The RF choke

The RF choke is the only component you will have to make yourself, but a quick look at the diagram will show that this is very easy.

The choke is simply a hollow bead (a torus) of powdered iron, and the power supply wire is wound as many times as you can around its circumference. The alternating power supply voltage has a frequency of between 50 and 60 cycles each second, and small inductance of the choke will have little or no effect at this low frequency. However, EMI harmonics have high frequencies, and it is these that we want to eliminate. At these frequencies, the electromagnetic coupling between the wire and the choke is significant, and generates eddy currents in the powdered iron. This dissipates the energy of these harmonics as heat in the iron. It is for this reason that powdered iron is to be used for this purpose. Ferrite cores are often used in radio and television circuits, but that is because such material is better "matched" to deal with extremely high frequencies. The harmonics generated by power switching transients are better dealt with by the relatively coarser structure of powdered iron.

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Brew Your Own Beer

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