I have this passively cooled mini-PC that froze every few days. After a lot of investigation I realized that it was overheating (running android studio and such) so I bought a fan, an arduino nano and everything else needed to build a fan controller. It works well, my question is why I have to set the threshold temperature so high? I originally programmed my arduino to keep the inside of the case at around 32 - 33 degrees (celsius) but this was back in spring when the room temperature was in the low twenties. Then summer came and the room temperature went up to the high twenties and my fan started running crazy fast to keep that temperature. My current threshold is 37 - 38 degrees which is good enough for pc not to lock up but I don't understand why it has to be so high? I would've thought cooling the case to room temperature would be easy for the fan because the case is a huge aluminium heat sink and I thought it would want to reach thermal equilibrium with the room. The outside of the case is now 34 - 35 degress, way above room temperature. One more detail: the fan is on the outside of the case centered above the cpu and it covers about 40% of the top area of the minipc, it's a big fan.
To extract large amounts of heat from a hot object requires that the temperature of the cooling medium in contact with the hot object be as low as possible, to maximize the difference in temperature between the hot object and the medium.
This is hard to do when using ambient air as the cooling medium, for two reasons. First, ambient air is "too close" in temperature to that of the CPU. Second, air is a poor heat transfer medium because its thermal conductivity is low and its specific heat capacity is low.
Efficient heat transfer using moving air as the transfer medium requires 1) maximization of the heat transfer surface area (typically done by adding cooling fins to the hot object), and 2) maximization of the airflow velocity over those fins, which requires a relatively powerful fan to drive the air (as in an air-cooled lawnmower engine, for example).
Neither of these can be effectively done with an ultra-slim laptop or mini-PC, especially since neither type of computer was designed with the capability of adding extra cooling capacity in mind.