[TowerTalk] Toroid Core Material Selection

[Jim Brown]

On Tue, 22 Mar 2005 07:31:04 -0800, Jim Lux wrote:

>with two different materials, you've basically got two inductors/chokes in
>series.  You'd have to evaluate them separately at the various frequencies
>and sum the contributions at each frequency.

Yes, BUT:  it is the COMPLEX SUM of their impedances that will result. A ferrite 
choke is basically a parallel RLC resonance having very low Q. At resonance (the 
peak of the Z curve) it will look resistive. Above resonance it will be capacitive 
and resistive, below resonance it will be inductive and resistive. If one of the 
chokes is below resonance and the other above, the L of one can cancel the C of 
the other, leaving you with only the R. 

Note that this is exactly opposite of what is commonly published on the datasheets 
for ferrite materials -- they show series R, X, and Z equivalents, all of which 
are curves that vary a LOT with frequency. But if they expressed them as parallel 
R, L, and C, the values would be much closer to constants. 

>If they're closely coupled, it will get a bit more complex, but ballparking
>it is probably close enough.

For all practical purposes, there is VERY little coupling between ferrite chokes 
wound around different cores (passed through different cores). That's because the 
u is much, much higher than air, so all the flux from one stays within its core. 

There's a tutorial on ferrites on my website. Although it was written for sound 
contractors, much of it is relevant to your interest. 

http://audiosystemsgroup.com/SAC0305Ferrites.pdf

Jim Brown  K9YC