Blown arduino board - grounding problem with h bridge

Using Arduino IDE 1.x
1

This was my first arduino project and my first time posting on the forums. I appreciate the opportunity to tap the knowledge of the community here.

I've just blown my arduino rev2 wifi (it overheated massively for a while, the on LED flickered for about a minute then the whole board just permanently died).

I've gone back through my workings and I can see one error I made but I don't know whether it would have been serious enough to overheat and blow the board.

I bought a Pololu md30a h bridge to control a 12v motorised ball valve. (The Arduino itself was powered by a separate 9v power supply). As per the diagram here https://a.pololu-files.com/picture/0J6505.1200.jpg I was meant to ground the left side of the h bridge to the arduino as well - the bottom left connection in the diagram. I did not do this, I only fed the PWM and direction connections on the h bridge to 2 output pins on the arduino. Everything on the right side of the h bridge in the diagram I did correctly. Would this lack of grounding to the arduino have been enough to overheat the board and blow it? I also had a water flow sensor drawing 15ma at 5v from the 5v pin on the arduino and every 3 seconds the arduino was sending POST data to a webpage through its wifi module all at the same time as I was sending output through the 2 pins to the h bridge to control the valve.

Thanks

2

Or if i can pose a much more simple question:

What are the dangers if you do not ground a pwm output from an arduino to the component you are sending the pwm signal to?

3

Normally an item would not work or would tend to be erratic without the ground.

Dont personally know of a way to blow something that does not have that ground.

Bob.