Misterious behaviour of the Servo

Forum 2005-2010 (read only) Hardware Interfacing
1

I have 30V / 3 A regulated desktop power supply, on which I can choose any voltage from 0 to 30V. Now, I set voltage to 5V and plug ground and positive into the servo, and I take control (white) wire from servo and plug it into the Arduino. Servo is 9gr mini type. What I want to achieve is to supply the power to servo from an external source and only the movement control from the Arduino. Since Arduiono can not supply much current anyway.

What happens, when servo is connected for power, to power supply, is nothing happens. Servo squeaks and stops. If I connect the power leads to the Arduino than servo moves just as programmed.

Well, in the above setup one mini servo already blew up. I took another servo, it didn't blow up, but otherwise things are still the same: if power comes from the power supply it doesn't move, if power comes from Arduino it moves just fine.

One last note is that when connected to power supply, current is 0.3A, voltage is set to 5V.

Does somebody understand why servo doesn't work when power comes from power supply, but it does when power comes from Arduino?

Same additional info:

The second mini servo takes only 20 mA from Arduiono, when burdened with small mechanic load (quite lightweight servo pan & tilt bracket). I find it difficult to believe that servos run on such a small current?

2

connect the ground of the power supply to the ground of the arduino and try again

3

weirdo557

I do not questioning your suggestion, but can you please explain why?

I am going to do it straight away, anyway.

4

weirdo557 many thanks, worked straight off the bat.

Basically I gave Arduino servo control wire a 5V high, but not 0V low. I understand now. Thank you once again.

5

not quite how it works. to make a circuit, you need to have a potential difference. there needs to be electrons flowing in and out. try hooking up a light bulb to a battery with only one wire, it doesn't light because there's no return path for the electrons and there is no potential difference from one side of the light to the other. same with the arduino, your servo can only see 5 volts relative to ground, so when there was no common ground, 5 volts is nothing because there is nothing to compare it to.