page_title: Common mode current
section_title: Definitions
hierarchical_section_title: Common mode current / Definitions
caption_attribution_description: English: current in two wires
context_page_description: Common mode current is portion of conductor currents that are unmatched with the exactly opposite and equal magnitude currents. Common mode current cause multiconductors to act or behave like a single conductor. In electromagnetic compatibility, there are two common terms that will be found in many electromagnetic interference discussions or considered as fundamental concepts, those are Differential Mode and Common Mode. Those terms are related to coupling mechanisms. Many electrical systems contain elements that are capable to act like an antenna. Each element is capable of unintentionally emit Radio Frequency energy through electric, magnetic, and electromagnetic means. Common Mode coupling as well as Differential Mode coupling can occur in both conducted and radiated way.
context_section_description: Differential Mode (DM) is where the signal or power propagation through a conductor and return using the intended path by the designer or flowing "differently" in opposition to each other. Meanwhile Common Mode (CM) is where the parasitic circuit (unwanted) is formed between the desired circuit (main and return path) and the structure of the circuit within which it is located. The signal or power propagates in the same direction in the same circuit.
  
  Henry Ott remarked something similar in his book. Differential mode is the result of the normal operation of the circuit and results from electric current flowing around loops formed by the electrical conductors of the circuit. Common Mode is the result of parasitics in the circuit and results from undesired voltage drops in the conductors.
  Clayton R. Paul provide a simple illustration that explain CM and DM terms on his book. A pair of parallel conductors with current Î₁ and Î₂ flowing on each conductor, which can be decomposed into CM and DM current respectively.
  As shown in the figure above, the relations between Î₁ , Î₂ and modal current are given:
  
  Î₁= ÎC + ÎD
  
  Î₂= ÎC - ÎD
  
  From those two equations, the modal current were obtained as follows:
  
  ÎD= 1/2(Î₁ - Î₂)
  
  ÎC= 1/2(Î₁ + Î₂)
  The CM current flowing in each conductor is equal in magnitude and directed in the same direction, while DM current has equal magnitude but  is directed in different direction.
  
  The radiated electric field from both conductors can be superimposed to obtain the total radiated electric field. For Differential Mode Current, since the conductors are not located in close vicinity, the fields do not exactly cancel each other, but the resultant is a small net radiated electric field. Different from DM current, CM current is directed in the same direction and results in a much higher electric field because fields from both conductors will be added. So a small CM current has a much higher potential towards producing radiated emissions compared to DM current.
  For conducted interference, if the interference doesn't appear between conductors, it will appear between each conductor to a third reference point, for example a structure near the conductor.
  Conducted CM interference causes more problems compared to DM interference because of the possible third reference point that could include any structure that is normally not designed for the purpose. Therefore: 
  CM current is difficult to be predicted and controlled;
  The interference varies with time because of the uncontrolled structural changes;
  Can pollute variety of unrelated equipment;
  The CM current can flow within a large and uncontrolled loop, increasing their potential for radiated coupling.
caption: Fig. 1. CM and DM Current Illustration on Pair Conductors.
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