How do common mode chokes reduce noise?

Common Mode Chokes are electrical components that are used to reduce electromagnetic interference (EMI) and radio frequency interference (RFI) in electronic devices and systems. They work by attenuating common mode noise, which is noise that appears on both signal lines or power lines simultaneously. This type of noise can be introduced into a system by various sources such as power supplies, motors, fluorescent lighting, and other electronic devices.

Common mode chokes are made up of a magnetic core (such as ferrite or powdered iron) around which two windings are wound. One winding is the differential mode winding and the other winding is the common mode winding. The differential mode winding carries the signal or power in opposite directions, canceling out the magnetic fields and reducing noise. The common mode winding is designed to pass the common mode noise through the choke, where it is then attenuated.

The mechanism by which common mode chokes attenuate noise can be understood through the concept of mutual inductance. When a current flows through the wire of the common mode choke, a magnetic field is created around the wire. This magnetic field induces a voltage in the other winding, which is the common mode winding. This induced voltage creates a current in the common mode winding that flows in the opposite direction of the original common mode noise. This effectively cancels out the common mode noise, reducing the interference that would otherwise affect the system.

In addition to canceling out common mode noise, common mode chokes also provide impedance to the common mode noise. The high inductive reactance of the choke at the frequency of the noise causes it to be effectively blocked or attenuated. This impedance prevents the noise from propagating further into the system, ensuring that the sensitive electronic components are not affected.

Overall, common mode chokes are a valuable tool in reducing noise in electronic systems. By effectively attenuating common mode noise through the mechanism of mutual inductance and providing impedance to the noise, common mode chokes help to maintain the proper functioning and reliability of electronic devices and systems.