EMC Question of the Week: August 12, 2019

Magnetic field lines encircling a printed circuit board trace

The magnetic field strength between a circuit trace carrying 1 amp of current, and a current-return plane directly beneath it increases when the

  1. magnetic permeability of the dielectric is increased
  2. transition time of the signal is decreased
  3. width of the trace is decreased
  4. height of the trace is decreased

Answer

The best answer is "c". The strength of a magnetic field line wrapping the trace is equal to the current in the trace divided by the length of the line. Wider traces require longer field lines to wrap them resulting in weaker magnetic fields. Note that the magnetic field is not constant below the trace. It is stronger near the trace and weaker near the plane. The other three options presented here have little or no effect on the magnetic field strength.

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