joining grounds

I have an L298 driver board http://www.solarbotics.com/products/k_cmd/ running a dc motor. The grounds from Arduino and the wal wart (18v) are joined on a breadboard. The pos from the wal wart is to the motor driver. USB is powering the Arduino. Is this the correct procedure? I have recently cooked several chips trying to organize these power sources.

Sounds like it. All you've connected is the ground, right? If so, you should have no problems (at least with that part).

That wall-wart... it is 18v DC isn't it? Not AC, I hope. (There should be a label on the device, although it may show "DC" with a graphic. If you see a little sine wave, you may have an AC output transformer... which WOULD fry the sort of chips we often play with.

Thanks for the advise. I seem to have it connected without the heat! Ruined one chip this morning.

Why not power the Arduino with the same power from the wall wart? BE SURE!!! to run the wall wart thru a regulator setup to drop the voltage to less than 12VDC.... maybe using a LM7812 TO-220? This would assure a set voltage level and a clean voltage from the wall wart.

Mixing grounds can cause all sorts of problems never anticipated.

Ken H>

A good practice is to use a 'star ground' arrangement such that high current loads don't 'ground bounce' the lower current loads (like the Arduino board). That implies having a single central ground terminal the the power source, board and motor all have their own ground wire run to a central point, no 'daisy chaining' allowed. Also be sure the power source and motor load wire size is appropriate for the current load.

Lefty

Why not power the Arduino with the same power from the wall wart?

Agreed, my current supply is a 9v, 3 amp switching supply, the arduino has its own regulator, safety, ect, and if that is not enough my extra stuff has its own matching setup 7805 w/ hs, diode protection ect

so everything is on the same page

BE SURE!!! to run the wall wart thru a regulator setup to drop the voltage to less than 12VDC.... maybe using a LM7812 TO-220?

this only effects you if 2 things are at hand

A) your using a Low Dropout Regulator B) your drawing more than 250ma without a heat sink

otherwise yes its a waste of power, but the 7805 can handle upto 18 volts, and with a chunky heatsink darn near 1 amp @ 12v

A good practice is to use a 'star ground' arrangement such that high current loads don't 'ground bounce' the lower current loads (like the Arduino board). That implies having a single central ground terminal the the power source, board and motor all have their own ground wire run to a central point, no 'daisy chaining' allowed

YES, always send ground to the lowest form, ie power brick to wall, do not chain, or it will bounce like a kick ball in a rubber room!