What is a Common Mode Choke?
A common mode choke is an electrical filter that blocks high frequency noise common to two or more data or power lines while allowing the desired DC or low-frequency signal to pass. Common mode (CM) noise current is typically radiated from sources such as unwanted radio signals, unshielded electronics, inverters, and motors. Left unfiltered, this noise presents interference problems in electronics and electrical circuits.
How do Common Mode Chokes Work?In normal or differential mode (single choke), current travels on one line in one direction from the source to the load, and in the opposite direction on the return line that completes the circuit. In common mode, the noise current travels on both lines in the same direction.
In common mode, the current in a group of lines travels in the same direction so the combined magnetic flux adds to create an opposing field to block the noise. In differential mode, the current travels in opposite directions and the flux subtracts or cancels out so that the field does not oppose the normal mode signal.
How do I choose a Common Mode Choke?The main criteria for selecting a common mode choke are:
- Required impedance: How much attenuation of noise is needed?
- Required frequency range: Over what frequency bandwidth is the noise?
- Required current handling: How much differential mode current must it handle?
Radials differ from axials in that the leads exit from a common side of the component, rather from opposite ends or sides. This gives the radial a vertical profile and a smaller printed wiring footprint.
