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Ferrite core variable inductor 
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#1
Jan1314, 05:33 PM

P: 11

Hi all, I'm working on designing a variable inductor using a ferrite core. The basic idea is that I have a large wound inductor on the central arm which I pass a DCcurrent through, and a smaller, variable inductor on one of the side arms. As you increase the current through the main winding, the inductance on the side arm decreases. Am I correct in saying that this is due to a drop in permeability in the core? As per the equation,
[tex]L = \frac{\mu N^2 A}{l} [/tex] What is the physical explanation for an increased current causing a decrease in permeability in the core? Image 1 shows something similar, except I'm only using one of the side arms and there is no air gap on the central winding. While the design essentially works I'm not satisfied with the accuracy. The biggest problem I've been having is that when I repeat my measurements I get quite a spread of results, which I find really interesting (image 2 shows the curve bounded by max/min measurements). Is there something about the physics of the core which makes it inherently difficult to control? I have noticed hysteresis but I'm not sure that it should effect repeatability. I'm also unsure of the shape of the curve, why does the inductance increase with increasing current briefly at the start? Any help/comments or links to other resources appreciated :) 


#2
Jan1314, 06:18 PM

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#3
Jan1314, 07:16 PM

P: 11

It seems like a pretty cool solution to the problem, but I can't really say it's that precise if it has such variance in testing. I was thinking by posting the problem here I might get a better understanding of the physics behind the design and the associated limitations. Would you be able to expand on why this method is imprecise/nonlinear? It seems pretty linear to me over a range of current values, although the type of curve doesn't really matter does it? Although I might have misinterpreted what you meant. 


#4
Jan1314, 08:05 PM

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P: 40,706

Ferrite core variable inductor
What is the application of this variable inductor? Can it be implemented with a gyrator circuit instead? 


#5
Jan1314, 08:46 PM

P: 11

A gyrator circuit was looked at but would require a digital potentiometer with higher resolution than is available. 


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