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I have the following circuit: Arduino circuit

And I know it works and that the diode protects the negative voltage from flowing back into the transistor. This is essentially the circuit: Circuit

Could someone illustrate what happens when the motor stops? The current normally flows from 5V to GND (and the electrons from GND to 5V, but this doesn't matter (I think?)), and then it reverses when you switch it off, so current comes out of the top of M1, and then what? I don't get it here.

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    This may not be the best suited for Arduino, it's more about Electronics.. – TheDoctor Oct 24 '15 at 20:25
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While the motor is being powered the current flows from Vcc to GND as in this circuit:

enter image description here

When the power is disconnected (such as the transistor in the ground connection in your circuit is switched off) amongst other things (including the collapsing of magnetic fields) the spinning of the motor turns it into a generator. The power generated is the opposite of what was being provided to make it move. As the magnetic field of the coils collapses additional high voltages are also generated.

When disconnected the motor can be represented as a battery connected backwards. In that configuration the current flows from the + of this hypothetical battery through the diode into the - thus stopping any current from going elsewhere, as in this circuit:

enter image description here

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    If it was only the motor turning into a generator, most transistors could cope with that, because the voltage generated by an open generator is limited. The residual current in the motor coils is the problem, because they provide a infinite voltage in theory when interrupted: U=L*dI/dt. In reality this voltage can reach some kV before it breaks down by some means. – Ariser Oct 24 '15 at 19:48
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    @Ariser One thing at a time for the poor learner's brain. – Majenko Oct 24 '15 at 19:49
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    Thank you. This makes a bit more sense now. I was wondering why current wouldn't flow into the arduino but it is shut down so it will obviously find the shortest way which is into the motor. Your representation of the motor as a battery helped me understand it. Thank you! – stenlan Oct 24 '15 at 20:44

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