How does this motor drive circuit work

"Image()":http://i.imgur.com/MTNK0Nx.jpg

The circuit is shown in the link above.



The Drive terminals are connected to a PWM signal generated by an arduino micro controller. The purpose of this circuit is to isolate the arduino from the motors to avoid too much current being drawn.

What I understand so far:

The motor will only turn when the diode is reverse biased. When it is forward biased this will short circuit the motor and turn it off.
The opto isolator (LED and NPN transistor) allows current to flow from the 6V rail towards the -6V rail whenever the LED is turned on.

The rest confuses me. Can somebody please walk me through step by step how this circuit works?

Thanks!

The 4n46-4 part is an optocoupler, wich is basically an infrared led and a photosensitive transistor in one package. Whenever the led is lighted up by the arduino, the transistor part starts conducting electricity. This construct isolates the circuit from the arduino in every way. The resistor Rx reduces the sensitivity of the optocoupler, so it doesn't turn on with the slightest current. It also makes the motor turn off faster. The current coming from the optocoupler then goes thru the 1K resistor, and ohm's law makes that create a voltage on the gate of the FET. The FET now starts conducting electricity in big quantities thru the motor, that will start turning. The diode does not conduct at this point. The capacitor shorts hight frequency pulses coming from the motor so you will not hear them on your radio.

When the arduino stops lighting the led, the transistor part stops conducting, the 1K resistor will no longer have a voltage over it and the FET stops conducting. The motor turns off at this point. Because electric motors have coils in them, the motor now start generating electricity in a reverse polarity, the (often very high) reversed voltage generated could damage the FET, but luckily it is now shorted with the diode.

Thank you! This has explained it perfectly. I just didn't understand the purpose of the diode, as I thought it would never be forward biased. I didn't consider the motor acting as a generator. I now understand.

Thanks!