Lots of good advice in this post.
See comments interspersed, lots of snips to save space in the digest.
On Thu, 6 Mar 2008 08:29:02 -0700, John Lyles wrote:
>Plate RF choke has an appreciable DC current component in it. This
>lowers the effective mu of the ferrite, and makes it less effective
>as a choke.
YES! The good news is that this can be predicted from mfr data sheets.
The bad news is that you probably won't like the prediction. More bad
news -- the DC will also cause heating, very possibly overheating. When
that happens you have no mu at all (until it cools).
>Ferrite-loaded inductors still do have parasitic resonances due to the
>stray capacitance from wire turns.
And also capacitive coupling from turn to turn and end to end through
the ferrite, which is also a dielectric.
>They tend to shift down in frequency so they still have to be taken
>care of that they are not excited at the normal operating frequencies.
>There is no free lunch here. Less turns of wire, for sure, but also
>the resonances are lower due to the higher permeability in the coil's
The measured data in my tutorial for coils of 1-14 turns on toroidal
cores of five different materials is a good illustration of this. The
tutorial also includes a discussion of how DC affects the behavior of
ferrite chokes and how to use the mfr's data to compute it.
The equivalent circuit of a wire or a coil of wire passing through a
ferrite core, or wound around a ferrite rod, is a parallel resonant
circuit. The R, L, and C of that resonance will appear in parallel with
the rest of the circuit, and do exactly what you would expect from R,
L, and C in parallel. The R will burn some power and lower the Q, while
the L and C will do some resonating.
The values of R, L, and C are not easy to measure, primarily because R
is usually very large and C is usually quite small (often much less
than the stray of the test setup). The tutorial shows one way to do it.
Jim Brown K9YC
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