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Topic: Connecting Grounds Together...why? (Read 4615 times) previous topic - next topic


Hey guys, i was reading the Arduinos Servo Library Documentation and i came across this..
Note servos draw considerable power, so if you need to drive more than one or two, you'll probably need to power them from a separate supply (i.e. not the +5V pin on your Arduino). Be sure to connect the grounds of the Arduino and external power supply together.

now this is not the first time ive read about connecting the Arduinos ground to the external power source's ground, i just do not  know, rather understand, why?
because as far as I understand, and i am just begging to self teach my self , in electronic engineering it is said that current flows from positive to negative, but in real life it actually flows from negative to positive, so if i am using lets say a 9v or even better a 12v battery and connect it to the Arduinos ground would'nt i in "real life" be sending 12v to the Arduino?
I could print the Arduino logo on a box of cereal and sell it as "Arduin-O's"


I'll try to explain it as simple as possible

A voltage is a difference measured against another line, normally the ground.
   (touching only one line gives no voltage, thats why birds can sit on electricity wire - http://www.youtube.com/watch?v=M_V-GI2BQss -

If you want to make device B sense the (output) voltage of device A, it is not enough to to just connect that line. (it will be a bird on a wire) Therefore the grounds are connected so both devices A and B have a common reference for voltages (signals).

Hope this helps to understand,

Rob Tillaart

Nederlandse sectie - http://arduino.cc/forum/index.php/board,77.0.html -
(Please do not PM for private consultancy)


a common ground with a 12V system does not send 12V back to the arduino, because the other pieces of the circuit (eg the insides of the servo or motor controller) are very careful not to let it do so...

Other than that, Rob's explanation is fine for "voltage" based signaling.  For current, a signal travels in a loop, so it needs a path back to the source.   Consider a lamp connected to the + side of a 12V car battery, and the - side of a 1.5V AA cell (with no other connection.)  Nothing will happen, because regardless of which way the electrons actually move, they need to go out one side of a battery and back in the other side...


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