I can power my project using a 9V battery going to VIN/GND on an Arduino Nano.

But if I try to put a 5V Power supply module inbetween the battery and the arduino then the project gets power but does not function correctly.

Why is this the case?

I read somewhere that you cannot supply regulated power to VIN, but can not find the reason why.

  • \$\begingroup\$ In my lack of understanding, I would have assumed that the power supply module would take the voltage down to 5V, and the arduino would pass that through it's built-in regulator unchanged because it is already at 5V. \$\endgroup\$ – Jethro Apr 23 '18 at 9:55
  • \$\begingroup\$ Can you elaborate on how the project does not function correctly \$\endgroup\$ – Impossibear Apr 23 '18 at 10:06
  • 3
    \$\begingroup\$ AFAIK Vin feeds a linear regulator. It would need up to ~2V of headroom depending on the model of the regulator (or even more if its an older one) for it to regulate to 5V (that is, 5V + ~2V, so 7V min). Since you are providing 5V, its not gonna regulate properly and probably outputs ~3V or something, maybe 4V. \$\endgroup\$ – Wesley Lee Apr 23 '18 at 10:14

From Arduino Uno R3 docs:

Vin: The input voltage to the Arduino/Genuino board when it's using an external power source (as opposed to 5 volts from the USB connection or other regulated power source). You can supply voltage through this pin, or, if supplying voltage via the power jack, access it through this pin.

5V: This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.

The Vin has a fixed voltage drop of 2V so to use Vin you would need a minimum of 7V.

While not recommend by arduino, If you know what you are doing have a clean 5V and you don't connect it backwards it will not matter that you bypass their regulators and connect it directly to the 5V pin. You just won't have training wheels that prevent you from damaging the board. Bypassing their regulators will bypass the fixed 2V drop.

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