Maybe it's because i'm super tired, but I can't work out what's wrong with this attempt to power my arduino. I'm powering 9 mini solenoids with a 12V supply and wanted to power the arduino with the same supply. I set up a little potential divider and attached a center positive jack to it to power the arduino. The multimeter reads a positive 8.9V when the red lead is in the centre of the jack and the black is on the outside but it won't bring the arduino to life? I tried it on a second arduino to no effect, which I know works when mains powered. It works when connected to USB. Any ideas?
The power supply reads +12V 1.25A, i'm using a 30k and a 90k resistor for the potential divider.