EMC ​​​​​​​

Learn EMC is a nice resource for EMC education. On their pages, I have found a quiz about return current: https://learnemc.com/identifying-current-paths The bottom example 1d) shows a twisted pair above a ground plane. One wire is driven with a voltage between wire and PCB and loaded on the other side PCB. The second wire is connected to the PCB on each side and both PCB’s are connected to the enclosure. Then they state that: “However at low frequencies, a significant fraction (perhaps most) of the current will return through the chassis grounds of each device.”

So, I have asked myself: what are low frequencies and what is a significant fraction? A quite simple task to find out by simulation. I have modeled a 25 cm cable, driven by 1 Volt and terminated by 1 Ohm. Below 1 kHz almost all current flows through the ground. At 17 kHz the current is equal in ground and the n wire. Around 500 kHz the current sum starts to decrease because the inductance is contributing significantly to the impedance. Even at 10 MHz there is still significant current through the ground plane, which is of course not ideal and has its own distinct resistance and inductance.

Therefore: Use educational resources to understand the basics and then apply simulation to check the actual values.

Models are attched!

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