Temperature Coefficent of Inductance and Mu factor - Page 1
Forum section: electronics / projects

author: CopperCone
author_rank: Super Contributor
timestamp: January 29, 2018, 09:51:36 pm
content: So I did some experiments on two different 10uH inductors I had laying around.1) Bourns 78F Series 78F100-RC2) 10uH from this sethttps://www.ebay.com/itm/1uH-to-1mH-Inductors-Assorted-Kit-37-Value-185-pieces/400295816919?hash=item5d337d6ed7:g:2xQAAOSwUdlWeewA1)Temperature : 22.9 C to 59.3 CInductance : 10.873 uH to 11.072 uHDelta (Percent) - 1.83Delta (PPM) - 18300 ppmDelta (Temperature) 36.4 CPPM/Degree C = 5022)Temperature : 24.5 C to 58 CInductance : 10.404 uH to 10.588 uHDelta (Percent) - 1.73Delta (PPM) - 17300Delta (Temperature) 33.5 CPPM/Degree C = 516https://books.google.com/books?id=R67HARlhisYC&pg=SA13-PA28&lpg=SA13-PA28&dq=inductance+temperature+coefficient+radio&source=bl&ots=TeQG6OCVtO&sig=nf-Bnc2vdC_KUZFGOhrJx3DAbbM&hl=en&sa=X&ved=0ahUKEwjO5OD2kf7YAhUEmeAKHSiKCnQQ6AEIXzAK#v=onepage&q=inductance%20temperature%20coefficient%20radio&f=falseIs this statement accurate? This would mean that the inductors I tested are built with a material that has a Mu of ~ 700?I measured this with a DE-5000 LCR meter. The inductors are in a styrofoam box with a light bulb that heats it up.Should I be looking for inductors that have a low mu if I want low TCL for stable filters?looking at this resource,https://books.google.com/books?id=UqJj9Ni37fEC&pg=SA2-PA22&lpg=SA2-PA22&dq=inductor+mu+handbook&source=bl&ots=dNoE0LXhFy&sig=hVyVi-HiXJb49lUkatcUYcjoQgk&hl=en&sa=X&ved=0ahUKEwifjPXmk_7YAhVjmuAKHVc3BRUQ6AEIMzAC#v=onepage&q=inductor%20mu%20handbook&f=falseThere seem to be a few options :Iron core - low permeability, 10, however I read that iron core inductors age, also, I don't think they work too good with higher frequencies?MPP & High Flux - Don't know much about them, have a low mu of 14, seems good? Am I understanding this thing correctly?

author: CopperCone
author_rank: Super Contributor
timestamp: January 29, 2018, 10:02:04 pm
content: Also, I missed this in the other threadQuote

author: CopperCone
author_rank: Super Contributor
timestamp: January 29, 2018, 10:04:17 pm
content: Tesla, what exactly happens in a ring inductor rather then a rod inductor (as the airgap does not occur?)If I got ring cores that are of very low mu material, could I wind inductors that have a tempco of like 8ppm (from the 14 material)?

author: Someone
author_rank: Super Contributor
timestamp: January 29, 2018, 11:06:58 pm
content: Its not possible to draw linear fits beyond a small temperature range, most core materials are very non linear with temperature. If you're in a relatively stable environment then you could try finding matched temperature coefficients, but more stable materials around that specific operating point could be easier to find.

author: CopperCone
author_rank: Super Contributor
timestamp: January 30, 2018, 12:42:21 am
content: I will repeat the experiment for a change of 10 degrees C.It's a bit bootleg because there is no feedback control, I just adjust a power supply to get some kind of final stabilization temperature after a while. I was using wider ranges to eliminate the thermal mass errors (I just let it settle for like 30 minutes but the room temp changes this is no real lab).. yet

author: Kleinstein
author_rank: Super Contributor
timestamp: January 30, 2018, 12:12:57 pm
content: There is the tendency for low µ materials to show less TC, but the details are quite complicated and it depends on the material. It is not only the temperature directly that effects the core material, but also mechanical stress that can have a significant effect - especially on some high µ materials.The stress effect is related to magnetostriction - so the worst materials in this respect are those with a high µ despite large magnetostriction. So something like the Metgalss alloy for transformers is about as bad as it can get, but also some high µ ferrites perform poor. As a warning example I one would a coil around a high µ ferrite, to find out that with 20 turns the inductance not went up by the expected factor of 400 compared to 1 turn, but only by a factor of about 20 - so the stress from the winding reduced the µ by a factor of 20 . On the other hand having a low magnetostriction often also leads to a higher µ - like with permalloy type materials. So these might be better despite of there high µ.There is also the option to have a core with air gap: here the gap is responsible for much of the magnetic resistance and thus the effective µ. So chances are that powder cores with a distributed gap might perform quite well.Due to a larger number of effects that come together, there is no universal simple description. So likely just look at the published data and hope they apply to the hole inductor and not just the core material.

author: CopperCone
author_rank: Super Contributor
timestamp: January 30, 2018, 10:38:54 pm
content: What do you mean stress from the windings? I find it hard to imagine a few loops of wire around a core put significant mechanical stress on it.Was this a heavy gauge wire wound with a machine?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 30, 2018, 10:54:23 pm
content: Quote from: CopperCone on January 29, 2018, 10:04:17 pmMaybe, what do the datasheets say?Note that low mu is more sensitive to the position of the windings:From:http://www.micrometals.com/appnotes/appnotedownloads/ipc4hqi.pdfWhich means, at some point, you get more tempco from copper TCE than from core properties.Tim

author: CopperCone
author_rank: Super Contributor
timestamp: January 31, 2018, 05:10:37 pm
content: Do you think copper tape stuck on the core wiuld be more stable tben wire because of mechanothermal motions of the windings? Or copper deposited by vapor deposition? I heard about silver vapor deposition inductors on ceramic for high stability, supposedly by nazis in the v2 program. Im thinking the plastic insulation might effect the winding position more then the tempco of copper metal. I kind of want to experiment with naked copper wound inductors vs insulated wire wrapped ones, and solid core vs stranded wire.A firm understanding of thr tendancies would help in the design of metroloical circuits operating at high frequencies at high signal to noise ratios where dsp does not cut it.I wonder if silver chemical deposition is possible to do without expensive equipment at home using masking techniques to make stabler inductorsOr should inductors be buit by "floating" them on a standoff and having the wire cage around it but not physically touch it so that the two mateerials are isolated from eachother to elimimate cross material strain and tension. This is the case with highly precise RTDs which basically have floating windings to eliminate the so called strain gauge effect. However this arangment is shock sensitive and a strong shock will permamently deform the winding.RTDs that are floating may be good till 800c while bobin wound ones are good till 450c and vapor deposition ones are good till maybe 300c. However i am unsure if this will be the case with inductors because of mutal inductance effect. I invision plastic insulation having some kind of hysteris while glue would increase the wire tension significantly with temperature change. Perhaps somome more mechanical then me could helpThis analysis seems difficult conceptually. Fun to think about however.Which configuration would be most fruitful to explore?Also can leakage inductance be explained by the fact that the flux density is high and that due to non zero magnetic resistance (better term?) there is a appealing "shunt" formed by the free space around the ring, kind of similar to magnetic fringing losses described in square magnetics?

author: CopperCone
author_rank: Super Contributor
timestamp: January 31, 2018, 06:01:05 pm
content: Ah i read the article. It appears that you have a capacitor form because of encapsulant acting as a dielectric, another factor to consider.Do dielectrics have thermal hysterysis? Foil capacitors are stable but i am a bit suspicous of glues...I wonder what happens to the dielectric constant when a material is soaked, like in electrolytic capacitors.Mylar and teflon are good but they are hardly glues.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 31, 2018, 07:52:19 pm
content: I suspect you are at the point were all questions are answerable with a few experiments. If not necessarily a fewquickexperiments.You're getting deep into the realm of material sensitivity, like how an assembly changes over environmental conditions, and how that will depend on the exact adhesives used.Likely, a lot of this info does exist ... deep in the trade secrets of the few companies that have run these experiments many times.Tim

author: Kleinstein
author_rank: Super Contributor
timestamp: January 31, 2018, 08:01:39 pm
content: Dielectric materials can also show a kind of hysteresis this is especially true for glassy plastics when slightly below the glass temperature (e.g. around 80-120 C for epoxy). There can also be an effect of humidity and thermal expansion mismatch (thermal expansion of plastics is usually much larger than that of metals or ferrites. Mechanical stress in plastics can relax on a timescale of hours to years. Humidity can increase the DK quite a bit. The DK of the insulator is mainly a factor if you are close to self resonance.Another odd effect that can happen with inductors is a mechanical resonance of a ferrite or similar core. This can give a short high Q resonance that may be overlooked. The high Q is used in some anti theft tags with a strip of amorphous material. The same effect can happen with ferrite cores too - possibly leading to a louder noise.For the mechanics is helps to isolate a ferrite core from winding stress - just the stress from hand winding can cause a large effect on some high µ materials. Due to the number of turns the forces can be quite large. So often inductors are wound on a plastic bobbin and the ferrite core more or less loose inside and sometime fixed with a kind of wax. Old radio tuners where sometimes covered in wax to prevent those air coils to move. Wax can also keep out humidity - however flammability can be a issue.If you don't need a core (e.g. low inductance), an air coil is usually more stable than an inductor with ferrite. If a ferrite is used it helps to not have a closed core, but an air gap. This way the size of the air gap can be the main factor, not so much the µ of the core.

author: CopperCone
author_rank: Super Contributor
timestamp: January 31, 2018, 10:44:28 pm
content: you are saying that the thermal air gap drift (due to distance change over temperature) is much less then the the torrodial core drift?I gotta try silver plating a core.