Radiation and Power
This content was originally featured on Amdmb.com and has been converted to PC Perspective’s website. Some color changes and flaws may appear.
Radiation: Radiation is an exchange of energy between objects that is based upon their temperatures relative to one another. All items that are above the temperature of absolute zero emit thermal energy via radiation. The amount of energy is defined by the Stefan-Boltzmann equation:
This equation defines the power emitted by a “blackbody” based upon its temperature. No material is truly a blackbody and the power emitted will be less than this maximum depending on the material’s emissivity. The s above is called the “Stefan-Boltzmann constant and equals 5.67*10-8 W/m^2-K. The size of this number means that when adjacent objects are fairly close in temperature, radiation plays a very small role in heating and cooling. For example, a typical heat sink may have outside dimensions of 60 mm by 60 mm by 45 mm. The net exposed surface area is 0.0153 m^2. At an average temperature of 50°C (323.15K), the total energy emitted is only 9.5 watts for a blackbody. Copper’s emissivity is approximately 0.03 meaning only 3% of 9.5 watts (roughly 0.3 watts) is emitted by the heat sink. If this number is not small enough to be insignificant, recognize that the heat sink absorbs almost 0.3 watts from the surroundings. The net effect of radiation is extremely small compared to the effects of conduction and convection.
Calculating Power
For electrical devices powered by direct current, power is nothing more than voltage multiplied by current. Power is measured in watts while voltage and current are measured in volts and amperes, respectively. As an example, a 60 mm fan may run on 12 volts and draw 0.3 amperes. The power used by the fan equals 3.6 watts.
Here we must make a distinction between steady-state power requirements and intermittent power requirements. The power used by the CPU will vary somewhat with processing load, but for the sake of determining cooling, we’ll assume it’s at full load all the time. The same is true for the graphics card. Other devices that operate at a steady load include constant-speed fans and most other motherboard components (bridges, clock-generator, memory, etc.). Many cards may also be assumed to operate at full load for the purpose of calculating cooling. Unless your computer is truly sitting idle or you run everything from a RAM disk, you may also consider your hard drive to be operating at a constant load.
Devices that do not operate at full load all the time are mainly drives. Both the floppy and CD/DVD drives tend to remain idle unless in use. At the moment they start, they draw a lot of power to get the drive rotating at speed. Once at speed, they operate with relatively little power. While the power supply must be able to handle the power surge needed to accelerate these drives, the cooling system generally does not.
It is not practical to assemble a list of typical power requirements for various devices. The range of power requirements is too broad and devices change almost daily. The preferred method is to look up the power requirements of the specific components you intend to use. This will work for everything until you start overclocking. This raises the big question, “What happens to power requirements when I start to overclock?”
I leave it to the reader to investigate the “stock” power requirements of the components you wish to use. What I will cover is how to calculate “actual” power requirements once you start to overclock things.
Overclocking Power
There are normally two things associated with overclocking that affect power. The first is voltage increases and the second is frequency increases. For DC devices, power is proportional to voltage squared. As frequency increases, current tends to increase linearly. The relationship between current and speed is not set in concrete; however, this rule-of-thumb is sufficient for estimating power. What does this mean for CPU power?
AMD’s technical document for the Athlon processor states that a 1333MHz chip running at 1.75 volts uses a maximum of 70 watts. If you are able to overclock this chip to 1400MHz without changing the core voltage, then the maximum power will be approximately 73.5 watts. To calculate this number, just multiply 70 by (1400 / 1333). If you must raise the core voltage from 1.75 to 1.8 volts to run at 1400, then the power goes up again by a factor of (1.8/1.75)^2. The resulting power is 77.8 watts.
The simple observation: Do not increase core voltage unless it is needed.
In this simple example, an overclock of 5% increases the chip power by about 11%.
This same method applies to other devices onboard, too. If you increase the DDR voltage from 2.5 volts to 2.8 volts, you increase the power by (2.8/2.5)^2 = 1.25, a 25% increase in power from an increase of only 0.3 volts. Combine this with a front side bus increase from 133MHz to 150MHz, and the total power increase becomes 41%.
