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Topic: Arduino Uno 3.3V regulator ain't regulating with Vin of +15Vdc (Read 9559 times) previous topic - next topic


I recently purchased an Arduino Uno. After playing with it for a few days I decided to run my LED blinking project  :smiley-roll: 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 !!!  :smiley-eek: 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!

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


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.


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

Wall-Wart Power Supplies:

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!" 


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. :smiley-roll:

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



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


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.
The UNO is solid.

Thanks again all for the replies,


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

Cool  8)  NOW I remember a similar situation in The Past (which, for me, goes a long way)  XD

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