I have an Arduino Uno project that uses an external 8 bank of relays which requires a 5v input. The 5v output on the Arduino is too weak to power the board so I need an external power supply. I'd really like to avoid having to use both a 9v power supply and another 5v supply for the board.
I know the Arduino has a Vin which accepts 5v so it seems logical that all I would need to do is split power from a 5v power supply to both the Arduino "Vin" and the board.
However I see lots of cautionary notes about using the Vin. It is a raw input so the power must be very stable and clean, etc.
What is a recommended method for powering both the Arduino and external board from one power supply?
Vin is wired straight to the power jack where you can plug 7V to 12V since it goes through a regulator or DC buck converter (depends on the board) to regulate Vin down to 5V so NO, Vin power does not have to be stable and clean.
The Vin pin is one way to get that 7V to 12V -before- it is regulated down to 5V. It does not go through the chip, it goes around.
If you have regulated (clean & stable) 5V power then THAT can be fed directly to the 5V pin.
Since your relay board will draw a lot of current you need to make sure you have a power supply that can deliver a good bit more just to keep the 5V going to the Arduino from dropping below 4.5V which will cause a reset.
For led strings I bought small 60W OEM (original equipment manufacture -- parts that get used in products) adjustable switching power supplies (no heavy transformer and higher efficiency than heavy old-type supplies) that can give 5V to 12V depending on how you set the adjustment screw. 60W is overkill for what I want but a power supply that loafs along will do so far longer than one pressed near maximum. If you need 2 or 3 amps, 5A or more rated is desireable.