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Part IV: A few notes on the design of the PowerBook G4

The chassi of the PowerBook G4 is very sturdily built, for some reason. Not that I understand why, since if the computer is dropped to the floor, walked upon or otherwise messed with, surely it won't be the chassi that determines the survival after the impact. Anyway, it gives some sort of primitive feeling of assurance to know that your computer is built of aluminium and magnesium alloy rather than the usual plastic, and in addition the metal helps to dissipate the heat from the processor and hard disk.

fig15

Figure 15. The exposed interior of the computer after the top cover and hard disk has been removed.

The cooling arrangement of the PowerBook is quite interesting. The processor is hidden beneath the ribbed metal plate as seen in the left part of Fig. 15. This acts as an extended cooling flange and heat distributor. However, the cooling fan is situated about eight centimetres away from the processor, with no direct channel for air transport. Instead, the processor is connected to the metal frame of the fan via a thick copper tube, which hence acts as the channel for heat transport. (You can see a part of this orange-brownish copper tube directly to the left of the fan in Fig. 15.) As the fan cools down its own metal frame, it is dissipating the heat from the processor as well, though only after the heat has been transported via the copper tube.

In fact, this arrangement works better than one might expect, and even when doing extensive numerical simulations the hottest area at the cover is closer to the fan, rather than directly above the processor. The only task of the fan is to cool down its own metal housing, and for this it does not even need an inclination of the fan blades, as long as the air is stirred around enough inside the frame. The air is then cooled down by the aluminium bottom of the computer, which hence acts a large secondary cooling flange, not for the processor but rather for the fan frame.

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Last modified Tuesday 15 Dec 2009