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Topic: Uno Reference Voltage of 1.1v (Read 3 times) previous topic - next topic

FigZ

just want to add my two cents - I am researching how to measure battery voltage through the bandgap method

if you connect the batterys to the vin barrel - you are not going to get the same voltage on vcc - far from it. The battery voltage goes through the voltage regulator. If at all possible - hook up your battery pack to vcc. (assuming it is not over 5.5 volts and not reversed!)
So the results you measure may actually be correct

hooking up an arduino to usb does bypass the voltage regulator
however - this does not garantuee a clean 5v vcc
when I hook my arduino up to a usb 1.1 hub - I get around 4,3 volts on vcc
it took me quite some time figuring out why analog temperature sensors would give bad results. I finally figured out a simple trick, measure the 3,3v line on an analog pin and use that to calculate the voltage on Aref. This did cost me an analog pin. I am looking into the bandgap method for future projects.

Thanks to coding badly and lefty for some great posts on the subject.

May I point to another great source on this subject?

http://jeelabs.org/2012/05/04/measuring-vcc-via-the-bandgap/
http://jeelabs.org/2012/05/12/improved-vcc-measurement/

retrolefty

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when I hook my arduino up to a usb 1.1 hub - I get around 4,3 volts on vcc


That is quite low, and below the USB standards of I think 4.75 to 5.25. Try plugging directly into one of your PC's usb ports instead of that hub and see if you don't get a more normal Vcc value.

Lefty

FigZ

way ahead of you there ...
when I use the computers usb ports all is fine!
just this usb 1.1 hub
I have not looked into it that much further - it doesnt bother me since I figured out a way to measure the vcc
which actually was a good idea since I need this when I want to run my arduino off batteries.

I just got fascinated by the whole bandgap idea because it doesnt involve sacrificing an analog pin.

thanks for your reply, and thanks for sharing your bandgap experiments

Coding Badly

#28
Jan 29, 2013, 12:34 am Last Edit: Jan 29, 2013, 02:53 am by Coding Badly Reason: 1
http://jeelabs.org/2012/05/04/measuring-vcc-via-the-bandgap/
http://jeelabs.org/2012/05/12/improved-vcc-measurement/


ARGH!  It's 1024!

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if you connect the batterys to the vin barrel - you are not going to get the same voltage on vcc - far from it. The battery voltage goes through the voltage regulator.


It that situation, use the regulated 5.0V (or 3.3V) supply on the Arduino as a reference and a voltage divider on the battery to get the voltage in range.

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however - this does not garantuee a clean 5v vcc


Use the regulated 3.3V as a reference.

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when I hook my arduino up to a usb 1.1 hub - I get around 4,3 volts on vcc


Yikes!  You need to drop that thing for a powered hub.  In my experience, this one is well worth the money...
http://www.usbgear.com/computer_cable_details.cfm?sku=USBG-7U2ML&cats=104&catid=104%2C653%2C210%2C212

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I finally figured out a simple trick, measure the 3,3v line on an analog pin and use that to calculate the voltage on Aref.


Or just use the 3.3 V as the reference.   ;)

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Thanks to coding badly and lefty for some great posts on the subject.


You are welcome.

retrolefty


way ahead of you there ...
when I use the computers usb ports all is fine!
just this usb 1.1 hub
I have not looked into it that much further - it doesnt bother me since I figured out a way to measure the vcc
which actually was a good idea since I need this when I want to run my arduino off batteries.

I just got fascinated by the whole bandgap idea because it doesnt involve sacrificing an analog pin.

thanks for your reply, and thanks for sharing your bandgap experiments


Your welcome, and bandgaping was fun but CB did most of the heavy lifting. I just wanted to see if it could be useful or not and wouldn't stop bugging him for help and ideas. To tell the truth I almost never find my projects using all the analog input pins (and if so I just switch to the mega board or my 644P soon to be 1284P board, so just tying the 3.3v to a AI pin and using that as a compensation/refence value is simple enough. Heck on one of my boards I have a jumper from the anode of the power led wired to a AI pin as a reference, it works.

But for serious ADC results I just use I2C ADC chips, either the TI 12 bit ADS1015 for 16 bit ADS1115, they are what real ADC is be all about. Adafruit has some nice breakout boards for them and has support library all set to go.
http://www.adafruit.com/products/1083
http://www.adafruit.com/products/1085

Lefty

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