One thing I'm uncertain about is if you feed the arduino 12v and it's a 5v arduino, what does it do with the difference on the pins that I always expect to do 5v when on and 0v when off. Either I have to take additional precautions (I'm thinking about heat generated) running him 12v or do I have to add something else to make sure the digital and analog pins only do 5v?
Asking here because I still feel like I should have learned more than I did from this. Thx.
Firstly you should realize how fragile logic devices are to over-voltage - the ATmega chips
used in most Arduinos are designed to be powered from 1.8V to 5.0V. They only tolerate
a continuous supply voltage of 5.5V (10% more). 6V will start to risk permanent damage,
12V will definitely destroy it in microseconds as all the transistors on the
chip blow their gate-oxide layers. Gate oxide layers are a few atoms thick...
So anything with > 5V on it has to be treated as "highly dangerous" and your should always
double-check your circuit before applying power - double-checking saves time and money.
The built-in regulator on the Vin pin will work from 12V, but its job is to waste the
excess voltage, so that a lot of heat may be generated - in fact the voltage-excess
times the current from the load (the microcontroller and anything you run from it).
If you are powering a lot of devices directly from the Arduino pins and the Vcc pin
all that current comes from the regulator and it might be too much when powered
from 12V on the Vin pin as 7V are dropped across the regulator. Power Vin from
7.2V instead and only 2.2V are "wasted" - so a lot more current can be regulated to
5V for the same heat generation.
If in doubt use an external voltage regulator to provide 5V for high-current loads
(LED array at 5V for instance).
One trick I use when running from 12V is to have an external 8V regulator chip
to power Vin - this shares the heat dissipation and helps out, especially as external
regulators can have a heat-sink bolted on. I then don't have to worry if the 12V is
12V or 24V, both will work (larger heatsink may be needed)