I have a raspberry pi 2 model B installed Raspebian powered via adapter of 5V and 750mAh. It is used to control 3 stepper motors via DIY motor driver circuits on GPIO pins powered by an external power source of 20V.
https://www.raspberrypi.org/forums/viewtopic.php?f=49&t=55580 is the guide I'm using for a single motor.
The issue here is that when I connect the power supply common ground to raspberry pi, the power supply voltage drops to 9V for 1 motor. It drops to 6.5V for 2 motors and 5.8V when all three are connected.
I have tried Arduino input to circuit with its common ground and that drives all motors successfully without any drop. I have tried the GPIO of Raspberry Pi and common ground of Arduino but it gives the drop all the same.
I have tried other power sources like 12V and 32V for motors and the drop is exactly the same i.e. 9V, 6.5V and 5.8V for 1,2 and 3 motors connected at a time respectively. Changing power sources of Raspberry Pi didn't help either.
I have tried different ground pins, the issue persists.
I need a significant voltage for the motors and a common ground is also necessary but everything I've tried so far hasn't helped.
All motors are bipolar.
Any other possible solutions? Or where should I look for the problem?