I have a Gadgeteer setup with power hungry modules. I normally power them by a 12V car battery into the USBDP module, and that works fine. Now I started using stepper motors and they require a seperate power source.
By splitting the power source into two barrels, one that goes into the USBDP and one that goes into the stepper motor controller does not work.
As soon as the USBDP is attached the voltage falls to about 8V not 12V - and the stepper does not work.
your 12v source still can’t deliver enough current. I have a 12v setup for my coffee roaster, and I draw ~2A constant when the motor (an old car windscreen wiper motor) is on, or well under that when not running. My 12v lines come from a car battery charger, they go in to two locations (sharing GND) one goes into a barrel jack into the USB DP module, and one goes onto a relay; the relay gets energized from 3v3 (via transistor and an IO pin from Cerberus) and the 12v then runs to the motor. Been working fine and I’ve roasted about 150kg of coffee in 2015 with this, so I know it’s been reliable!)
I assume the 2 grounds are common to both supplies? The only time this cannot be is if the motor driver is optically isolated but I assume in this case it is not.
Is the motor controllers input isolated from the USBDP output? In other words, does powering up the motor controller feed power back out to the 5V and or 3.3V that the USBDP normally supplies? You can’t have 2 power supplies feeding the same power rails, not without diode protection at least.
Post some more info on the motor controller so we can see how the power input is arranged?