1
\$\begingroup\$

I'm using Arduino to control 2 stepper motors via two l293D drivers. I'm using a separate power supply for the steppers (6V). I'm looking forward to expand the system to 3 up to 10 motors.

Now i have read that i should share the grounds so i have connected the ground of the Arduino (powered from the USB cable) with the ground of the secondary power supply. It works all right. I haven't tested disconnecting the grounds. Now i read in an answer here in this site which is selected as the correct one (albeit for a different question) "Never share grounds."

Powering a Servo: Do I need a separate power source?

Can someone explain a bit what is the case?

Thanks for the answers. Now please a bit more so to get it, I'm a newbie.

enter image description here

Now in the l293D (2 of these) i have connected the grounds all together (4,5,13,12).

In one of the answers the author talks about opto-isolation of the 2 power supplies (control and load). This means that if i raise the voltage and the current of my load i.e. use more motors, the circuit in it's current stage may misbehave and i should opto-isolate the grounds for precaution?

\$\endgroup\$
1
  • 1
    \$\begingroup\$ "... i should opto-isolate the grounds for precaution? The opto-isolators completely isolate the circuits from one another. There is no electrical connection between the two. (You probably understand this but haven't expressed it properly.) \$\endgroup\$ – Transistor Jul 2 '16 at 23:39
4
\$\begingroup\$

If you are sending a signal via wire from the Arduino to the motor controllers then the current requires a return path to the Arduino. The common ground provides that.

In many applications (particularly industrial) it is common to isolate the power supplies. In that case an opto-isolator is used and the return paths are local to each side of the opto-isolator.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. Opto-isolated connection allows isolated power-supply grounds.

The risk with common grounding in your case is that under some fault condition large motor return currents use your micro as the return path to the motor power supply. This could, for example, burn the relatively light PCB traces on your micro or introduce significant noise on the ground lines and interfere with operation. Good wiring layout can minimise the risk of this.


The concept of separating power and logic power supplies is very simple.

schematic

simulate this circuit

Figure 2. Bad wiring practice. All the motor current returns through the micro-controller board risking interference and overheating of low-current traces.

schematic

simulate this circuit

Figure 3. Good wiring practice. Here all the motor current only flows in the 12 V loop. The micro and motor control board power-supply grounds are connected at one point only (shown in green) so that the control signals have a return path.

\$\endgroup\$
8
  • \$\begingroup\$ Time for sleep in my country so I might expand in the morning. Please un-accept the answer for a day or two. Accepting so quickly will discourage others from posting (possibly) better answers. You can re-accept in a day or two if nothing better shows up. \$\endgroup\$ – Transistor Jul 2 '16 at 23:41
  • \$\begingroup\$ "Should i use an opto isolator when connecting the 2 grounds together?" No. Either connect the grounds (and do direct wiring from one board to the other) or use opto-isolators to transmit the signals from one part to the other (without wires). Think of the opto-isolated circuit as having the controller on Earth and the motors on the Moon with communication between them using light. No wires. Fully isolated. \$\endgroup\$ – Transistor Jul 2 '16 at 23:44
  • \$\begingroup\$ A good PCB layout would make sure that the current from stepper motor will not return through the uC/procesor.For a begginer, I recommend to understand the whole concept about ground loop. \$\endgroup\$ – Stefan Merfu Jul 3 '16 at 1:10
  • \$\begingroup\$ @JohnAm: No, you don't need opto-couplers. Just make sure that your motor controller grounds go back to the motor power supply and not through the Arduino. Connect the two power-supply commons at one point only. \$\endgroup\$ – Transistor Jul 3 '16 at 23:14
  • \$\begingroup\$ Nope. Those four pins will be connected internally and act as heatsink / ground. See ti.com/lit/ds/symlink/l293.pdf. I'll update my answer in < 1 hour. \$\endgroup\$ – Transistor Jul 4 '16 at 11:34
0
\$\begingroup\$

Each electric circuit has its own reference potential. If it is floating, then this potential can gain an arbitrary voltage. Since these circuits are always on the planet Earth, whats more even connected between them, there is a potential difference between the. When the voltage is beyond the whitsanding voltage the insulation breaks. Basic rule is that you always have to reference the potential at some specific level, called ground, earth, GND,...The goal of galvanic separation is to break the ground loop.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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