Take the 2-minute tour ×
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free, no registration required.

So I have an Arduino and an L293D. The L293D logic is powered by the +5V output of the Arduino, while the DC motor is powered by an external power supply.

Here's the part I don't get. If I connect the Arduino and the power supply grounds together the circuit works, otherwise it doesn't. Can you explain me why?

share|improve this question

1 Answer 1

up vote 6 down vote accepted

While a schematic would have helped describe the problem statement better, one key concept might help in clarifying this matter:

  • A voltage is the potential difference between two points in a circuit, it is not an absolute value of any physical characteristic at a single point in a circuit. Thus, there is no absolute potential involved, it is relative value, a difference.

How this applies:

The control side of the L293D is actually powered by the +5V from the Arduino only when the +5V has a reference ground available, that corresponds to that particular +5V supply, in other words, the ground of the Arduino board.

The L293D does not have a separate drive-side ground pin, just the "Heat sink and Ground" pins, which are also the ground reference for the \$V_{cc1}\$ pin.

If you note the schematic on page 3 of the datasheet, the ground references for \$V_{cc1}\$ and \$V_{cc2}\$ are one and the same, the GND pin(s). Thus, that reference needs to be connected to the Arduino's reference ground.

share|improve this answer
Thank you for the complete answer, I get the problem now ! So do you think it's a good idea in that case to plug the L239D +5V to my external power supply too ? In that case I would only have the control pins connected to my arduino ? (I've notice that if for example I don't put capacitor on my DC motor inputs I get a lot of noises across the whole circuit, so I would like to decouple it from the arduino as possible) –  Emmanuel Istace Dec 22 '13 at 10:02
No, the +5V should not be connected to the external power supply at all - else you'll have either a blown regulator IC on the Arduino (most likely) or a blown power supply (not so likely). Just the ground pins are to be interconnected. –  Anindo Ghosh Dec 22 '13 at 10:23

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.