Arduino Uno maximum voltage

Preliminary note: this is a purely theoretical question, I don't have any practical usage for this.

In the process of trying to strengthen my electronic skills, I've started to study the schematic of the Uno board (found here : http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf), and more specifically the power supply part.

On the main page for the Uno (http://arduino.cc/en/Main/ArduinoBoardUno) it is stated that the absolute maximum input voltage via the DC jack is 20 volts, and it's a figure I've found cited in numerous places as well. I guess this comes from the onboard voltage regulator, as it's datasheet also gives a maximum input voltage of 20V.

However, upon visual inspection of my own board, confirmed by the photographs of the above linked page, the capacitor PC1, which is located before the regulator, as a maximum voltage rating of 16V.

So the question is, shouldn't the absolute maximum voltage rating of the board be 16V instead of 20V ? (I've on purpose neglected the voltage drop around the input diode to avoid over-complicating the question). Or is there some kind of black magic in operation that I'm unaware because of my noob status ?

So the question is, shouldn't the absolute maximum voltage rating of the board be 16V instead of 20V ? (I've on purpose neglected the voltage drop around the input diode to avoid over-complicating the question). Or is there some kind of black magic in operation that I'm unaware because of my noob status ?

No, your thought process is correct. If that cap is indeed rated as 16vdc max then that is the highest value that the external DC voltage should be, minus the small voltage drop of the series polarity protection diode.

Lefty

Even better, the cap should have its maximum voltage de-rated for longevity, to say only use 75% of its max capability. So 12V as a max to board, and if the supply occasionally exceeds that you have some performance margin.

Thanks.