Powering duemilanove with a 12v taken to 9v w/pot

Hello everyone!

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.

Huge thanks!

-Susan

A resistive divider cannot never in his life power such high power things like an arduino, with your resistance values the max current that it can provide is: R=U/I => I=U/R => I=12/30k = 0,0004A, thats less than 1mA, use an LM7809 and use the arduino jack or use an LM7805 and connect it to the Vin pin.

agreed, things like pots are generally made to be passive components effecting signals, it would burn up under any semi serious load

i'm using a 30k and a 90k resistor for the potential divider.

So in effect you have a 30K in series with the power supply. Suppose your arduino draws 100mA then in order for the resistor to supply that current you would need an input voltage of 3000 Volts !!! Generally you need 10 times the current down a potential divider than you are taking out so in your case the two resistors would have to be 3R (ohms) and 9R and have 1A flowing through it. They would then get very hot.

As the others have said you can't power things with a potential divider it is way too inefficient.

Ah! thanks so much everyone. I did it on a recommendation from a colleague, obviously it wasn't the right thing to do. I'll look into finding a proper voltage regulator.

A center-positive AC adapter should be easy to find. Take your arduino to a thrift store and USD$2 will get you one. I would try not to use the same supply that powers solenoid to power arduino.

Take your arduino to a thrift store and USD$2 will get you one.

She's in London, the air fare might just put a dent in the price. The UK doesn't have these sorts of shops. ;)