xxmamakinxx:
Everything works fine with a small 3V toy motor, but when I hook it up to my 9/12V - 3 amp car seat motor, the Arduino crashes, and can't control the H-Bridge. It just turns the motor off and the Serial Monitor stops reading. I am using the Vin to power my Arduino and motor from a bench top power supply. I replaced the small protection diode by the barrel jack with one that can handle high current, but I'm still having the same issue. Any recommendations?
Well without seeing the whole circuit I've nothing much to go on, but it sounds
like you have massive interference going on, which could indeed be caused by
powering the Arduino from the 12V rail - so that's the first thing to correct if so.
However there can be other routes to interference once you have large currents
being switched, perhaps the next most common being bad circuit layout. Current
loops in high-current circuits are to be avoided since they radiate magnetic flux
which can link with other more sensitive parts of the circuit. In practice make
all cables compact, twisted pairs for instance.
You also must keep high current ground wiring away from the low current
electronics, sharing grounds at a single point is a good plan.