I'm driving a 12v, 1.5A (no load) motor from a 12v lead acid battery. From this same 12v source, I connected a 5v boost/buck power regulator behind a few diodes to drop the nominal 12.9v from the battery to ~11.5v to keep the voltage on the regulator in spec. Off the 5 volt, I power a Raspberry Pi 2 (drawing about 200mA), which then closes a relay to power the motor.
The RPi2 starts great off the regulated voltage, and independently the motor works great off the battery. When I hook it all up, however, and the relay is closed, the RPi2 shuts off. The 2nd time this happened during a test, it appears the power regulator got fried, as it no longer supplies 5v.
I'm guessing the motor draws too much current to start, dropping the voltage too much on the regulator. But why would this burn out the regulator? How can I prevent this from happening? Would a large cap across the power regulator input save it?