This is a pretty standard design for H-Bridges that I drew and will be using. I picked mechanical relays because of availability, expandability for an eventual larger robot, my arduino motor shield is already intended for my CNC project, and I didnt want to keep buying transistors should I miscalculate the amperage. In this design, I went for a single voltage input (+V) to supply the relay coil, Va, and Vb.
Summary:
Va is default choice. The relay is set to Normal Close (NC) with Va supplied to the motor for the default normal (Forward) direction. The diode is for back-voltage for the DC motor. The NPN transistors are set up as a Darlington series for PWM input from the Arduino microcontroller pin (PWM) that operates at either 3.3-5 V to control the speed.
Vb is second choice when the relay coil is energized for the motor to move in reverse. The coil is energized when the microcontroller pin (REV) enables High through the Darlington series. The coil has 200 ohm rating. The relay will then switch the motor input to Vb reversing the direction. The diode is again intended for back-voltage for the DC motor. The PWM input from the Arduino microcontroller pin (PWM) will control the speed.
NOTE: I still have to determine if the diode is in the right spot or should be moved to a point after the Darlington PWM series and between +V. I still may need to add resistors to the coil for equal voltage division through the motor.
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