Force external power supply while usb is connected

Hi,

I would like to supply my Arduino project with an external power supply, however I still need the USB connection for communication with a pc. I read elsewhere that the auto-select chooses USB for power even with an external supply connected. Is there any way to change this behaviour?
I'm worried that I cannot get current from usb for all the components.

Cheers
Nadja

I read elsewhere that the auto-select chooses USB for power even with an external supply connected.

No it is the other way round. External PSU is used if it is present.

That'd be great news! So the information in this thread http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1275565622 is wrong then?

Cheers
Nadja

Yes it's wrong. if you look at the schematic you will see that the output of the regulator chip is selected when Vin is greater than 6 volts. This is derived from feeding the 3V from the USB bridge to a comparator with the other side coming from a 50% potential divider driven by the Vin line.
So if Vin is greater than 6V this is selected in preference to the USB power.

It's all there in the schematic.

However I am not sure what you are that interested because the current you can get through the regulator is about the same as you can get through the USB lead. So there is not much advantage to using one over the other unless you just want to take the load off the USB.

Ok, thanks.
I don't really know too much about electronics. I thought USB only supplies about 150mA. So if I have 60 LEDs with 20mA each, wouldn't I need 1200mA?
Or am I on the wrong track completely?

Thanks
Nadja

USB will supply up to 500mA, the internal regulator will supply about the same. If you have 60 LEDs at 20mA each then you have to supply an external regulated 5V supply to provide power to both the LEDs and the LED drivers. You need to connect the ground of this external supply together to the ground on the arduino.
http://www.thebox.myzen.co.uk/Tutorial/Power_Supplies.html

Thanks a lot! I'll check the tutorial out.

Cheers
Nadja