The good thing is that now known that supplying 5 volt via the 5 volt pin is now officially not OK.
I have a powerplug going from the 12 volt to the Arduino. However when I use this plug the Arduino becomes hot (At the power conversion chip)
The 5V on the USB cable simply connects to the 5V rail on the Arduino - its equivalent to directly powering the 5V pin.
There is nothing wrong in powered the Arduino via the 5V pin - but you _must_ ensure it is a regulated 5V which never ever goes above 5.5V or so... If you have a cheap-and-cheerful "5V power supply" its up to you to verify it does actually put out a well-regulated 5V. You need to measure it with a multimeter under no load and under load (not your Arduino!) to check the DC output is 5.0V +/- 0.1V and that the AC component is small (< 50mV is advised).
I'm an IT guy not a electronics guy; so I don't dare to say you are wrong but the info you provide is seem conflicting to me with other info available.When looking at he schematics http://arduino.cc/en/uploads/Main/Arduino_Uno_Rev3-schematic.pdf I can not conclude Vin and USB power line are connected.
I'm an IT guy not a electronics guy; so I don't dare to say you are wrong
but the info you provide is seem conflicting to me with other info available.
I don't know what you mean with equivalent.
These requirements seem tight to me
Arduino explicitly states that Vin does not go over the regulator