Is it safe?

I recently purchased a Lenardo to work on a pendant for my CNC machine. I have a question about the 5vdc input......

The board has a 5vdc barrel connector which I plan to hook my to my 5vdc power source. I know that is perfectly fine, but when I use the usb cable to plug the board into the pc for updating, that USB connection also brings in 5vdc via the USB port itself. Is there going to be an issue, smoke , burnout, etc.?

In this case there would be two sources supplying 5vdc to the board. and the 2 grounds would also be together.

Or must i cannibalize a USB cable to stop the 5vdc from the port?

Thanks
Henri

Which board has a 5V DC barrel connector? The barrel on the Leonardo requires a minimum of 7V.

It's not my area of expertise, but I would not do it. I have a similar situation for a project with a Mega and will use an DC-DC booster to create 9V from 5V to feed the board.

Connecting power to the barrel connector is ok, the board will switch from USB power to external power and not fry anything. But as sterretje satd, it requires a minimum of 7V. This is because it’s equipped with a voltage regulator that must have some voltage drop to work. The actual switchover point is 6.6V if I read the schematic correctly.
If you connect only 5V, it will keep on drawing USB current.

Yes, it is safe. The Leonardo, same as other boards (genuine), has on-board power switch which selects the source between barrel jack and USB. Barrel jack takes precedence before USB and of course, minimum voltage on barrel jack must be above ~6.7V. On the other hand, the voltage above 9V can cause that on-board 5V regulator will get heating.

Thank you both for your comments. I will rest easier knowing I will not damage the board. I presume there is some limit on what can be drawn from USB, which would force having an external supply. Guess I should consider building a little 9vdc supply, just in case.

Yes, there is a limit on the USB, e.g. up to 500mA for USB 2.0. However, max. current for on-board regulator is also 500mA. In any case, it is better to offer cheap external power supply than USB port if something will went wrong. Especially, if you are working on new project.