Using 5v 3A to power nodeMCU?

Hi guys i need help. So i have always used arduino for projects but i always got it plugged in my laptop so i never have to worry with my power supply. But now i need to power my project without it and i wanted to use my old computers power supply it has a bunch of outputs and im thinking of using the 5V 3A to power my vin nodeMCU and my Arduino Mega. Now what i want to know is if this is ok will it not cause any damage to the boards and will i still be able to use the 5V and 3.3V in the boards output pins to supply some of the Sensors? Thanks

I don't know the availability in your area, but I recently got a couple of USB wall chargers at the DollarTree for $1 each. I have a ESP8266 WeMOS D1 development running off of one with no issues. Easy and inexpensive power source.

I assume you will use the barrel connector or the Vin pin for the power input it won't work. You will need at least 7 volts for that input. However if you connect to the +5V pin it should work just great. The board will only have the 3.3 volts available, the 5V pins are connected together and to your 5V power supply. Your current limit is now the external power supply not the on board protection. This response is to help you get started in solving your problem, not solve it for you.
Good Luck & Have Fun!

I use an adjustable buck regulator in my esp8266 RGBW LED controller.

“Vin” on the NodeMCU is essentially the same as the USB connector, so supplying 5 V to it is correct.

“Vin” on the Mega 2560 is pretty useless, you should always supply it with regulated 5 V via either the USB connector or the “5V” pin and ground.

I am not sure what “old computer” uses a 5 V 3 A power supply, for most PCs it would be nearer 30 A. However as long as it is a properly regulated 5 V, it will be just fine. It should not however be connected while the Mega or NodeMCU is connected to a PC via USB.

The maximum rating of the 5 V supply is of no concern as long as it is more than the current actually required, however it may be a good idea to put a fuse in series with it at the power supply - say a 5 Amp fuse - not to protect your boards as such but to prevent the wiring catching fire if that 30 Amp supply sees a short circuit.