Ti LDC1000EVM A Question on Inductance

I suspect that the problem is that the steel core is much more lossy than the ferrite at the operating frequency of the chip, or possibly that the inductance has increased too much and is out of range.

What happens if the steel slug is  close to the coil but not inside it ?

MK

Hi John,

Very interesting results!

It is impressive that the LDC1000 circuit can reveal so much detail about inductance and the effect of cores. However, interpreting this is hard I think.. : (

I also think the same, the steel has caused the inductor to become lossy, i.e. it now has significant resistance at operating frequency. According to the LDC1000 datasheet, it computes inductance by observing resonant frequency. With the lossy inductor, a higher current is needed to maintain a particular amplitude oscillation, and this can be observed by calculating Rp which is the parallel resistance of the resonant circuit.

There is a formula on page 12 of the datasheet which shows how to see the Rp value, I put that formula in Excel, and this was the output (x-axis is sample number, y-axis is calculated Rp in kohms) -

the value dropped from 11kohms to just over 7kohms.

The graph above initially shows nothing close to the coil. As the steel was brought close to the coil, the Rp dropped. I then removed it, hence the graph went back up to 11kohms.

The graph below shows the result with ferrite, where the only relevant bits are at the beginning and at the end of the graph.

At the beginning, there was nothing near the core. At the middle, I was busy securing it to the coil using tape. The end of the graph shows the actual

result, and it is at a higher value (about 12kohms) than at the beginning (11kohms). I only used a tiny piece of ferrite (ferrite type 61 - there are various mixes of ferrite possible).

Another way we could dig deeper is by subjecting that coil to a vector network analyzer (VNA), which would basically show us the equivalent circuit of the inductor with the steel or ferrite. I do have a (home-made) VNA but it needs calibration (and some practice) to be useful, and I still need to do that : (

Hi Michael,

Thank you for your reply. As the steel core is brought closer to the coil it gradually shows a drop from approx 20 uH to the approx 17 uH. Your idea that the steel core is introducing core losses into the setup is probably the source of the problem. With its granular nature the ferrite would not be as susceptible as the solid piece of steel to eddy currents. I will do some more experiments as, for me, this exercise is about learning about inductance and how this circuit works more than any real application for it. Thanks again for your insight.

John

Hi Shabaz,

Thanks for the quick reply and nice analysis of my question. I can see that your graphs show the same phenomenon that I was observing and confirm the suspicions of michaelkellett I have to confess that last night in my eagerness to play with the new toy I did not read the Data Sheet which is on the schedule for today before I ask anymore questions I also have a 14 page article on inductors that I want to review Thanks again for introducing the  LDC1000EVMLDC1000EVM in your blog

John

Hi John,

The datasheet, while good, could maybe do with more detail on examples and their explanation (like you're trying to do). Still, the datasheet is pretty good - I'm being over-critical.

Most of the analysis in my comment is still vague guesswork to be honest, since I'm assuming that the change in Rp has had sufficient effect to however the LDC1000 internally measures the inductance (and Rp has 16-bit resolution), but now I think of it, perhaps the

change in Rp, and the change in drive to maintain oscillation level may affect the resonant frequency, e.g. the parallel capacitance may change slightly. Often this can be seen by a LC oscillator when the voltage is varied - the

frequency does shift slightly (but this is a guess. I don't think that bit alone is enough to explain 20uH -> 17uH).

A good book is "RF Circuit Design" by Bowick, by the way.. it has a good (although brief - the entire book is short, but easy-to-read) section on inductors and resonant circuits.

Hi Shabaz,

Thanks for the book recommendation. I have ordered a good used one through Amazon. As is the case with most questions involving multiple variables it is difficult to get an exact answer but rather we approach the answer slowly by degrees as we learn and understand the system better.

John

Hey thanks! A good reason to re-polish my passives theory. Gone are the days when I could do the calculations (and pass the exam ) myself.

I'll be looking for some reference to revamp the theory and the brain.

On a side note - but related: I did the online Georgia Institute of Technology's "Linear Circuit" training on Coursera last year. A great refresher.

Hi John,

After reading your post I remembered something I wrote in my book about how the crystalline structure of iron behaves under a magnetic field.

The ferrite core is a very homogeneous material, so all of the induced currents will flow in the same direction and initiate an induced magnetic field, which is why the measured inductance goes up.

In steel, you have iron mixed with other metals, forming irregular crystals.  When you apply the magnetic field, you get interactions at various angles depending upon how the crystals are aligned inside the steel core.

The various induced magnetic fields will interact, creating an impure conductance path, hence the drop in apparent inductance.

Your experiment does a good job of showing how important the alignment of the material and crystal patterns can really affect magnetic fields.

Nice job.

DAB

Hi DAB,

Thanks for your insights on the Physics behind the scenes. I have been digging through the old college physics book today to refresh my understanding of inductors.

John

John;

Any chance that it's stainless steel?