Electronic – arduino – 12V supply to Vin Pin on Arduino

arduinopowerpower supply

I'm trying to understand the following section of the Arduino Uno's schematic:

Comparator section of Arduino schematic

(Here's a link to Arduino Uno R3 complete schematic)

The Arduino's site recommends that Vin should be between 7 and 12V. However, I was reading the datasheet for the comparator (LM358) and it seems that the voltage range is -0.2 to 5.7V, where 5.7V is absolute maximum for the input pin.

As a result, wouldn't 12V damage the chip since 12V would input 6V into the comparator?

I tried asking this question in another forum. However, no one really knew the answer why, and tech support at SparkFun was also stumped.

Thanks for the help in advance!

Best Answer

The very same schematic was used for the original Uno32 from chipKIT. As it happened, someone managed to blow it up by applying 15V to the barrel jack. That was perfectly within the specs of the regulator, but the comparator ejected all the Magic Smoke™ and ceased to work.

After some diagnosis of their board I did the same calculations as you and came to the same conclusion. 15V, while within the limits of the regulator, overwhelmed the input of the op-amp.

I subsequently recommended a pair of clamping diodes on that input (now in Rev C of the Uno32), and since then there have been no reports of dead boards from too much input voltage.

enter image description here

(Note: I don't know if it's just a labelling problem in the PDF version of the schematic, but the VIN on the upper diode should actually be Vcc so it's clamped to the same supply voltage as the comparator.)

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