Cooling
Modern PCs will have at least 2 cooling fans: One for the power supply and one for the CPU. A third cooling fan can sit on the graphics card to cool the graphics core (e.g. 5VDC at 60 mA=0.3W) or you could hang a twin fan between the expansion cards ('buss' cooler 3,2 W). An extra cooling cooling fan could be mounted on the motherboard to cool the North Bridge chip (also 0.3W). Naturally, PCs and servers with dual or more processors need a cooling fan for each processor. And if all this doesn't help, a booster fan ('system cooler' 10-12 W) could be added to cool everything that is in the casing.
Before the 486 processor, the power supply fan (up to 4-5 W, RPM control internally or by South Bridge chip on the motherboard) and passive heat sinks (finned blocks of aluminium or copper near the CPU and other hotspots) used to be enough to cool the PC. Now, active CPU cooling has become (almost) a necessity.
For Pentium III and comparable, the CPU cooler uses some 1,2 to 1,5 W (12VDC at 85-130 mA) to keep the CPU at lower than 40 oC. For P4 (Thermal Design Point at more than 70 W with 180 nm technology) also fans at 3,6 W as maximum (12VDC at 300 mA) exist. Note that these are maximum power requirements: the CPU cooler fan speed controlled on the basis of temperature (control by South Bridge chip on the motherboard), so the actual power will be a fraction of the rated power.
Alternative CPU cooling methods are with a heat pipe (usually combined with natural convection in an aluminium casing and/or with a fan blowing at the hot end of the heat pipe), water cooling ( this system employs 2 heat exchangers at either end, a circulation pump and a fan placed outside the PC), a 'refrigerator' (with compressor, evaporator, HC or HCFC fluid, etc.) or a Peltier cooler. These refrigerator solutions exist, but are rare and used mostly with overclocked CPUs. Depending on the heat load that has to be cooled, the refrigerator may use up to 200W (Windows desktop) or 300 W. A water cooler might easily take some 20-40 W and ofcourse the heat pipe solution with natural convestion doesn't use any energy at all. The reason why designers choose e.g. the heat pipe has nothing to do with energy (yet) but is instilled by the desire for an absolutely silent machine.
Courtesy of the French Energy Agency ADEME, Future Electronics project.
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