Editor’s note: This question was asked in response to Interference Technology’s recent webinar by Keith Armstrong. To view the webinar, click here.
Question: Could You Show the Path of the 310uA Current through the Victim (E-Field Coupling)?
Answer: Good question, because being able to accurately visualise where the stray current flows is an important part of the skills we need to develop to be effective EMC engineers!
Unfortunately it is impossible to say where this stray current will flow, because it depends on all of the physical details of each specific situation – and the sketches that I used in my PPT slides didn’t include sufficient information.
For example, the victim circuit that I drew could be connected to the same power and ground rails as the source circuit; or it could be powered completely separately and only share the same chassis as the source circuit, or it could be completely floating – unconnected to the source circuit. These three different situations could easily have very different stray return current paths from each other.
However, we do know, from EMC Physics (i.e. Maxwell’s equations) that the stray current will preferentially flow through paths that result in the least magnetic field energy – and these will be the paths with the lowest overall impedance.
These paths could be along any conductors or through the air (i.e. via stray capacitance once again) – whatever returns the stray current to its original source with the least field energy.
It takes a full-wave field solver computer simulator that is supplied with the complete three-dimensional structure and all the electrical parameters of the materials used, to fully analyse where stray currents will prefer to flow.
Experienced EMC design engineers are – to some degree anyway – generally able to visualise the strongest paths taken by stray currents, but they have to beware of structural resonances that can what appears to be an important low-impedance path have a very high-impedance instead, and can make what appears to be a negligible (high-impedance) current path actually have a very low impedance instead – at certain specific frequencies.
To help with determining the actual stray current path, we generally use a range of diagnostic tools such as close-field probes, clip-on ferrite chokes, copper tape with conductive adhesive, etc., etc.
Of course, if we happen to have a complete 3-D characterisation of the situation and know what the various materials are (as we do (or should do!) in any competent product design project), and if we also have access to a full-wave field solver and know how to drive it – we can let the computer work it all out for us!
Suitable field solvers are very costly items, but the time they save in design, development, compliance, and getting to market usually makes it possible to justify their cost on the basis of a payback on the first project they are used on! Financial managers cannot resist such arguments; if they are presented correctly (I have an article on how to do that, if you want).
-Keith Armstrong
