Connect 5V pin to external 5V regulated power


I see some people saying that what I will say can damage my arduino or my USB port, but other people say it's totally safe.

Here it's: I am powering my arduino with a 5V regulated external power using the 5V pin of arduino. But sometimes I need to connect arduino to my computer using USB to debug my code. My question is: keeping arduino powered by 5V external regulated power at the same time the USB is connected, will damage my arduino?

The "10 ways to destroy your arduino" does not say anything about that and many people say it could damage and others say that looking at the datasheet of arduino Uno (I think it's the datasheet of the atmega) it will cause no problem.

So what is the truth?

The truth is both :grinning: ::slight_smile: :astonished:

I do it all the time, but it has consequences.

1 ) A 5V regulator power supply can introduce a lot of noise, or the voltage can be 5.5V. There are many bad power supplies out there. Use a good and safe and certified power supply.

2 ) In some cases the voltage regulator on the Arduino board was blown. To prevent that, I add a diode (a normal 1N4007) from 5V pin to VIN pin. Or I solder that directly over the 5V voltage regulator.

3 ) Current can flow into the USB bus to the computer. The Arduino Uno checks VIN, and if that is above 6.6V, the USB power is turned off. However when power is applied to 5V pin, the VIN is low, and the USB power is not turned off. That could result in a current through a mosfet and to the USB bus and into the computer. That's not good, but I don't care. Perhaps my 5V power supply gets a little hotter than it should. Some of my 5V power supplies have lots of amps, but there is still the polyfuse in the path to the computer.

Some regulator chips really don't handle back-powering well, so there is a risk the
one of the Arduino board doesn't. Its not a normal case for the output voltage to
be above the input voltage.

Behaviour depends on which regulator, and that varies between boards and versions.
The diode across the regulator prevents Vin from being more than 0.5V below the 5V pin.
It seems this helps (in particular the regulator's internals are properly powered up in this

Personally I've not experienced issues, but I mainly use Arduino clones of my
own fabrication using AP1117-5V regulators and I never power the 5V pin directly
if USB power is applied - one or the other. My boards don't auto-switch.

1 ) I was talking about a power supply like a wall wart or so. I assume your ESC give a good 5.0V voltage. So you can forget this number 1 )

2 ) A diode can be used to prevent damage. If you don't know where to put that diode, just ask.

3 ) I assume your ESC is battery powerd. So it is possible that power from that battery is flowing into the computer. If you don't mind that, you can ignore it.

See Fig 22 for why to put 5V (anode) to Vin (cathode) diode.

I've got an old arduino demulove (sp?) that the on board regulator chip provided 8v+ to the board 5v bus when 12v was connected the the external power jack. I used an external 7805 regulator to power the 5v bus.


I’ d like to connect a leonardo using a usb connection on a hub which is self powered. So there will be maximum 100 mA for the board. I think it is not enough. I’d like to avoid using the barrel jack. So I think I will use a regulated power source connected to the +5V.
Actually according what I read on this topic and on other on the subject, it may be dangerous.

How can I make it works?

I read it was possible to use diode or modded usb cable… but I have no idea how to do.


Some regulator chips really don't handle back-powering well, so there is a risk the one of the Arduino board doesn't.

If that were genuinely the case, it would be dangerous to use USB power, as that does in fact always, "back power" the regulator.

The actual concern, is that applying 5V suddenly to the 5V terminal will cause a significant current as it tries to charge the input capacitor on the regulator. The 47µF capacitor may or may not be a critical value for the particular model of regulator (refer to the datasheet where this is discussed) but in general, if the 5V terminal is already connected to the supply before it is powered up, the rate of voltage increase on switch-on (and particularly for a switchmode supply) is extremely unlikely to be a problem,