This is a question about electronics theory that I haven't been able to grasp. I was testing a stepper motor wired like the following: https://learn.adafruit.com/adafruit-tb6612-h-bridge-dc-stepper-motor-driver-breakout/using-stepper-motors
The only difference was that instead of wiring my Yaeccc MS-305D DC power supply to the arduino Vin, I wired from the power supply to the motor power on the driver. The sketch failed to work, and I measured the voltage from the cathode to ground as around -2.7 V and the voltage from the arduino output to ground as the expected 5 V. Roughly, it looked something like this:
simulate this circuit – Schematic created using CircuitLab
After unplugging and replugging the ground 1 or more times, the circuit began to work. I figured the arduino ground and my power supply ground should have been at the same potential level so it should have worked initially. I don't know why this was the case.
Then, upon unplugging the ground from the power supply, the circuit still worked. I thought that a circuit has to be completed for current to flow (with the exception of discharging) Can anyone explain this?
//Clarity edit
Here is a more detailed schematic initially:
But now, the circuit still works as following:
I'm not understanding how current flows from the power supply to power the motor when the circuit is not completed.
The circuit works in all of the following configurations:
Best Answer
The lab power supply has three banana sockets, red supply output, black supply return, and green mains earth.
When you have the metal bridge in place, it connects mains earth to black supply return. Since your PC that powers the Arduino has a metal case that is also mains earthed, it means that all the return currents from the 9V lab supply returned via motor driver to Arduino, and from there to PC via USB ground, and from PC back to lab supply black terminal via earth wiring in the house mains wiring.
It is just hard to see as you did not draw the PC in there, or mains earth wiring between PC and lab supply.
That is exactly why the grounding should be considered extremely carefully in a system. Fortunately, most likely nothing got damaged yet, but imagine if the lab PSU currents had been larger.
So take the metal tab off so it does not short the green mains earth and the black return terminals. Sometimes this kind of floating power supply setup is extremely convenient.