I guess there is an electronic magic beyond that, but when I power my arduino UNO with USB from a macbook and put the simple sweep movement I can run many servo (HITEC HS-322) at the same time.
But, if I put a more powerfull power supply like 5v from a real PC power supply that could deliver like 20amp, then my arduino can't run the same code! It do jerky small moves.
I know I should wire all the 5v from servo directly to the PC power supply but how is it possible that the usb port of my macbook works better?
I suspect the voltage you're getting from the supply is unstable (I've heard people proclaim that the output of computer power supplies is unstable at light loads, or if the 5v is loaded but the 12v isn't), and when the servo turns on, the voltage falls enough to reset the arduino (and/or the servo's control electronics), or you have it connected incorrectly.
Where are you connecting the 5v from the power supply to?
For future reference, this question should have gone in either project guidance, or maybe general electronics (but don't go posting a new thread over there - that would be cross posting, which is prohibited by forum rules)
I connected the 5v of my power supply to arduino power jack first. Then tried Vin, which is the same.
Now I tried wiring the 5v of the power supply to every 5v of each servo wich looks to be better. I did the same with ground and I worked well with 4 servo. I need 9 do you think it will be ok?
Ok, I will post in project guidance or electronic section's thread next time
Aaaha! Vin goes through the regulator, but the regulator has almost 2v of dropout, so the 5v output was not 5 volts (probably like 3.5ish), which explains why things weren't working. If you're supplying a regulated 5v, you need to put it to the 5v pin, not Vin/Barrel Jack. Vin and the barrel jack (which are connected) will only work with supplies of 7-15v, via the regulator on the board.
You may have problems with the Arduino resetting when the servos fire - often people recommend using separate supplies for the arduino and the servos. If you have that issue, the path of least resistance would probably be powering the arduino through Vin with the 12v output of the computer power supply, and using it's 5v just for the servos (in which case the 5v of the power supply must not be connected to arduino's 5v, else the power supplies will fight)
