for a project, i'm using 18V 800mah Nicd batteries. my problem is to how i'm going to power the arduino itself. all the arduino is doing is running code, everything is running on a separate 3.3V or 5V regulator. everything would be great if i could just put 5V from my regulator into VIN, but I've heard that that also runs through the onboard regulator. however, when i looked at the schematic for the arduino, it appears as if the VIN pin just terminates and isn't connected to anything.
You need to look harder.
i'll be sure to do that. but do you have any solution on how to power this thing?
Well, both the 10k and 2009 boards connect Vin to the input of the onboard voltage regulator, as you'll see in the reference schematic on the arduino website. According to its datasheet it tolerates a maximum input of 20V.
So if the arduino just runs code and needs just a few mA, you can feed your 18V into Vin or into the external power plug. Same thing, except for the missing reverse voltage protection diode. If you feel uncomfortable about it, feed your Arduino with 5V and connect that to the 5V pin on the board.
thanks much! this should get me well on my way to finally completing an arduino-related project.