Building a fully proper motor control circuit on Raspberry Pi is literally an introductory course in power engineering. The protective circuitry literally is a low-voltage AC power adapter between your motor and your computer. The first part that piques my interest: upon a careful analysis of the diode protective circuits in the L293D, I found out it was actually a diode bridge rectifier between the motor and its power source.
So if you turn the motor on its own, it is a generator that supplies power to your computer. Of course you can’t control your motor at the same time it is being used as a generator.
Kids, don’t try this on commercial consumer electronics at home. I did, and the electronic in question almost turned into a brick. That’s because in the interest of cutting costs, they eschew on proper electrical isolation in both their motors AND AC power supplies. Lucky for me, the particular electronic did have some alternate design wherein there was effectively a polyfuse that would not let too much current flow from the motor into the rest of the circuit, when tripped it would refuse to provide any power to the entire device circuit.
But, you should still be forewarned, in college classes on electrical/power engineering, professors do note to students that popular consumer electronic gadgets omit some protective circuitry from the AC power adapters in the chargers in the interest of saving cost/bulk/weight and reasoning that such power fault events are rare. “Let the buyer beware,” but especially for home use, the buyer is but one of the many people who use it.
Key components of protective circuits in power systems: (1) diode bridge rectifier, (2) flyback diodes, (3) decoupling capacitors, (4) voltage regulators, (5) polyfuses, (6) large grounding plane, (7) isolated protective ground.
Commercial electronics cut costs by omitting components #2 and #4 as they apply on the high-voltage side, since these are also larger and add more bulk and weight to your mobile device’s charger.
Speaking of volage regulators, here are some useful info sources for the sake of building really cheap voltage regulators. Voltage regulators are, of course, good for electrostatic discharge protection.
20191115/https://en.wikipedia.org/wiki/Voltage_divider
20191115/https://en.wikipedia.org/wiki/Zener_diode