[Elecraft] RE: OT: Effect of Compression and Expansionon
the Inductance of Toroids?
Jim Brown
jim at audiosystemsgroup.com
Tue Aug 30 11:23:05 EDT 2005
Ron said:
>>I'm confident that the added capacitance by squeezing the turns
>>together is not what is causing my L-meter to show increased
>>inductance. If anything, the capacitance would tend to cause
>>the L-meter to show lower inductance.
On Tue, 30 Aug 2005 07:00:35 -0700, Cortland Richmond wrote:
>Inductance is a result of the magnetic field intercepted by each
>turn.
Having recently completed a LOT of research on how coils wound on
ferrites behave, I think I have a handle on what is going on.
Several basic principles are at play.
1. When the mu of a ferrite toroid is much much larger than air, it
contains virtually all of the flux. This is the case at frequencies
where LOSSES in the ferrite are low.
2. Ferrites chosen as cores for resonant coils and transformers are
usually chosen to have low losses (high Q) in the frequency range
where they are used (for example, in the K2).
3. Conversely, ferrite cores used for RFI suppression should be chosen
to have HIGH losses (low Q) in the spectrum where suppression is
needed.
4. Many ferrites are semiconductors (that is, between conductors and
insulators), so they also have permittivity, and they will act as a
dielectric. So even if all we do is pass a wire through a long ferrite
core (like a bead or a clamp-on), there will be capacitance through
the core between the opposite ends of the wire. This capacitance will
be in addition to the capacitance between turns.
5. Ferrite parts also can exhibit a DIMENSIONAL resonance, whereby
standing waves are set up in their cross sectional dimension at the
half-wave frequency. This mostly happens with LOW frequency ferrite
materials (MnZn).
6. The equivalent circuit of a ferrite choke or coil is two parallel
resonant circuits in series. One resonance is the DIMENSIONAL
resonance, the other is the CIRCUIT resonance between the coil and the
stray capacitances (of #4). Both of these resonances have significant
R components as well.
So the apparent change in L as the turns are expanded or compressed is
simply the CIRCUIT resonance moving as the stray capacitance changes.
For all practical purposes, L does NOT change. C changes.
There's a lot more about this in an applications note on my website,
that also includes some references to the literature.
http://audiosystemsgroup.com/SAC0305Ferrites.pdf
Jim Brown K9YC
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