To insert 100mA on to the positive line of usb cable. (The usb positive on my tablet puts no power out but my midi wants 100mA) a 10v zener diode stops power from shorting into the android. The negative on the inserted batter goes direct to negative usb line.
In this demo the lamp represents the midikeyboard. The android is "simulated" by the square circuit of 12v battery/resitor/led. The 9v battery on the right is the battery to insert the power and with the resistor/transistor/resistor//z-diode the power is conditioned to put out at 100mA required to power up the midi keyboard without shorting to the android circuit.
The common ground is common ground and the 2 data lines need no cutting or interupting in any way.
When hooked up the usb connection is detected, just no power is supplied to the machine via the tablet, but is supplied when using the phone. The internet suggests quite a system of otg > usb y with mobile charge battery > hub > device. To me that seems way overkill.
The goal of 100mA to the midi and 0v flowing back into the android device seem to be satisfied with this demo, and thus i shall see if feedback comes in saying that yes or no this will or will not work, or improvements suggested.
