Hi, im new with all this and i starting to work with servos, but the servo that i have ( MG90S, tower pro) present some problems, when i load the program to my arduino the servo doesnt work and when pass some time it starts to work very well, its confusing, but worst of all, after a time or if i load a new program to my arduino the servo doesn't work again. I use the usb conection to supply the servo and try with a external supply (9v battery) but nothing. Thanks.
You have not shown us your code or your wiring diagram so it is hard to help.
Do NOT power the servo from the Arduino 5v pin. Give the servo its own power supply with the servo GND connected to Arduino GND.
The small PP3 9v batteries are useless for an Arduino.
...R
external supply (9v battery)
Supplying 9v to the servo might damage them and cause erratic operation as they are probably rated for 4.8v-6v. you might want to try 4 AA batterys for a power source.