Common & Differential Mode Choke Symbol & dots meaning - Page 1
Forum section: electronics / beginners

author: 2X
author_rank: Frequent Contributor
timestamp: March 12, 2025, 08:52:20 pm
content: Hello,What are the "formally"(most common used) symbols for Common Mode & Differential Mode chokes. In some designs on the internet I have seen in the Common Mode choke the two inductors are tied parallel close and in the Differential Mode choke the two inductors are parallel with a big distance between them (Image 1).Also, I have seen in other designs that both Common Mode & Differential Mode chokes use the same symbol and dots orientation like these in image 2.In the image 3 I have designed all the chokes where I have seen on the internet.On the transformers as I know the dots declare where the current flow; on a transformer in the primary side the dot declares in which end of the winding the current will flow. At the secondary winding the dot declares the current that will flow out where was caused from the current in primary side (with dot) at the same time (in theory at least "the same time").1. I want to design a schematic like this in image 1. There is a “formal” or most popular at least symbol for a common mode and for a differential mode choke respectively (both chokes I have seen them in one package and have four pins).2. On the chokes dots what declares?Images sources:Effectively protecting super-speed interfaces against EMIhttps://efficiencywins.nexperia.com/efficient-products/protecting-super-speed-interfaces-against-emiWhy do common-mode EMI filters lack damping networks?https://electronics.stackexchange.com/questions/554395/why-do-common-mode-emi-filters-lack-damping-networksWhy are differential mode chokes (inductors) not generally coupled?https://electronics.stackexchange.com/questions/618107/why-are-differential-mode-chokes-inductors-not-generally-coupled

author: tooki
author_rank: Super Contributor
timestamp: March 12, 2025, 09:21:28 pm
content: Quote from: 2X on March 12, 2025, 08:52:20 pmUhhh... no.If that's whay you "know", then what you know is all wrong.Current flows in one end of a winding and out the other. Then half a cycle later, it flows in the opposite direction, because it's AC, and transformers and chokes are fundamentally AC devices. This applies to primaries and secondaries of transformers, as well as to choke windings.The dots indicate "polarity" in that they indicate the direction of the winding. The dots are in phase with each other.Look carefully at both parts of Figure 5 onhttps://article.murata.com/en-sg/article/basics-of-noise-countermeasures-lesson-13Notice how the bottom winding of each choke is wound differently.

author: 2X
author_rank: Frequent Contributor
timestamp: March 12, 2025, 10:20:56 pm
content: Thanks for your reply.You have right... my mistake is that I tried to discribe the dots in respect to currents (quora link below) in order to explain the polarity (phase) between input signal and output signal.So the dots indicate the direction of magnetic flux from the coils:If the direction of magnetic flux of each winding/coil is the same the choke is a common mode (Dots are in the same side between the two coils). -Correct me if am I wrongIf the direction of magnetic flux of each winding/coil are not the same the choke is a differential mode (Dots are in different side between the two coils). -Correct me if am I wrongOn a transformer the black dots declare the voltage polarity/phase between primary and secondary winding. -Correct me if am I wronghttps://www.quora.com/What-is-the-dot-convention-of-a-transformerUnderstanding Dot Convention in Transformershttps://circuitdigest.com/article/understanding-dot-convention-in-transformersVery detailed Video:EMC Filter Design Part 6: Common Mode Choke Operation

author: PGPG
author_rank: Frequent Contributor
timestamp: March 13, 2025, 07:58:12 pm
content: Quote from: 2X on March 12, 2025, 10:20:56 pmI understand dots in voltage.When voltage at one coil gets + at dot (versus its other end) than the voltage at the other coil also gets + at dot (versus its other end).I think that dots also mean coil beginning so when coil has many layers the dotted end is internal of winding and not dotted is external.In DCDC supplies you frequently have jumping voltage at one coil end and not jumping at the other. If you have jumping voltage at dotted end then it is shielded to some extent by less jumping coil layers.

author: 2X
author_rank: Frequent Contributor
timestamp: March 14, 2025, 10:37:41 pm
content: Transformer PolarityPolarity Test of a Transformer – Circuit Diagram and Workinghttps://www.electricaltechnology.org/2022/03/polarity-test-of-transformer.htmlANSI Transformer Standards for Lead Identificationhttps://metapowersolutions.com/ansi-transformer-standards-for-lead-identification/Polarity Test by DC Source (Battery)QuoteSource:https://www.electricaltechnology.org/2022/03/polarity-test-of-transformer.htmlThe above technique with battery is it trusted to find the polarity of windings?