I'm using an Arduino Mega to drive 7 servos (only 3 shown in my schematic), and I'm having some problems with the power management. Worst case scenario, the servos will draw about 4 amps, so I got a 7.5V @ 4A power source that I can connect to an independent DC jack on my custom shield.
Now the problem is that since I command the servos through USB, the arduino has to be connected to the USB, which puts a voltage of 4.3V on the Vin pin.
The circuit you see above is what I originally thought. The diode was supposed to keep the servos from pulling current from the arduino when both of them are connected. However, an obvious problem now is that if both sources are connected (USB to the arduino, and my external power source to the DC jack), then you are asking for a ~2.8V drop across that diode.
While I could disconnect Vin entirely, I want the capability of just having the DC jack connected, and still power the arduino so I can control the servos with a pre-programmed routine. But it still has to work when the arduino is connected to the USB.
Putting an extra resistor in series with the diode seems like a very ugly an inefficient solution. Any ideas on how to make this work?
Thanks in advance for your answers!