strange problem with servo power source

[fugrammer]

hi, i am sending the servo signal from my pic mcu. the servo works fine when i power it with the same source as the mcu, however if i power the servo with another battery, the servo will keep rotating towards its right in a slower speed. can someone help me solve this problem? muz i power the servo with the same source as the mcu? thanks a lot.

[dunk]

nope. this should work fine.
you want to connect the negative terminal on both power sources together.
as long as you have done that i can't think what might be causing your problem.

hope you work it out.

dunk.

[Admin]

yea I think dunk is right, you just need to make your grounds common. 

[fugrammer]

hi guys, thanks for helping me out. it works. i am curious that why i need to make their negative teminal common when the power supplied to the servo does not connect to the circuit of the mcu?

[dunk]

so i don't know how your high school physics went but a batteries voltage can be described as Potential Difference.
ie, the difference in volts between your positive and negative terminal.
your negative terminal is the reference voltage. often referred to as 0Volts.
your positive terminal is a set voltage above the negative terminal.

if you don't connect all your negative terminals together the various pieces of electronics will be using a different reference point.
think of placing 2 rulers side by side. connecting the negative terminals together makes sure the 0mm (or 0 inches if you prefer...) point is at the same place on both.

dunk.