I was just looking at the power supply part of the Arduino Uno, but I am a bit confused:

1) Why the opamp LMV358 with VIN as input (USA)?

2) Why the opamp LMV358 with SCK as input?

3) Why an LP2985 to convert 5V into 3V3?

Here is a screenshot of the schematic.

Thanks! power_supplies



1) It is a comparator. It is switching the MOSFET on and off depending whether or not the Vin is present in order to choose to power the board from Vin or from USB.

2) It is a buffer separating between the SCK pin and the LED. The LED is intended to blink while the board is being programmed.

3) It is converted to have 3.3V supply on the board in order to provide it to potential sensors/shields requiring it (and as the reference to the comparator above, but it is not necessary as it can be taken out of voltage divider).

| improve this answer | |
  • \$\begingroup\$ The ATMega on an Uno is driven from 5V. The 3.3V regulator is there to supply the 3.3V power pin for shields which may require it. On the early versions, the 3.3V was sourced from the FTDI chip which was limited to about 50mA causing people to fry it by drawing too much current. Later versions corrected this by rather more sensibly using a separate regulator. Though looking at the screenshot they have also since used the 3.3V as the reference for the op-amp (it used to be a potential divider from 5V). \$\endgroup\$ – Tom Carpenter Jul 20 '15 at 20:54
  • \$\begingroup\$ So you have, the edit hadn't shown up when I was writing the comment. \$\endgroup\$ – Tom Carpenter Jul 20 '15 at 20:56

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