[...] last robot I fried two servos [...]
The first servo that I bought, was a Hitech311. I tested it once and it worked. I did something else, and tried to test it again. I connected it to a breadboard, itsetlf linked to an external source of power and an Arduino. Nothing happened. There was just a little smell of something burning. I quickly removed the servo, but that was too late. I have done new tests and that servo doesn't move. I guess it is fried. Maybe, the polarity was wrong, or there were too many volts. Then I tested servos from other brands and everything was ok. So, I don't buy anymore Hitech servos. I know I am stupid, because there are zillions of people who have no problems with them. This is an absurb fear (or maybe some servos are better protected than others against hazards).