886
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Actually, you're all wrong about heat in a Thermodynamic sense.

Heat transfer is an energy transfer method across boundaries of a system. Heat is not an energy in itself.

Kinetic energy is the energy in the motion of an object with a particular velocity. Since your computer has no associated velocity (it's stationary) it's kinetic energy is zero.

Internal energy is the energy of an object due to chemical forces, nucleic forces, and the random motion of particles which contributes to a net velocity of zero on an object. That small-scale motion is what you are referring to as heat and temperature.
 

JingP

Member
Mar 13, 2002
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Question:

I want to reduce fan noise on my PSU. I'm quite confident that All these different designs (pabst...etc) are pretty samiliar so I'll try to do it myself without buying another fan.

the current fan moves about 8.3 cfm, so I figure if I reduce it about 50%, the Decibels will also be reduced a samiliar amount. Seeing that decibels is a log function, i'm guessing a 10dba from original 28 reduction will result in 50% noise decrease, making it virtually inaudible.

So given that, i'm setting out to put a resistor on the 12V to make it around 7V or so. I'm pretty sure i'll trip to start up. Question is:
(i know i can use the 12-5 trick, but I want to use a resistor because i'm a nerd)

what kind of resistor do I need, i haven't had physics since high school but this is what I remember:

I'm guessing because V=IR, and if I want (7/12) of the current, I need 12/7 of the resistance...right...right.

to find the current resistance, I take P=IV, 2.4W=12V*I, and find that the current is 2.4/12 A.

So I take that and plug into V=IR, to get R=144/2.4.

So thats the resistance of the wire Right...?

So I just take 12/7 of that and subtract 60 and get my new resistor right?
it comes out to 42 ohm. So a 40ohm resistor would do the trick right?

(these are all questions in cause you don't know)

But my question is...if the wire itself is a resistor, it should dissapates heat itself right?

So my initial calculation based on the power consumption of the fan is somwhat faulty...my question is...how negligable is this number.

my intital equation should have been (input power)=heat disappation+transferred energy to the fan.

Heat dissapation of a resistor is P=I^2*R and the transferred energy should be p=IV
so the sum should equal 2.4W

Using this function, I get the R is only 30 ohm, so my new resistance should only be 15 ohm???

Can someone verify this for me?
 

jteef

Golden Member
Feb 20, 2001
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200mA is an awful lot for an 8.3cfm fan. Try to avoid using power ratings as they are largely unreliable.

standard 5% resistor values dont come in 40 ohms. you can get 39, 43 or 47 which of course is not a problem.

if you have to buy in groups of 5 or 10, I would buy 10 ohm resistors to give you more flexability. If that current is correct for your fan, you'll be dissipating over a half watt in that one resistor so be sure to use a 1 or even 2 watter or parallel larger values. the resistance of the wire is unimportant.

jt
 

Freejack2

Diamond Member
Dec 31, 2000
7,751
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Actually systems these days are pulling a lot more than 45-100 watts.
My system chews up over 300 watts.
Hard drives do close to 50 watts.
Cpu is probably 80+ watts.
Graphics 15-30 watts (gf3 ti200)
ram 10 watts (3 sticks ddr)
mobo 10 watts
other 20 watts

These are approximations and I suspect I'm estimating low as my system can't run with just a 300w power supply.
Monitors though should hopefully be on the fall with flat panels. My current monitor pulls 175watts, I suspect someday when I replace it with a flat panel, it will require a lot less juice...
 

Clopedion

Junior Member
Jan 23, 2002
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Your first calculation was correct assuming that the motor can be treated as a resistor, which may or may not be true.

The essential idea is that you want to reduce the current to 7/12 of its current value; again, assuming that the motor can be treated as a resistor, increasing the resistance to 12/7 of its current value will do that.

The second calculation would be correct if the resistance of the wire was non-negligible and if you could measure the total power consumed by the wire and motor together, but typically, wire has negligible resistance.

That is, if you put a resistor in there, the total power consumed by the circuit would be divided among the motor and the resistor, however it would not be 2.4 W; imagine sticking a huge resistor in the circuit, so that next to no current flowed; power would be close to 0.

This is all theory, though; I've never actually done it. The Cases and Cooling forum might be able to help you more, I think.
 

rgwalt

Diamond Member
Apr 22, 2000
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<<

<< All the energy your PC sucks in turns into heat and is dessipated into your room. >>


Some is converted into kinetic energy.
>>



You are right... It takes energy to overcome the frictional losses when spinning the platters in the HD, it also takes energy to spin the CDs in the drives, the floppy disks in the floppy drives, etc. Also, fans convert the electricity into kinetic energy by moving the air.

As far as what heat is, etc... Heat is a form of energy related to molecular motion. However, you can have energy (heat) transfer through radiation, which doesn't require a medium.

Blah, too tired to discuss...

Ryan
 

Agent004

Senior member
Mar 22, 2001
492
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<< All the energy your PC sucks in turns into heat and is dessipated into your room. >>



Some is sound, ie the fan, the spinning of the hard drive platter.



<< Heat is not an energy in itself. >>



You are wrong, heat is just a 'generic' term for thermal energy. It's just as the same eletricity, as we say eletricity powers the light bulbs commonly, but never say eletrical energy powers the light bulb.

An example of common generic term is 'weight' and mass, those of us know weight is really a force, but when you go around taking 'weight' of people, you say they 'weight' 56 kg, when you actually mean they have mass of 56 Kg.

You see ;)



<< Internal energy is the energy of an object due to chemical forces, nucleic forces, and the random motion of particles which contributes to a net velocity of zero on an object. >>



Why make it sound so difficult, internel energy is the sum of total kinetic energy and total potential energy.