power supply - Is it safe to use a voltage divider for Arduino and DC motor controller?

I have ordered a Dual H-Bridge 12A continuous (30A peak) Polulu motor driver shield for my upcoming (first time ever, a 1.2m long tank made of wood) robot project.

I was thinking about how to supply the power to all the components in the circuit without wasting much power, but the only thing I can come up with is this simple voltage divider connected in parallel to the motor drive:

Knowing the Arduino can handle max 5V I did make sure to make it a bit lower.

But my wonders go to the fact that I am suspicious that connecting the motor driver and Arduino to the same power source isn't a great idea? Or am I paranoid?

And what about power waste by the voltage divider, how can I reduce/minimize it? Make the resistors smaller? (15/10 ohm - but more error and higher current)

Is there a formula to find the most effective R1 and R2?

Do I have to worry about current spikes?

Any other things I could miss in this simple-yet-suspicious circuit?

Also, if I wanted to simulate any possible scenario, which components should I choose in the circuit to keep it as simple as possible (for simulating in Multisim 13)?

Answer

Using a voltage divider as a power source for almost anything is a Very Bad Idea. The actual voltage supplied to the load will vary with the load current. I expect that the current drawn by an Arduino will be Much, Much Greater than is drawn by your 10 meg resistor, so the voltage will be much less than you expect.

You should use a DC-DC converter (AKA switching regulator) to drop the 12 volts down to 5 for the logic. A linear regulator such as the LM7805 could also be used, if the 5 volt current demand is low, but a linear regulator will waste the excess power as heat.