Hi - I tend to develop projects using a 5/12/variable bench PS that can deliver 3 amps (with a wall wart for the arduino). All good but when it comes time to deploy the project I need to come up with mixed voltage/current sources and that is where I get stuck for a good solution.
In a current project I am driving a big stepper with a 20v PS, and also need to drive 2 servos and a 24 v fan. I am going to control the fan off the 20 V PS using a TIP120 transistor and am resigned to using a wall wart for the arduino.
The problem is the servos - each one draws @ .25 A with a peaks @ 1 A. I have tried using one 7805 V reg off the 20 V PS to power both but the 7805 gets very hot and (I assume) goes into thermal shutdown: combined current draw peaks > 1.5 A.
I am going to try using one 7805 for each servo to see if that works. Short of getting a 3rd PS for just the servos, is there a better way? I don't like running the 7805s so hot and wasting all that power.
Long winded... sorry! 
If you are creating a regulated 5V supply for the servos you can use that for the Arduino, too. Just hook it to the 5V pin.
That 7805 might not be the best chip for your conditions. This model can take up to 35V input and produce 5V at 2A output:
STMicroelectronics L78S05CV:
http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=497-1468-5-ND ($0.76 each)
I am going to try using one 7805 for each servo to see if that works. Short of getting a 3rd PS for just the servos, is there a better way? I don't like running the 7805s so hot and wasting all that power.
I use the 7805 to power my pan/tilt servos (best with one per servo). A servo under a load will make them hot. I put a diode in series with the 7805 ground to up the output voltage to ~5.7v for better servo performance.
Thanks for the reply. FWIW - I just did a test running the 7805 w/ 20 V input (using all the filtering caps as specified here: http://www.play-hookey.com/analog/experiments/plus_5_volt_ps.html). I hooked up an LED to the 5 V output (along with the servo) and while the servo motor runs the LED dims. I expected the caps to prevent that sort of brownout - so I'm not sure running the Arduino off the 7805 would work.
The 7805 gets pretty hot too. Will put on a heatsink, etc. but may have to try another chip as you suggest (though 7805 chips in hand vs mail order delays, etc.)
Jameco has the L78S05CV for $.35 each but you have to order 10, (but why not if the postage will actually be more). I'll get some of these on my next order. I've powered my arduino from a regular 7805 without any noticed issues.
https://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_889461_-1