wmorr:
Hi,
Thank you thus far for your input.
Sorry not posting the schematic. I've added it to the original post. Won't make that mistake again.
pwillard and MrAl answer explains the confusion around the diode. Electrons won't hop onto the anode because the cathode has the higher positive charge. Thank you. Karma given to both.
With some of my semantic mixups (e.g. calling the common ground a floating ground) and lack of schematic there would be some confusion around my other question about the looping of circuits. I actually think pwillard's response here may also satisfy my question, but I'm going to take time to digest that and read more about the impact of common grounds.
In the meantime, I'm definitely interested in reading your follow-up responses (thanks again) with the schematic added. Just having difficulty seeing a return path of electrons back to both power sources.
Regards,
Chris
Hi again,
Now that we can see your schematic it is more obvious what the diode is for.
The diode is there to prevent the inductive kickback from the motor coil which acts like an inductor. When the MOSFET is turned on, the current flows downward in the motor and the motor runs normally. But when the MOSFET is turned off, the coil in the motor has stored energy that it wants to release in the form of a reversed voltage. That's the same idea that makes a boost circuit work, but here we dont want a higher voltage so a diode shunts the kickback current around the motor so it can eat up that stored energy. If it were not for that diode, the motor could develop a very high positive voltage at it's lower terminal in the schematic, which could easily blow out the MOSFET.
In the drawing you'll see the motor replaced with an inductor. A motor has series resistance too which isnt shown, but the coil acts very similar to that inductor and when the MOSFET turns off the inductor creates a very high reverse voltage as shown with the polarity symbols plus and minus. That makes the diode conduct, and that eats up that stored energy so the 'inductor' does not blow anything out.
Note that if one lead of that diode becomes disconnected, the MOSFET might blow out unless we get lucky and the inductor saturates before the voltage gets too high.
That diode is a common thing to see in motor circuits and relay circuits too as relays have inductance too.
You can see the path the current takes by looking at that red arrow. It loops around back to the other terminal of the motor. The motor has been replaced with an inductor because that is the aspect of the motor we are most concerned with when analyzing the action of the diode.
As a final note, you might want to notice the file size of the attachment, which is very small. That's because the jpg format works better for drawings that have a very varied color scheme. The original was something like 800k bytes while this one looks almost the same but only uses less than 40k.
