I am trying to understand how common-mode chokes work. I get the basic concept: the signal lines are joined by a transformer, such that each line will aid or impede current in the other. But I am hazy on the details.
My main question is:
- When allowing signals to pass, do the magnetic fields cancel out or add?
Google has turned up contradictory answers. This page offers an illustration:
The diagram says "flux adds to impede common-mode current". Well, to me, the phrase "flux adds" means that both sides of the coil are driving in the same direction, and the ferrite core is gaining a magnetic field. Conversely, in differential mode, if - as stated - "flux cancels", then I imagine the two sides are driving in opposite directions and generating no net magnetic field.
Except this understanding seems to contradict Wikipedia:
The convention is that current entering a transformer at the end of a winding marked with a dot, will tend to produce current exiting other windings at their dotted ends.
By this definition, I would expect that (in the illustration) current entering terminal 1 would induce current exiting terminal 2. If, as illustrated, current were applied to both terminals 1 and 2, they would attempt to generate magnetic flux in opposite directions, and thus oppose each other, and result in zero net magnetic flux.
This would seem to contradict the assertion that "flux adds" in this scenario. Am I missing something?
EDIT: Seems I need to clarify my question, to avoid unhelpful responses like "a choke is not a transformer" and "try thinking".
First, a choke is a transformer.
Second, if the transformer is wound such that a current from 1 to 4 would result in a current from 3 to 2, then a current from 3 to 2 would result in a current from 1 to 4.¹
Therefore it makes sense that these two currents are associated with the same polarity of magnetic flux.
Therefore applying both of these currents should result in an addition of flux.
Therefore common-mode operation should result in a cancellation of flux.
Which of my statements is incorrect?
¹ If this statement is wrong - i.e. the windings are not wound that way - then THE DOTS ARE IN THE WRONG PLACE in the illustration.