Improve this schematic to guarantee 12V and 70 mA in the fan
At a low current of 70mA, a simple lineair voltage regulator like the 78L12 could be used.

In the worst-case scenario (+VBAT = 16.8V ) the power dissipated by the regulator would be: ((16.8V-12V)*0.070A)+(16.8V * 0.0045A) = 0.412W. 
This means that the small TO-92 package (Pmax = 0.600W) could be used.

At at thermal resistance of 180°C/W (package to still air)  0.412W would result in a temperature of 74°C. Hot enough to burn your fingers. So maybe place the voltage regulator near the fan. :)







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