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miner_tom , 10-18-2024, 12:08 AM

In the video with Eric Bogatin "what every PCB designer should know - return current path" at about 10:50, Eric mentioned Maxwells 4th law and the displacement current, as the means by which the signal flows down the transmission line and returns down the return path to the source. https://www.youtube.com/watch?v=icRzEZF3eZo&t=392s

However, my understanding of the displacement current part of Maxwell's 4th law is basically that the curl of the magnetic field is the result of the changing electric field. It is also my understanding that a current does not pass directly through a capacitor. Yes, I know that in the transmission line representation there is not a "real" physical capacitor but the transmission line is a representation of an infinite amount of infinitely small capacitors across the dialectric (between the signal and the return). Not that I wish to argue with Eric Bogatin, but if there is no actual current flowing through the dislectric then the only current in the return path is caused by the curl in the magnetic field which is the result of the electric field change. This would make the animation somewhat misleading. Perhaps the animation would be less misleading if it could show the return current in the return plane being generated by the curl in the magnetic field.

I hope that someone can clarify my understanding of what is actually happening.

Thank You
Tom

QDrives , 10-21-2024, 08:08 PM

A very academic way of describing things.
But I think you have a problem - nobody has been able to explain the electric and magnetic fields extensive enough (for this question). At least, I have not seen or read anything to that level.
Then again, most of them have the electric field at the front edge and not so much the magnetic.