External vs arduino power supply for servo motors?

Question

I recently built a project that involves the control of three SG90 servo motors. I started off by controlling this through the 5V output pin of the Arduino Uno, with the Arduino powered through the barrel plug with a 9V mains power supply.

However, the servos couldn't perform the full 180° sweep, it would appear to be due to the high torque at the end of the motion for my application.

I then switched to powering the servos separately to the Arduino using four AA batteries in series. I now had no issues with the servos stalling and could perform the full sweep.

I am not sure why this is though.

Four AAs in series will result in 6V, which is not that different to the 5V Arduino output pin, and it seems the current is the issue with servo stalling. Surely the number of batteries makes no difference to this as connected in series it stays the same. Is the current of the AA battery just higher than the 5V Arduino output pin?

Any help would be greatly appreciated, I'd really like to have a better understanding of this.

Answer 1

SG90 servo motors pull 360mA at stall, so 3 of them will pull up to 1,080‬mA. The MC332690-5.0 in the Arduino can only supply 800mA. So you will need an external power source if you want to fully power 3 of these servo motors.

Answer 2

Because the 5V from arduino which powered by Vin will regulate by 5v linear regulator that can provide 1A of current. If you power servo with separated power source, it nothing to limit the current except the power source itself.