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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:

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.

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  • \$\begingroup\$ Simply apply the right-hand rule and (conventional current) you will see in which case the flux will add or cancel. \$\endgroup\$
    – G36
    Sep 14 at 13:55
  • \$\begingroup\$ @G36 which doesn't explain the inherent contradiction of the dot placement. \$\endgroup\$ Sep 15 at 17:33
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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.

This is correct.

Therefore it makes sense that these two currents are associated with the same polarity of magnetic flux.

This is exactly anti-correct. Look, let's say it's a 1:1 transformer. The two sides are perfectly symmetrical. And think of it as a solenoid instead of a toroid: the windings go the same way, the dots are on the same end and the no-dots are together on the other end. A current from 1 to 4 flows from dot to no-dot. A current from 3 to 2 flows from no-dot to dot. Therefore they set up exactly opposite fluxes.

Look, a transformer is just an inductor where part of the winding is electrically isolated from another part of it. And what does an inductor do? It opposes a change in current.

  • If you set up a current from dot to no-dot, it pushes back and creates a voltage from no-dot to dot, opposing that current.

  • If you put a load on the other winding, the induced voltage creates a current from no-dot to dot in that winding, and we call that a transformer.

  • If both windings have a current from dot to no-dot, then the inductor opposes both of them at once, and you have a choke. You can forget about transformer stuff, it's just an inductor where some of the winding is DC-isolated from some other part of the winding.

  • If the two windings have equal and opposite currents, they set up equal and opposite fields, which cancel one another, so the overall voltage induced is 0.

  • If the currents are something other than perfectly equal or perfectly opposite, you can consider them as the sum of a common-mode part and a differential-mode part, which makes analysis easy (the CM part sees an inductor, the differential part sees nothing), or you can try to look at it as a transformer being driven from both sides, which makes life hard, but eventually comes out with the equivalent answer, supported by the wikipedia quote, that the fields end up working to make things "more differential".

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  • \$\begingroup\$ Your answer is so logical and well explained. It makes sense now. I didn't realise where I was going wrong until now; but all this time, I was under the impression that a transformer's secondary emitted current in the same direction as the primary current. But I tested this with Falstad and it doesn't. I see now why the dots are correct. Thank you. \$\endgroup\$ Sep 16 at 21:20
  • \$\begingroup\$ For the record, it was your statement "it pushes back" in bullet 1 that got me questioning the basic assumption I had always made about transformers, allowing me to understand. \$\endgroup\$ Sep 16 at 21:32
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Flux adds in the common-mode case and cancels in the differential-mode case, and there's no contradiction with the quote from Wikipedia, you've just misunderstood slightly.

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.

This means that the current flowing from 1 to 4 produces an emf that would create a current from 3 to 2 (if operating as a transformer), but in this case it ends up resisting the current from 2 to 3. Likewise the current from 2 to 3 produces an emf that resists the current from 1 to 4. This is the phenomenon we want out of a common-mode choke.

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  • \$\begingroup\$ No, that doesn't make sense. If, when used as a transformer, current flowing from 1 to 4 causes current from 3 to 2; then by that same mechanism, when current is applied both from 1 to 4 and from 3 to 2 (i.e. normal operation), the flux created ought to add. \$\endgroup\$ Sep 15 at 17:10
  • \$\begingroup\$ I have added clarification to my question. \$\endgroup\$ Sep 15 at 17:15
  • \$\begingroup\$ @SodAlmighty no. Action and reaction have opposite sign. When I push down on the earth, the earth pushes up on me. \$\endgroup\$
    – hobbs
    Sep 15 at 17:19
  • \$\begingroup\$ So....you're saying that if I apply current to both windings of a (normal) transformer, and in the case of the second winding, this current is applied in the same direction that it would ordinarily be induced, no inductance will occur? \$\endgroup\$ Sep 15 at 17:20
  • \$\begingroup\$ @SodAlmighty I feel like that question easily gets tangled up in definitions, but basically yes. Look at the treatment of an ideal transformer (100% coupling, 0 loss) with a shorted secondary. The flux in the core goes to 0. \$\endgroup\$
    – hobbs
    Sep 15 at 17:30
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Common mode currents produce flux that is additive: -

enter image description here

This makes the toroid act like an inductor and this tends to block CM currents or, act as a high impedance. The quote from Wikipedia is not applicable because it is considering one source voltage applied to one winding with a load on the other winding i.e. it is considering a regular ideal transformer with negligible magnetization current. A CM choke cannot be treated the same way as an ideal transformer with a supply on one winding and a load on the other. It is inapplicable.

Thought experiment

enter image description here

The same voltages are applied to both halves and the same current flows down both halves and there is no difference in the inductance should the two halves be insulated (yellow line) or electrically connected along their length.

Nobody can sensibly argue that current in the orange D section will be in the opposite direction to current in the ochre D section when an insulator (yellow) is used? Accept this and move on.

First, a choke is a transformer.

No it isn't. A transformer works with two current types; (a) magnetization current and (b) load current whilst a CM choke has only one type of current; magnetization.

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.

It's not a transformer. If it were and a load was connected to the lower winding then that would be true but, it isn't a transformer.

Therefore it makes sense that these two currents are associated with the same polarity of magnetic flux.

If it were a transformer then those two currents produce cancelling magnetic fluxes i.e. NOT the same polarity flux. Load current in the secondary produces a flux that is totally cancelled by the reflected load current in the primary. If it didn't, then as soon as you add a load, the flux would increase and the output voltage would increase and the output current would increase and the flux increases more and, pretty soon you'd get meltdown. Transformers don't work this way. Accept this and move on.

Therefore applying both of these currents should result in an addition of flux

Incorrect.

Therefore common-mode operation should result in a cancellation of flux.

Incorrect.

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  • \$\begingroup\$ Again, the answer is attempting to bludgeon me into understanding. Phrases like "accept this and move on" aren't helpful. Clearly some part of my fundamental understanding of transformers (yes, TRANSFORMERS, which common-mode chokes are!) was wrong. Rather than trying to identify this misunderstanding (which Hobbs did), the answer persists in simply demanding that I accept assertions without understanding them. This is NOT a good way to help someone learn. [Edited by a moderator.] \$\endgroup\$ Sep 16 at 21:26
  • \$\begingroup\$ CM chokes ARE NOT transformers. [Edited by a moderator.] \$\endgroup\$
    – Andy aka
    Sep 17 at 10:02
  • \$\begingroup\$ @SodAlmighty - Hi, I see you have issues with the answer. You are allowed to raise constructive criticism of the answer (which the author may, or may not, choose to act upon to modify the answer) - but you should not criticize the person writing it (see the Code of Conduct). Therefore I have edited your comment to address the answer, not the person. If you are unhappy with the edit, you can delete the edited comment and write a new one (which must comply with the Code) - your choice. Thanks. \$\endgroup\$
    – SamGibson
    Sep 17 at 13:12

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