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[on:]

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.

Regards,
Ru55

[Reply #1 on:]

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!

I'm not sure if its the same one, though.

[Reply #2 on:]

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?

http://arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf

[Reply #3 on:]

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

[Reply #4 on:]

It's real. I was able to replicate the regulator error. No USB connection. No other devices or shields attached.

Test Setup:
Black wire: GND
Purple Wire: Vin
Red Wire: +5V
Orange Wire: 3V3
http://www.surfruss.com/Arduino/Connections.JPG
http://www.surfruss.com/Arduino/ConnectionsCloseUp.JPG

Wall-Wart Power Supplies:
http://www.surfruss.com/Arduino/Wall_Warts.JPG

First test with the 12Vdc PS:
Vin: 14.37Vdc     http://www.surfruss.com/Arduino/12V_Vin.JPG
+5V:  4.98         http://www.surfruss.com/Arduino/12V_5V.JPG
3V3: 4.16Vdc    http://www.surfruss.com/Arduino/12V_3V3.JPG

Then the 9Vdc PS:
Vin:  8.38Vdc  http://www.surfruss.com/Arduino/9V_Vin.JPG
+5V: 4.98Vdc  http://www.surfruss.com/Arduino/9V_5V.JPG
3V3: 3.30Vdc  http://www.surfruss.com/Arduino/9V_3V3.JPG


"Nothing up my sleeve, presto!" 
Regards,
Ru55

[Reply #5 on:]

Wow, Interesting question, here..

Try to find someone with an adjustable power supply and see if there's a "break point"??

Hope this is not a just-uncovered UNO problem.

AND: WhatIf there's a LOT of AC ripple on the walwart??  Try reading AC volts as well. What do you see??

Hmmm....

[Reply #6 on:]

Just a thought-- Try putting a load on it, maybe a 1k resistor.

[Reply #7 on:]

Thanks Terry - I checked the ripple on the 12v supply. There wasn't anything significant.

But..Good call baum!
Putting a 1K ohm load on the 3.3V supply pulled the reading down to 3.30 Vdc.

<grovelling> Whew! Thanks man! I though my new board was toast.
Apologies for the false alarm.  I wonder why the no-load output floats like that with a larger Vin.
</grovelling>
The UNO is solid.

Thanks again all for the replies,
Ru55

[Reply #8 on:]

Good call baum!
Putting a 1K ohm load on the 3.3V supply pulled the reading down to 3.30 Vdc.

Cool    NOW I remember a similar situation in The Past (which, for me, goes a long way)