3
\$\begingroup\$

I am new to Arduino. I need to use Arduino Nano in one application in which we will be supplying the 5V regulated power from the external world. At the same time, we need USB communication also.

The official website says

"The Arduino Nano can be powered via the Mini-B USB connection, 6-20V unregulated external power supply (pin 30), or 5V regulated external power supply (pin 27). The power source is automatically selected to the highest voltage source."

It did not say anything when both the voltages are equal.

In my application, both voltages are 5V(USB and external regulated 5V)

May I know will it make any issues like USB communication failure.

\$\endgroup\$
3
  • 1
    \$\begingroup\$ In general, connecting multiple sources is a bad idea. Voltages are never identical (because real world), and you don't want to have slightly higher voltage source sending current INTO second voltage source (which has slightly smaller voltage; besides, voltages fluctuate). Unless arduino board has some protection to prevent that, which I'm not sure about. Potential risks include damaging voltage sources. Most likely, it will actually work for some time, but you never know when or if it kills something - maybe you'll get lucky and it will work ok indefinitely \$\endgroup\$
    – Ilya
    Commented Aug 27, 2021 at 11:24
  • 1
    \$\begingroup\$ As for USB, I don't think it should cause trouble for USB communication specifically, although all these currents running back and forth may cause some unexpected voltage and current fluctuations back and forth between two supplies -> random noise of random magnitude (maybe insignificant, maybe gamebreaking) for the USB. Again, hard to tell. Just as a rule, better not to connect stuff like that if you can avoid it (unless it has some protection from this sorts of things, of course) \$\endgroup\$
    – Ilya
    Commented Aug 27, 2021 at 11:27
  • 2
    \$\begingroup\$ Link the schematic. From what I remember, the Arduinos use some cheap LDO and those need a bit of voltage drop across to work. 5V in, 5V out won't work on a LDO. \$\endgroup\$
    – Lundin
    Commented Aug 27, 2021 at 11:30

2 Answers 2

Reset to default
2
\$\begingroup\$

Connecting an Arduino Nano to multiple supplies is somewhat complicated. It is NOT as simple as the Arduino.cc statement if you want reliable and accurate results.

  1. The 5VUSB is connected via a diode. With the Arduino operating at 40mA+ you might expect to see a 320mV drop across the diode. This means the VCC is about 320mV less than VUSB, which itself may be in a range of 4.7-5.1V depending on the USB Host supply.

  2. If you feed a 5V supply into the direct 5V line on the Nano, that will have a range (which you have not defined). Likely you will power the Arduino via this, but if your lower range of the supply is anywhere near 4.7V then it's anyone's guess which will supply power.

  3. If you have both the USB and external supplies connected, operation could be unpredictable since you cannot reset the Arduino by dropping either supply (USB or Ext). You can only reset the device using the pushbutton or DTR#.

  4. If your external 5V supply is turned off and there is other logic and devices connected to it, it may shutdown incorrectly or not shutdown at all. The VUSB supply will attempt to supply all the current needed, but may shut down if the current is too much. How much you can draw from the USB depends on the Host system. If the spec is tight then you should only get about 100mA, but many host systems will supply 300-500mA without negotiation.

  5. If you intend to use the A/D on the Nano, it is worth noting that the VCC is the Analog reference. So you must calibrate your conversions since you do not have a 5V reference from either supply. If you want at least semi accurate results you should supply a good +5V supply as the VUSB supply is likely not very consistent. The best results are likely to use Vin as your supply (6-12V) through the onboard regulator.

If you want to be sure that the USB does not provide current to the Arduino and potentially the external logic you may have, you need to remove the diode D1 (or alternatively F1) from the Nano.

You should always be aware of the variant of Nano you may have, the official schematic is shown below. YMMV if you have a clone. enter image description here

\$\endgroup\$
2
\$\begingroup\$

Arduino Nano has just a Schottky diode from USB VBUS 5V input to the onboard 5V pins.

Which means that if you supply external regulated 5V to the 5V input, it is higher voltage than typically available from USB via the diode and thus external 5V supply is used.

However, if you turn off the external 5V supply or it is lower than available from USB via the diode, the whole Arduino and all your devices connected to the 5V pins of the Arduino will be powered from USB.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.