Hey guys,
I'm trying to wrap my head around how my circuit will behave with a certain layout but I'm stuck.
Let's say I have a 12V motor, and 2 12V LED bulbs I want to control with rudimentary PWM. They are connected to a 10' long cable with four conductors. One of the conductors is 12V. The other three terminate at my Arduino, where I have three NFETs to pull them to ground when I want to turn the device on.
Now, I need a flyback diode for that motor. And I'd like to put it at the Arduino end of the cable.
The problem is, I'm not sure what's going to happen to that 12V line when I shut the motor off.
If I had the diode at the motor, then when I shut off the motor and the magnetic field collapses, the + and - on the motor flip, and the current flows through the diode and back into the motor until the motor's resistance dissipates the power. There's still 12V connected to one side of the diode, but the current doesn't seem to care about this and still flows through the diode.
I think what's happening here is that to the motor and diode, 12V has become the new ground, and the other side of the diode is at such a high voltage that the potential difference allows current to flow through it. So I think that nothing else on he 12V line will see the voltage dip.
Anyway, to get right down to it, what I'm trying to figure out is if my two bulbs will shut off momentarily every time I turn off my motor if I have that diode all the way at the other end of a 10' cable.
I don't think they will, but I can't be sure.
Another thing I can't be sure of is that they won't see a high voltage reverse spike. Like, when the motor flips, if it can't pull 0V up because of the long wire and its inductance then maybe it will pull the 12V line to -24V or something and blow the LEDs which aren't rated for that kind of reverse voltage?
Anyway, I'm sure half of what I just said is crazy, but I'm hoping someone can explain how this thing would behave.