Why is my Arduino is getting power from the 5V pin?

Question

enter image description hereI have an Arduino that is controlling two 6V DC motors through a L298N motor driver. The motor driver is being powered (from its Vin pin) by a 7.4V external source.

I then have the 5V pin from the motor driver connected to the 5V pin on an Arduino UNO to power the logic board.

Now, when I plugged in my Arduino to my computer to upload the sketch, everything was fine. However, when I unplugged the Arduino from the laptop, the Arduino was still being powered somehow from the 7.4V external source, though that source should only be powering the motors. I thought that unplugging the Arduino from the laptop would have cut power to the Arduino, but it was still somehow being powered by the external source.

What is going on here?

• jsotola: What is going on here? ... we can only guess ... you provided almost zero information ... please draw a clear diagram of all of the connections
• Johnathon Bulman: @jsotola I added a diagram.
• Gil: How about a schematic the frizzy picture does not help me much as I do not recognize the motor board.
• Edgar Bonet: It could be that this driver board is designes to supply (rather than consume) power through its 5V pin. Do you have a reference to this board?
• 6v6gt: What @EdgarBonet said is the most plausible scenario. It appears that at least some L298 modules have a jumper selectable 5 volt regulator which can be used to supply the 5 volt logic side of the L298 chip but only if the motor side voltage is less than 12 volts. If the motor voltage exceeds 12 volts, the jumper must be removed and the L298 module 5 volts must be supplied from the MCU board. See the example here: wiki.sunfounder.cc/index.php?title=Motor_Driver_Module-L298N . TL;DR ? understand and check the module jumpers.

Answer 1

The Arduino Uno can be powered from the 5 volts present on the USB cable or from an external 5 volts connected to the Arduino's 5V and GND (ground) pins.

In general, power supply outputs should never be connected in parallel unless they are designed for such an unusual configuration. From the description in the Question, it appears the USB power and the L289N power supplies were connected in parallel. Fortunately, the Arduino design include a power cut off shich. Look for part "T1" in this schmatic. If power is applied from both the 5V pin and the USB port, the 5V pin is used and the USB power is cut off.

To answer your question specifically:

• Assuming this is your board's schematic (note, this is a link to an RAR file which contains a JPG of what is likely the wb291111 board's schematic) and assuming you have J6 jumper installed.

• If the Arduino 5V and GND (ground) pins were connected to the L289N board and the L289N board was supplying 5V power to the Arduino board, the Arduino board was never powered from the computer's USB port as transistor "T1" would have disconnected the USB power from the Arduino.

• Johnathon Bulman: Is there another configuration to send 5V to the motor driver logic board? The plan was to have a separate 5V power supply for the Arduino, and have the Arduino send power to the logic board on the motor driver. The 7.4V external source is only to power the motors.
• st2000: It looks like disconnecting J6 will remove the source of power of the motor driver board's 5V regulator. Test this (the "disconnected" 5V pin of the motor driver board) with a volt meter ... as the schematic is a bit hard to read (doesn't look correctly labeled). But why use 2 power supplies if you can get away with only 1 (the motor driver board power supply)?