Pro mini runs sketch when connected with usb but not when powered externally

Hey there!

So I'm having such trouble with this particular issue that I finally relented and decided to create a forum account to see if you wonderful people had any suggestions XD

So I'm working on a project that requires a servo to be controlled by a variable potentiometer (slide resistor in my case - connected to pin A0) that in turn rotates a valve. The sketch I have written up is pretty simple and just uses the analogread command and a loop that sets the value for the servo which I have connected to pin 9. Both the servo and pot are connected to the 5v output and ground on the board.

The sketch will run fine while the board is connected to the PC but the problem arises when I try to power it externally using the VCC pin. When not powered by the USB cable the lights on the board become dimmer and flash. Along with each flash the servo will move in pulses and does seem to be moving towards the value dictated by the pot position. Its almost as though the sketch is looping at a much slower rate than normal. I initially was using a 12V to power the whole project (since I also had other components that needed 12V to run and wanted to use a common power source) with a 9V and two AAs in series but to try and isolate the problem I moved to a variable power supply and adjusted the voltage between 5 and 18 volts with no change. I even removed the board completely from the circuit and just connected the servo and pot but it still exhibits the same behavior - it works with a PC connection but doesn't when powered externally. :~

I cant think of what would be causing this and was wondering if anyone had seen this sort of thing before? Any suggestions would be greatly appreciated!


I am having trouble visualizing this. Can you do a sketch or something of your circuit?

I moved to a variable power supply and adjusted the voltage between 5 and 18 volts with no change

Through the VCC pin?

Flat Batteries

Flat Batteries

Yep, sounds like a brown out (see datasheet) or low power to the micro. You don't say what kind of servo and with what load but in most cases you should have a separate power supply for the servo.


Dead give away, when you said 9v with 2xAA, since most 9v are < 90mah regardless of how beefy your 2xAA , you drained the 9v just with the board.

Get a decent power source, don’t mix and match battery technologies, better to have one dead power source $2 than one broken Arduino +$10.

Tried running the servo through a separate power supply and seems to working fine. Thanks for your help guys!


I have the same problem. When connected to the PC works OK. When connected to external power servo just twitches. Now external power is batteries (2 x 18550 7.2 V) or bench power supply is the same thing. OK, if nothing else I could power servo on separate power, but I would like to understand why does it work on PC but not on external power.

Best regards,

How many milliamps can the bench power supply provide? The Arduino is not really intended to be a power source for motors.


Bench power supply can go up to 40A. So there is no shortage of A or in that matter mA.
Well this is my first attempt with arduino and servos. I know. I should power motors with separate power supply.
But here is the thing:
I disconnected everything that only one servo is connected. I load up sweep program from examples. Nothing. Then I build exactly the same on arduino Uno and Nano. In both cases works just fine with external power.
So is it possible that I have a faulty arduino pro mini ? If that is the case how come it works with USB connected ?
I don't understand it.
Any idea ? I would really like to understand why. Oh why? Why ?! :stuck_out_tongue:

Best regards,

Because when the ProMini is connected to USB then that's the 5V, the USB doesn't power (or go through) the ProMini 5V regulator.
USB has more capacity than the ProMini 5V.

Oh now it makes sense...!
Thank you very much Runaway Pancake!

Best regards,

Is your pro mini a clone?
Does it have a RAW pin?
If so, power it externally through that pin.