12V power source for Arduino


this might be a stupid question, but I just started with Arduino. I want to control a 12V pump with an Arduino and I want to use the power source for this pump for the Arduino as well. But when I connect the 12V to the Arduino using the pins on the board, the program seems not to be executed. Nothing happens. When I connect the USB the program runs fine. I also tried the power connector without luck. In all cases the power led on the board is on. What is the problem?

thx, mschub

Which pins? If you use the Vin pin (+) and Ground (-) that should work the same as the power jack.

I use the 9V and GND pins right next to the analog in. (Arduino NG)

That should work just fine.

Are you using Serial or something? Because the Arduino can't run Serial commands when USB isn't connected...

I do not use serial. For testing I wrote a program that only turns an led on for a second and then turns it off for a second and so on. Again, if the Arduino is connected to USB, the led blinks just as expected. If I use the pins on the board (9V and GND), the power led on the Arduino is on, but the led is not blinking. Maybe my board is broken?

mschub: I use the 9V and GND pins right next to the analog in. (Arduino NG)

The Arduino NG has a power jumper next to the USB connector. Did you move the jumper from the USB to the side to the EXT side? I think you must have if the power LED lights up, unless maybe you have any of the data pins connected to external power.

Yes, I did move the jumper. The power LED on the Arduino lights up. There are no data pins connected to external power. There is only one LED at pin13.

The only error I can imagine that would cause the symptoms you see is a bad connection between the voltage regulator ground and the Ground pins of either the processor or the LED that is not lighting. It sounds like a hardware problem and you will need a multimeter to get much further. Using the resistance setting you should check to see that all the ground pins are connected together.