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I have an old 56k6 modem, powered via USB-B cable, which I want to repurpose to make another device. Basically I'd like to use the enclosure to house an Arduino Nano and keep the existing power connection and LED indicators, while removing and blocking off the other connections such as phone line and audio jacks.

To keep the power connection, I desoldered the USB-B jack from the PCB to be mounted on the PCB I'll need to create for my own project.

As far as I can see there are two ways to power my Arduino Nano with the 5V coming from the USB-B jack:

  • Connecting 5V to the 5V pin from the Arduino Nano (and GND to GND)
  • Soldering the 5V to the back of the USB-plug on the Arduino Nano

Because the latter is such a tiny connection, I've opted for the first option.

Online I found that this method does work, however it is not recommended. Therefor I'd like to know if there are any precautions I can take, and if there are, what they are.

Keep in mind that the device will always be powered from a USB port on a monitor or PC (which would mean the incoming 5V rail would be a stable one).

Is this a safe way to connect the Arduino Nano, or do I need extra safety features?

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    the Arduino Nano has a USB connector ... why not make a USB cable that plugs into the Arduino? Commented Jan 23 at 17:44
  • @jsolola Because the enclosure (the old modem) has a USB B connector and I want to reuse that enclosure. A second reason is that the USB B connector would be soldered in such a way that it only provides power to the Arduino but not data. Having the Arduino plug accessible would mean that anyone could plug in a USB cable and reprogram the Arduino (not that big of a deal, but I prefer it this way). Commented Jan 23 at 18:34
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    ".. this method does work, however it is not recommended", it works until some magic smoke come out, it is not recommended to directly feeding a 5V to the Nano 5v pin. I'd suggest you take a look at the Nano schematic to see how it feed the VBUS to 5V pin via D1. BTW, when you are not providing a schematic of your circuit, you need to be more precise on the statement like "Connecting 5V to the 5V pin from the Arduino Nano", where is the first 5V from? VBUS is 5V, it could be between 4.8V to 5.2V as per USB spec. Commented Jan 24 at 2:39
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    @hcheung Although it is stated on docs.arduino.cc/learn/electronics/power-pins that it is possible, you are right in saying that it is not recommended. I read about differences in clones and after powering my Arduino Nano via the 5V pin (some) magic smoke indeed came out. Fortunately the Nano survived (there was enough magic smoke left to work properly). I understand what you're saying about connecting 5V to the 5V pin, but there is no circuit or schematic to be honest - the original 5V comes from the USB B connector which is supposed to be a regulated 5V. Thanks for the schematic! Commented Jan 25 at 15:57

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If you look at the Arduino Nano schematics, you will see that the 5 V bus is connected to 5 V pin, to VBUS on the USB connector and to the regulator which takes input from the Vin pin. There is a diode between the VBUS and the 5 V bus. This is to avoid feeding 5 V to USB when the Nano is powered from 5 V pin or from Vin pin.

So you can power the Nano over the 5 V pin from USB. Just don't connect other power sources to the Nano at the same time.

enter image description here

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A word of warning for other users who want to power their boards via the 5V pin.

Even though the official Arduino documentation (https://docs.arduino.cc/learn/electronics/power-pins/) states that it is indeed possible to power an Arduino Nano from the 5V pin, I found out the hard way that it is not the way to go.

I accepted the answer provided by Juraj because it is the correct answer for official Arduino boards.

In my project however, I was using an Arduino Nano clone. Some posts I found already mentioned differences between official boards and clones, and after providing 5V (coming from my PC via the USB-B cable) to the 5V pin, my Nano powered up but I immediately noticed the on-board LED wasn't lighting up the same way as usual, after which I saw some magic smoke coming from the board.

Fortunately I was able to yank the USB plug from my PC and the board wasn't damaged - after powering the Nano via a normal USB mini cable the sketch is running just fine and no more smoke is escaping. Guess I got really lucky this time, but suffice it to say that I won't be powering any more board (even official) this way.

The reason I posted a separate answer is only to post the solution I went with in the end: in order to keep the USB-B plug and the enclosure, I cut off a USB mini cable plug and soldered its wires to the USB-B plug, leaving the data wires not connected. This way the Arduino Nano gets power via its USB mini plug via the USB-B plug, and there is no way to (re)-program the Nano without opening the enclosure.

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