Choosing components and calculating turns for crossover inductors - Page 1
Forum section: electronics / beginners

author: wastenotwantnot
author_rank: Contributor
timestamp: August 06, 2021, 11:30:19 am
content: Hello.I need to create several inductors for loudpseaker crossovers, both air and ferrite core.I am struggling to calculate the number of turns, size of rods, bobbins etc. I was recommended to use a particular sowaftre called Loudspeaker Labs to calculate this, but it doesn't seem to be developed anymore and all the download links for it seem to be gone. I googled and found several alternatives, one of which is Coil32, but I am not sure if I have the calculation correct.I found some 13mm diameter 75mm long ferrite rods on eBay, so I used this as a starting point, leaving some space at each end.Using the Coil32 programme:This seems to indicate that there is only a single layer needed, and only 22 turns.If I use a smaller rod of say 8mm and 40mm winding length I get this:And these values would be higher due to the ferrite core... The thing thats perplexing me is that looking at various images of 3.3mH coils on google images with a similar guage wire, it looks like a VAST difference in wire quantity.... although most of those pictures are wound on bobbins, as opposed to directly on the rod.What am I doing wrong with these calculations? Is this Coil32 app unstuiable for this?Edit: Sorry the images aren't inserting inline for some reason :/

author: mikerj
author_rank: Super Contributor
timestamp: August 06, 2021, 05:01:58 pm
content: The immediate thing that stands out is you are calculating for a 3.3 microhenry inductor which sounds way too low for a loudspeaker crossover. Are you sure you don't need a 3.3 millihenry inductor? (i.e. 1000 times higher)I had a quick look at the Coil32 web page and I didn't see anything to say it supports ferrite cored solenoid style inductors? I only see ferrite toroids mentioned.

author: Jwillis
author_rank: Super Contributor
timestamp: August 07, 2021, 02:23:40 pm
content: Go herehttps://coil32.net/online-calculators/ferrite-rod-calculator.htmlYou need to know the magnetic permeability of the core . The data sheet for the core should have this information.The down loaded coil32 application does not provide that variable for anything other than toroid cores . So the assumption is that all others will have a permeability of 1 . This is only good for air coils .If your using an unknown material this can make things much more difficult to calculate . You can fudge it an assume the permeability is at least 500 to 1000 for ferrite cores .

author: wastenotwantnot
author_rank: Contributor
timestamp: August 07, 2021, 04:39:42 pm
content: Ah! Yes that’ll be it thanks @mikerj school boy error@Jwillis yes that looks better for ferrites, the only thing is I cannot find the permeability because the rods I was looking at are old rods made by Mullard from eBay:I’ve googled, but not much has turned up.Is there any other way to find out?

author: TimNJ
author_rank: Super Contributor
timestamp: August 07, 2021, 05:05:37 pm
content: The rod geometry is a little bit tricky because, unlike toroids, the magnetic flux lines are not contained entirely to the core. This gives rise to a large external "air gap" (if you will) in the magnetic flux path. Air gaps reduces the effective permeability of the core (Ueff) to something much lower than the material's initial permeability (Ui). The analytical ways to find the inductance of a rod core inductor is largely an approximation (curve fitting), although it can still be done fairly accurately. The design can be a little recursive in nature because the effective permeability depends on how much length of the rod is utilized among other things.If you don't know the Mullard material type, it will be pretty tricky to calculate. I'd prefer to use a known material type from a manufacturer who provides some documentation on their materials in this application. For example:https://www.fair-rite.com/product_datasheet/PN4077375411.htmlOf course, you can just do it experimentally if you have an LCR meter.

author: mrkev
author_rank: Regular Contributor
timestamp: August 07, 2021, 06:29:15 pm
content: Quote from: TimNJ on August 07, 2021, 05:05:37 pmJust to clarify, you really want an air gap in this kind of solenoid, because it vastly extends the loading capabilities of the inductor. After all, that is one of the main reasons why they make crossover inductors with air gap.Quote from: TimNJ on August 07, 2021, 05:05:37 pmYou'll probably end up doing this anyway. The calculation is going to be just an aproximativní and you'll have to fine tune it with LCR meter. If you don't have one, you can use signal generator and scope.

author: wastenotwantnot
author_rank: Contributor
timestamp: August 07, 2021, 06:37:59 pm
content: @TimNJ Thanks for the explanations. I do have an LCR meter on the way, although as I understand it I would need to unwind the wire from the spool and lay it out in a non-inductive fashion in order to test. This could be a bit troublesome with tens of meters of it! Maybe I could do the air coils first to trim that down as much as possible. I’ll have a think about what option is best.@mrkev About that air gap, it’s been recommended that I use a drilled out dowel as a former (if that counts), but I imagine without the proper tools (pillar drill?) it would be a bit of a pain, especially if the spacing between the wire and rod is to remain as even as possible. I’ve been thinking about alternatives such as plastic pipe, something reclaimed/repurposed etc. What is the goto DIY choice? Any tips for using the LCR meter without cutting the wire too short in the process?Also looking for suggestions for homemade air coil bobbins. I can get premade for cheap enough, but if there’s a homemade suggestion that’s free and recycled, I’d prefer that.I’ve found some enamelled copper wire here:https://www.wires.co.uk/acatalog/TX_0750-2000.htmlnothing special, is this suitable?

author: TimNJ
author_rank: Super Contributor
timestamp: August 07, 2021, 06:55:29 pm
content: Quote from: mrkev on August 07, 2021, 06:29:15 pmRod cores have an extremely soft saturation characteristic because the "air gap" is so huge, and thus the path reluctance quite high. This can be advantageous, yes, but you still need to be cognizant of drop in inductance in any case. Drop in inductance will cause shift in your filter parameters. Ferrite is cheap and rod cores are easy and cheap to wind. Once the design is figured out, they are fine.Personally I'd gravitate towards powder alloy cores like moly perm-alloy, mega flux, etc. Low permeability like 60u. The magnetic flux is contained and it's easy to design based on Ae value.

author: TimNJ
author_rank: Super Contributor
timestamp: August 07, 2021, 08:31:50 pm
content: Quote from: wastenotwantnot on August 07, 2021, 06:37:59 pmI'm actually not sure what you mean. The test you are talking about sounds like the "self-inductance" of the wire itself. I don't think you really care about this? Also, self-inductance is pretty well understood from tables of diameter vs inductance.The suggestion is to just start adding turns to the ferrite core until you've reached your target inductance, and making sure that the wire size is suitable for the current. Just a trial and error approach.

author: wastenotwantnot
author_rank: Contributor
timestamp: August 07, 2021, 09:37:04 pm
content: Quote from: TimNJ on August 07, 2021, 08:31:50 pmSorry, please let me explain. I have a spool of wire with say twice as much as is needed.I start winding round the former, and after x turns want to test. I don’t want to just cut the wire as it could be too short. I connect one probe to start of the wire and then where to connect the other? The wire is covered in enamel. If I scrape it off, isn’t this now unsuitable? I was told that I would need to basically connect from the other end of the entire length ie undo the entire spool of wire and lay it out in such a way that it did not add to the inductance value. It seemed extreme but plausible to me.Judging by your confusion, I’m thinking this might very much not be how to do it. So just scrape the enamel and test then keep winding if it’s not enough?

author: Jwillis
author_rank: Super Contributor
timestamp: August 08, 2021, 04:44:19 am
content: Not sure if this will help but I found some information on Mullard FerritesMullard_QuickReferenceGuide_Check out page 236. There seems to be only a few rods that Mullard made and use Ferroxcube grades 4B1(NiZn) and 3C6(MnZn). Check the dimensions . Maybe the ones you have match.GradeCross reference3C6 cross to NEOSID F5https://www.neosid.com.au/files/F5.pdf4B1 cross to NEOSID F14https://www.neosid.com.au/files/F14.pdfgood luck

author: TimNJ
author_rank: Super Contributor
timestamp: August 08, 2021, 06:22:32 pm
content: Quote from: wastenotwantnot on August 07, 2021, 09:37:04 pmAh, that’s a reasonable approach. Don’t scrape the enamel midway through the winding. I think you don’t have to worry too much about non-inductive positioning of the un-wound wire if your target inductance is in the mH range. But it’s good practice. If anything, lean towards too high of an inductance. Then, if you need to make a final correction, just remove a turn or two.

author: wastenotwantnot
author_rank: Contributor
timestamp: August 08, 2021, 09:51:40 pm
content: Quote from: Jwillis on August 08, 2021, 04:44:19 amThanks, I'll take a lookQuote from: TimNJ on August 08, 2021, 06:22:32 pmThanks for the advice. I am still waiting for a couple of things to arrive then I can get started. I have gone for just enamelled wire, not the stuff that heat bonds, but I have read that this could allow vibrations in the coils to cause noise and distortions.. Do Ireallyhave to be worried about this? Or would heatshrinking the coil afterwards be adequate?

author: wastenotwantnot
author_rank: Contributor
timestamp: August 10, 2021, 06:04:38 am
content: I am struggling to find data for those Mullard rods. They don’t appear to match the sizing in the brochure. However, I did find some ferrite bobbins which appear to be the ones used (or similar) by many of the well known crossover manufacturers:https://powermagneticsstore.co.uk/shop/owd56x35-tf5.htmlManufacturer: HitachiMaterial Grade = MSB-5SMagnetic Characteristics:uiac = 3000Are these suitable?

author: Jwillis
author_rank: Super Contributor
timestamp: August 11, 2021, 03:59:34 am
content: Quote from: wastenotwantnot on August 10, 2021, 06:04:38 amThat doesn't surprise me since much of the New Old Stock from Ebay is obsolete. You might just be better off going with new ones since more information is available thus making the calculations much easier. Was there any mention from the Ebay seller of a permeability ?On the other hand , if your getting a inductance meter anyway you could wrap ten turns on a rod and see what the inductance is and work from that value. That might get you close to what you require without baring the wire every so often. I've done that before and came real close plus or minus a few turns . Worked out better than guessing.Pretty much any ferrite core will work since your working with in the range of hearing (20Hz to 20KHz) . So anything above 20KHz isn't a factor. It all depends now on how much power you intend on driving through your cross over . More power the bigger the wire needs to be and the bigger the ferrite needs to be to accommodate the number of turns . Last thing you want is to spend all that time just to have your cross over burn up when you crank the volume .

author: wastenotwantnot
author_rank: Contributor
timestamp: August 11, 2021, 10:49:51 pm
content: Quote from: Jwillis on August 11, 2021, 03:59:34 amYeah that would not be fun.I can message the seller and ask if they know the permeability value.The wire being used is 1.0mm diameter.The completed units are going to be around 40 watts, all drivers are near to 90dB sensitivity so reasonably efficient.

author: wastenotwantnot
author_rank: Contributor
timestamp: August 13, 2021, 08:23:02 pm
content: @Jwillis Finally heard back from the seller, he messaged with photos of the catalogue, but I didn’t see the rods in question. Nothing on any of the pages matches the diameter and length, so let’s forget those. I tried 10 turns and it wouldn’t even register on the LCR meter. I confirmed the meter isn’t faulty with a known good coil from another crossover.Moving on, I’ve been offered some of those square bobbins with the sheets of silicone steel in. Are these garbage? I’ve only seen them on Chinese units:A quick Google seems to suggest silicone steel is one of the oldest commonly used materials:https://coefs.uncc.edu/mnoras/files/2013/03/Transformer-and-Inductor-Design-Handbook_Chapter_2.pdfMaybe I should just say yes to these and see how they turn out. Having a bobbin already sorted will save having to drill into dowel or something else to make a former. Any good reason to turn these down?