I recently purchased an Arduino Uno. After playing with it for a few days I decided to run my LED blinking project in a stand-alone mode. I read the power requirement specification for the Uno online at http://arduino.cc/en/Main/ArduinoBoardUno. It says any voltage between 6 and 20 volts as Vin will work. 7 to 12 volts is highly recommended. I hunted down a few wall-wart power supplies. I first found a 12Vdc 1.2A supply with the exact adapter plug.(what luck!) I hooked it up and like any good hack, I whipped out the Fluke and measured the on-board supply voltages. Here's what I found:
Vin: 14.47 Vdc
+5V: 4.98 Vdc 3V3: 4.7Vdc !!! WTF? (What the Fluke)
I unplugged it and hooked up choice wall-wart #2 which was a 9 Vdc 300mA RS special. (RS is a registered trademark of Tandy Corporation)
Vin: 9.2Vdc
+5V: 4.98 Vdc
3V3: 3.3Vdc
It looks like there may be a 3.3Vdc regulation problem when Vin is above +12 Vdc.
Odd. I checked out the Uno's schematic in hopes of finding the 3.3V regulator, but I'm not to good at reading schematics. I found the 5V reg., which has a 20V max. Just out of curiosity, though, sparkfun's 3.3V regulator has a max input of 15V!
That is very odd. On the schematic it shows the 3.3vdc regulator (U2) as getting it's input voltage from the +5vdc, so I don't see how your +5vdc can be OK and the 3.3v output not OK?
Thanks for the reply. It is weird since the 3.3V regulator (U2 on the Uno schematic) is fed from the +5Vdc supply. I did not have the USB connected so I'm not sure what caused this. I measured twice to be sure I was on the 3V3 pin. I will attempt to recreate the condition.
-Ru55