Hopefully this is the right place to ask this: Is the 2.1 mm barrel jack connected to the Vin header? I ask because I'm wondering if I could power the Arduino off of the barrel jack (at 9-12V), and then power a case fan (for a motor with rotation speed sensor) off of the Vin header, as a convenient pass-through?
You are correct...please note that the trace to Vin isn't very large, so there is a limit to the power you can run through it. A case fan should be okay.
Thanks. Yea, they're powered by traces (don't know how big) on a computer motherboard, so it should be okay. The one I was using says it draws 120mA, so not too huge.
I rigged it all up, and running the fan at 12V, I was reading ~80hz off of the speed pin with a little program I wrote. Translates to 2400RPM - reasonable for a case fan. Now to see if I can do any fun POV tricks with it. 