I have a wall power adapter that converter from 240v AC down to an output voltage of: 3, 4.5, 5, 6, 7.5, 9, 12VDC (selectable) at 600mA (Its of these: http://www.jaycar.com.au/productView.asp?ID=MP3310). So far I've set it at 7.5 and powered my Arduino (since spec says input 7-12v).
Now I have two 3v hobby motors I'm also running. I know I can't run the motors off the Arduino board because it's pins can't supply the current so I've run the motors of two AA batteries on a separate circuit controlled from the arduino a n-channel mosfet. But I hate using up batteries when the power converter should have more than enough juice for both. I'm just unsure how to go about powering the 1.5-3v motor from the 7v plug. My high school level electronics says V=IR but since the current changes as the motor spins up/stalls that means I can't just chuck in a specific resistor to drop the voltage from 7 to 3v right? Are there simpler alternatives? Or does it start getting complicated and expensive and I should stick to the two AA batteries?
The simplest solution is to use an ohm-meter to measure the resistance of the motor (this will be through the brushes and the armature). If you take 3V and divide by that resistance, that is your "stall current" and it is the most current the motor designer wanted the motor to handle. Now, take 7.5v and divide by that stall current. That will give you a resistor you can put in series with a 7.5V supply that will cause the supply voltage to be zero at that current. Now put that resistor in series with your 7.5v supply to your motor, and your motor will run safely under all conditions.
That is the simplest solution, but it is not the best one. The best is to measure the current and cut off the voltage when it approaches the stall current. That is called a "chopper" driver.
A little bit of 'sideways' advice. These variable voltage power supplies can be unreliable. Check the output with your multimeter. I had one that pumped out 9v at 6! I also have a non variable one here that starts out at about 13v then after a minute or so drops down to 10v.
I discovered this by measuring everything (or so it seemed) because I'm so nervous of burning things out - perhaps its because I'm fundamentally a mechanical engineer and avoided electronics at uni (never did manage to see an electron so don't trust what I can't hit with a spanner :. )