How much current is it safe to pull from the 5V pin? I'll be powering my board with an external 12V and want to use the 5V pin to drive two IC's, combined current around 10mA. Also, is there any detailed documentation on the specifications/low level hardware of the Arduino Duemilanove?
If you look at the Hardware section of arduino.cc, you can find the schematics of the boards. The regulator on the Duemilanove is rated for up to 800mA. This of course depends on how much heat the regular dissipates which depends on the input/output voltages and how well it is heat sinked.
Since there is no heat sink (except for the PCB itself), you will probably get a couple hundred mA out of it @12V. However, I wouldn't expect much over 500mA in most cases.
Just after posting I read some of the FAQS relating to this issue. According to the info there, the board is assumed to draw a maximum of 200mA from the 5V regulator. If a 12V supply is used the issue then becomes one of heat dissipation by the regulator in order to drop the voltage down to 5V. The result of this is that only a few 10's of mV's would be available before heat becomes an issue. Given that, if I drop the voltage to say 7V-8V to Vin does this mean I can potentially draw more current?
Yes, you can draw more current if you lower the input voltage, because the regulator has to dissipate less heat.
Since you’re not drawing much current, I’d suggest putting a 2W resistor of about 10-15 Ohms in series with the power lead. That’ll lower the input voltage to about 8V or so, dissipate a lot of the heat away from the board, and leave the regulator with enough headroom to ensure a stable output voltage.
If you’re going to be using the Arduino for other projects that need more power, spend the 5 bucks or so to buy a 7V regulated supply from a surplus dealer like bgmicro.com or goldmine-elec.com.
If your 12V supply is not regulated, you may need to do some experimenting with the resistor value, because unregulated supplies can put out a lot more than their rated voltage if they’re not loaded down to their rated current.
Thanks for the info! :) Never considered the power dissipation task of a voltage regulator when supplied with 12V. I intend to power the Arduino and attached circuits with a 12V car/motorcycle battery. Some of my circuits are powered with 5V regulators, others with unregulated 12V because they need at least 9V to work, and because I am allowing the battery voltage to drop to a minimum of 10V I need to do away with voltage regulators for these circuits. That means a dedicated regulator for the Arduino alone, I think I can pick up a 7V one easily enough.. ;)