Low-currrent regulator and high-current motor driver in parallel

I'm trying to control a large motor with an Arduino Micro. From the power supply, current goes through a power switch and a diode before reaching a branch. The two branches lead to: (1) An appropriately rated motor driver breakout (11.8 V, as much as 5 A). (2) A voltage regulator to drop down to 5 V before going into the Arduino's Vin pin (maximum 1 A).

Theoretically, I wouldn't expect the high current draw across the motor to overload the voltage regulator, since they're in parallel. But is there any chance that it might? I ask because I've had a few voltage regulators work just fine for a little bit, and then start acting very strangely: (1) Vin and GND somehow got internally shorted, which caused a series of VERY BAD THINGS. (2) Vout produces 0 V. (3) When I tried just feeding the Arduino 11.8 V, hoping that its built-in regulator would handle it, the 5V pin started outputting around 8.5 V.

I'd like to sort this out before I destroy any more regulators.

Thanks, everyone!

Please do not continue this way.
Is there someone who can check your wiring ?

When using 11.8V to power the Micro, the 5V should still be 5V.
Was there something connected to the pins ? Did you have a ground wire ?

I’m afraid that you have connected a motor driver in a wrong way, and that current is going back into the Arduino board.

Always work on a clean part of a desk. No loose wires, or components or metal tools near the Arduino board. Check everything before you turn on the power supply.

Sounds like you've got high voltage spikes on that power line from the motor.

In general sharing supplies between motors and other circuitry leads to grief.

Without an oscilloscope its hard to diagnose for sure. What actual 5V regulator are you using, some are good for 26V input, others much lower.

Since you have a diode before the regulator/motor then any inductive spike from the motor has nowhere to go but into the regulator.

You haven't told us which motor driver you're using - please provide such details its important to be able to check on their specs. For instance does it have decoupling?

The driver board I'm using is the Pololu VNH2SP30: http://www.pololu.com/product/706 The regulators I've tried are the Pololu S7V8A and S7V7F5: http://www.pololu.com/product/2118 http://www.pololu.com/product/2119

The inductive spiking seems very likely, and I'm kicking myself for not seeing it earlier. So the solution then is to remove the diode from the power supply and place a flyback diode in parallel with the motor driver? If so, what kinds of specs does my flyback diode need? I have some 50V 1A ones kicking around, would they suffice?

Thanks for the help.

The solution is to use a separate supply for motor and logic, ideally.

You haven't said what your supply is, if its anything like 12V then just use a 7805 which is good to 26V for logic power? Just noticed this comment:

A voltage regulator to drop down to 5 V before going into the Arduino's Vin pin

but the Vin pin needs 7 to 12V...

Unfortunately, project constraints prevent me from using two separate power supplies. I did mention that the motor was getting 11.8 V - this is 3 x 18650 cells. I have been getting the Arduino to run fine on about 5 V input - there is a small drop, but it's easily compensated for. Which is good because the Arduino's onboard voltage regulator seems to be shot.

I've removed the Schottky diode from the power supply and placed it in parallel across the motor driver. So far, this seems to be doing this trick. Thanks again!

Strangely, at the same time that I put in the flyback diode, the two Hall effect sensors in my system started acting weird. They're analog sensors, so with a 5 V input, they should have a baseline output of 2.5 V, which they have up until now. Now they produce 0.6 and 0.8 V. I tried removing the flyback diode, but the problem persisted. I tried checking the input voltage right on the sensor, and I still got 5 V, so that isn't being attenuated in any way. They still respond to magnets, so they're not completely broken, but this suggests to me that something has gone wrong inside the sensors themselves. I can replace them, but I'm concerned that if I don't know the cause of this, it will happen again to the new sensors. Any thoughts?

Your problems are all because you are feeding 5V into the Vin jack. Stop doing it. You might have blown your arduino doing this.