Hello Giuliano.
The Netduino faces an issue related to the maximum Vin voltage, because the op-amp used for automatically switching between the barrel power input and the usb. A simple calculation yields to a rough 10V as upper limit for Vin. Above this value there is a (weak) current flowing through the opamp, because the substrate diodes.
Okay, just for your knowledge!
The above issue won't damage anything on the board: at most will stress a bit the opamp when the Vin is pretty higher than the nominal value.
I believe that Chris fixed 9VDC because he guarantees the reliability of the board. I would have done the same, considering the opamp issue.
So, what's the real limit?
The 5V regulator imposes about 20VDC as maximum limit, and this value is mandatory. Voltages above this value may lead easily to some damage.
There is another problem to take in account, though: the temperature of the 5V regulator.
As you know, the heating power is the voltage drop across the regulator (Vin - 5V) by the current flowing.
Now, the chip is very small and the ability to flush the heating away is very limited, thus also the heating power must be limited. The regulator itself embed a thermal protection, so that the current is automatically limited upon the internal temperature.
If you take the Vin up to 16V, for instance, you'll get a drop of 11V across the regulator. From the chip specs, upon a 11V drop, the available draining current is even lower than 200mA (in a perfect condition). Think that it's just what the board itself asks for running alone!
At that point you won't be able to wire anything: no leds, extra logics, etc. Moreover, the regulator will be warm enough.
A even worse context is when you take the board and leave it running in a pretty warm environment (e. g. during summer or a radiator). That will limit the current even more, and maybe the board could get stuck.
A very simple rule is to put your finger on the regulator chip: if you can keep on it without problem, your regulator will run forever!
Hope it helped.
Cheers
