# Ferrite core variable inductor

1. Jan 13, 2014

### jrand26

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 DC-current 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,

$$L = \frac{\mu N^2 A}{l}$$
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?

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2. Jan 13, 2014

### Staff: Mentor

It looks like you are wanting to use saturation of the core to reduce the effective inductance. This is not a very precise or linear way to accomplish this. Is there a reason you are wanting to control the inductance this way? What is the context of this circuit, and what is the application?

3. Jan 13, 2014

### jrand26

I'm an EE student doing vac-work at a place where they essentially want a variable inductor, but controllable to really precise levels (up to 0.01 mu H). Someone here before me did their thesis on methods for doing this and they determined that this was the best solution, and my project here is to continue on from there.

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/non-linear? 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. Jan 13, 2014

### Staff: Mentor

Can you post more about the specifications for this variable inductor? What range of inductances are needed? What is the amplitude of the AC signal that needs to "see" this inductance, compared to the range of DC saturating current that you are using? How quickly do you need to change the inductance? (Like, could you just use a moving slug variable inductor, or do you need to be able to adjust the inductance elecronically fairly rapidly?)

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

5. Jan 13, 2014

### jrand26

Range is about 50-750 mu H, application is in simulating vehicle passing over in-ground inductive loops. I'm not aware of any restrictions on the AC signal or DC saturating current. It needs to change quickly electronically (I'm doing that with a DAC).

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