are there 2 voltage regulators on the Uno? One for USB and one for the 2.1mm input jack? I have a project ( xmas lights of course ) where I can power it fine from the USB but anything I put on the 2.1mm power jack doesn't quite drive the relays enough....the lights will come on but they won't always trip. It's fairly consistent..and if I tap the relay that doesn't trip it will trip. After a few tries with different power inputs, I decided to try another board and it works fine. Could it be a bad regulator?
The USB does not go through a regulator as it is regulated by the source.
If you put 5v into the power jack, it passes through a regulator that drops voltage. This socket is meant for 7v or more to allow for the regulator loss.
Thanks for the info...I actually saw that the regulator was only for the jack after I posted. I tried 12v 1.5a all the way to 20v 1.5a wall warts....with the same result. Would it be worth replacing the regulator? Or for the cost...should I just replace the unit?
There are 2 regulators - 5V,800mA, and a 3.3V 150(?)mA, that is fed from 5V.
7.5V DC input on the barrel jack works great. With 9V, 12V, the 5V regulator can get quite hot if driving a lot of LEDs or using 5V off the power header to provide current to offboard components. http://www.dipmicro.com/store/DCA-07510
If you need 12V for an LED strip, okay, then a 5V linear regulator to power the uC makes sense, & control some MOSFETs to switch the strips on & off. But that only needs some 10's of mA of 5V current.