Micro Servo 9g motor rotates automatically when powered on without PWM signal

Hi all,

I am a new to arduino uno. Recently I start to play with Micro Servo 9g motor, but it began to rotate 15-20 degree after connected the GND and VDD(5V from another board). When put to VDD again, it rotated another 15 degree.

I think the motor should stay stalling when just connecting to VDD and GND, then rotating after receiving PWM signal normally. I searched on the Internet about the possible reason, but it seems no one have the same issue. So I would like to ask where goes wrong, if the motor is broken or something else?

Thanks a lot in advance.

If there is no connection to the servo signal cable the internal electronics will not know what to do so it may well behave randomly.

I presume, when you say PWM signal you mean the output from the Servo library?

The servo should NOT be powered from the Arduino 5v pin, it should have its own power supply with a common GND with the Arduino.

If those comments don't make sense please post your program.