BEGINNERS: Where did my volts go?

There are an unusually large number of questions about 'how big is my power supply', or 'the motor crashes my Arduino'... why does USB work, but not my battery?

The power input pins are explained in great detail elsewhere - try a search, but here is a quick guess at what's going wrong.

The pins are designated as 0V / GND as a REFERENCE to their 'potential'.
The DC power you can return via those pins is expressed as Watts (Volts x Amps), or fractions of that - but this isn't really any use to you, as in reality that 'current' in Amps (or milliAmps) limit will be determined by the weight (thickness) of the copper tracks, and their ability to get rid of heat...!

This also true for the 5V and 3V3 pins, except their limits are imposed by active components (the voltage regulators). The source of expensive white smoke and smells.

So, If you have a suitable power supply that ties the 0V / GND pins of the Arduino to a common 0V rail across your whole project, which is shared with your various power supplies, sensors and output devices.
The current will flow 'outside', around the Arduino board, using the heavier rated common supply wires. The 0V pins are only there to make sure the Arduino knows where the 5V and 3V3 voltage levels should be in relation to a reference 0.

This is why it's important - e.g. if you have a 12v motor, or 1W LED using a separate higher rated 'power' supply - to tie the 0V rails together (under almost all circumstances).

The higher power (W=VxA) will flow through the motor via FET, relay or h-bridge, and not through the Arduino. There are other techniques to limit the current sourced - or sunk - through the I/O pins to feed the driver.

If you're drawing 5A from an external supply to drive some device, it's probably not a good idea to return that current 'through' the Arduino 0V pins... use that external 0V/ GND rail to carry the heavy load away, off the board - using wires directly from the power supply 0V to the shared 'common' 0V/GND rail.

IMPORTANT: You cannot tie different voltage rails together - even if they are the 'same' voltage...!
The two supply regulators will be chasing each other... so don't.

NOTE: This is a summary of concepts - there are unusual circumstances where ground-effects can interfere with operation at higher frequencies and in other operating modes - but get the LED to blink first before yo start studying power supply physics!

I'm not sure this Thread is as wise as your others. The topic is rather too complex to explain to a newbie in a few words and s/he will probably not want to read more than a few words.

To my mind a simpler presentation would be sufficient - how much current can be drawn from the Arduino Vcc, 5v and 3.3v pins and how much current can be drawn from, or fed into, an I/O pin.

...R

@Robin2 - agreed, but rather than expressing ‘limits’ which can vary over time with different and newer products, I was trying to reach the fundamental idea of common grounds, and moving current around the Arduino - not through it.

"IMPORTANT: You cannot tie different voltage rails together - even if they are the 'same' voltage...!
The two supply regulators will be chasing each other... so don't. "

Sure you can - as long as they don't share the same ground. I do it all the time to create a bipolar power supply to feed the gate of a depletion mode GaN device.

Most of the twinkies that need these threads won’t understand what you have done with bipolar supply rails - and will smoke their ‘duinos.
If you look at the whole OP, there are mentions of tying grounds together, and other events ‘in some circumstances’... these are rarely for Ardu-beginners.

lastchancename:
Most of the twinkies

What's a "twinkie", in this context please?

The thread is titled for BEGINNERS: