Connecting grounds together...why?

Question

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? which would ruin it!

Answer 1

Hello Eddie,



Remember back to physics class where you learned about potential energy of a falling object. The potential energy gets higher as the object is lifted higher from where it will fall.



Voltage is like this - it is a potential difference. If you had a 1,005 VDC generator and had a 1,000 VDC generator (that shared a common ground) you could connect the 1,005 volts to the VCC pin on the arduino and the 1,000 volts to the GND pin and it would work fine because the difference is 5 volts! The difference between the voltages is what matters. This is why having a common ground matters - when you have a 12v source and a 5v power source you know they are delivering 5V and 12V respectfully.



As for the negative to positive thing, yes this is true, but chip makers follow the electrical engineering conventions. VCC is connected to the positive side of the battery, GND is connected to the negative side. Hope this helps. Let mr know if it doesn't.

Answer 2

Most things will work ok with multiple grounds, but ground loops occur which usually involve millivolts and milliamps, unless lightning strikes. For very weak signals these ground loops can cause problems. If you can give us some details about Arduinos perhaps we can help with the last part of your question. Neil

Answer 3

Connecting the grounds establishes a reference for all for the electronics, a common rail.

Conventional flow is from positive to negative for electron flow while actual electron flow is negative to positive at the atomic level.

The word says it all though, " Convention ". Its a ' conven - ient ' way of dealing with a circuit. If you want to use electron flow use it instead. Both are equally right.