[HN Gopher] Making Ferrite Core Inductors at Home
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Making Ferrite Core Inductors at Home
Author : hasheddan
Score : 96 points
Date : 2026-02-02 12:07 UTC (3 days ago)
(HTM) web link (danielmangum.com)
(TXT) w3m dump (danielmangum.com)
| jacquesm wrote:
| Interesting, I'm right in the middle of this, so very timely,
| thanks for posting.
|
| As a surprise to me, the price of variable capacitors (which you
| can pair with an inductor as described to get a particular
| resonance) has gone completely through the roof because
| apparently nobody uses them any more. So what used to be a 20 ct
| part is now $7,50 quantity one and quantity 50 as well!
|
| You can see a couple in the picture of the regency radio, the
| circles with a screw in the middle. The ones in the other radio
| in the little metal cubes are adjustable coils, they have a split
| ferrite element that screws up and down to extend the length or
| contract it.
|
| As for stripping the conductor of the enamel: the best and safest
| way is not to scrape or sand (which will affect the conductor by
| reducing its strength as well as its cross section) but to simply
| boil off some of the enamel in a bit of solder stuck to the tip
| of your soldering iron.
|
| That way you only strip just as much as you need to and there is
| no mechanical damage. You also should not twist the two wires the
| way it is done in TFA because these coils will pick up LF noise
| as well and that will end up rubbing the wiring to the point that
| you can get a short. The easier way is to just put a drop of
| superglue on the paper that you're going to use as coil body and
| then to stick the winding end in it and wait for it to dry (which
| should be really quick). After that you can start winding.
|
| Wind nice and tight and make sure to not leave any air between
| the coil body and the ferrite, this will result in 'microphony',
| your core will end up modulated by sound waves! That's why if
| you're going to wind around paper you should wind with the paper
| already around the ferrite as tight as you can make it. If the
| paper slides on the coil too easily then you may have to
| mechanically affix it to make sure that the microphony effect is
| minimized.
|
| If you need to cut ferrite realize that the stuff is super hard.
| It is easier to score it using a grinder wheel (a big one, or one
| on a dremel) all the way around and then to snap it off. After
| that you will still need to dress the ending, the fragments are
| super sharp so be careful you don't cut yourself.
|
| Finally, I hotmelt a little piece of perfboard onto the ferrite
| and then solder the coil wires onto that and exit using thin
| silicone wiring. That gives a very flexible interface which will
| result in fewer broken coils.
| hasheddan wrote:
| Author here -- thanks for the comments and feedback! I
| typically use the solder stripping technique as well when
| actually incorporating the inductor into a circuit. Interesting
| you should mention variable capacitors as I just went through
| the process of sourcing 5 of them (will be discussed in a
| future post) from Amazon, and not only were they expensive, but
| they also took significantly longer to ship than was originally
| estimated.
|
| I am somewhat surprised that the small loops would pick up a
| meaningful amount of noise, though I suppose meaningful always
| depends on the use case and tolerances. Do you have any
| references for how significantly this can impact inductor
| performance? Or is the primary concern just the rubbing and
| potential shorts?
|
| Despite ferrite core inductors and loopstick antennas being
| old, fairly well understood technology, I have found there to
| be very little in the way of hands-on experiments with tangible
| results available. I'm hoping to continue testing various
| techniques and posting about them -- you've already provided
| some great variations for me to test out!
| jacquesm wrote:
| Hey there, fellow hardware hacker :)
|
| > Interesting you should mention variable capacitors as I
| just went through the process of sourcing 5 of them (will be
| discussed in a future post) from Amazon, and not only were
| they expensive, but they also took significantly longer to
| ship than was originally estimated.
|
| It _really_ took me by surprise, I was ready to buy a bag of
| them and then I realized that the price was for single
| pieces! And those were the crappy foil kind, not even the
| cermic /silver versions which seem to be unobtanium.
|
| > I am somewhat surprised that the small loops would pick up
| a meaningful amount of noise, though I suppose meaningful
| always depends on the use case and tolerances.
|
| So was I :) But the spectrum is extremely polluted on the low
| end, just about everything contains a switching inverter
| these days and they're not exactly clean so you have to pay
| extra attention to this on the receiving end. Higher Q helps
| (more coil, less C), up to a point. Above 500 KHz or so it
| cleans up a bit but below that it is a real problem, because
| it is not just the fundamentals but also the harmonics which
| for a switched coil go up very high (in theory: to infinity,
| in practice, easily to 10x the switching frequency before
| they fall off to the point that you can ignore them).
|
| > Do you have any references for how significantly this can
| impact inductor performance?
|
| For me it made the difference between having something
| working and not being able to recover the signal at all. In
| the end I chose an extremely high impedance pre-amp and a
| coil/cap combination that is on the edge of what you can
| still make at home without having to worry about stray
| capacitance. It helps that I know exactly what the pattern of
| the signal is that I'm looking for as well, so maybe if you
| end up having problems you can use that kind of trick to
| raise your signal above the noise floor.
|
| > Or is the primary concern just the rubbing and potential
| shorts?
|
| That's the secondary concern. Also: beware of using metal in
| your fixtures and metal objects near to the receiver, that
| can have a massive effect (imaging, signal attenuation, or,
| if you're really lucky, signal amplification). The potential
| shorts issues usually only show up over the longer term, you
| can compensate for quite a bit of that by mechanically fixing
| your windings using either glue or by dipping the whole works
| into inductor resin and letting it dry.
|
| > Despite ferrite core inductors and loopstick antennas being
| old, fairly well understood technology, I have found there to
| be very little in the way of hands-on experiments with
| tangible results available.
|
| True, that's because these are the ways of the past, the
| modern way is to get out of the analog domain as fast as you
| can and then to do the rest digitally.
|
| > I'm hoping to continue testing various techniques and
| posting about them -- you've already provided some great
| variations for me to test out!
|
| Enjoy this, it is very rewarding to pick signals out of the
| air and there are a lot of interesting lessons to be learned
| by doing this the 'hard way'. Feel free to mail me if you
| want to take this off-line, email in profile.
|
| best regards from NL,
|
| Jacques
| hasheddan wrote:
| > it is very rewarding to pick signals out of the air
|
| I could not agree more :)
|
| ----
|
| This is quite possibly the most positive interaction I've
| had on HN -- thanks Jacques!
| pkolaczk wrote:
| If you can afford losing a bit of frequency stability, you
| can use a varicap instead and control it by voltage.
| Precise multiturn potentiometers are much cheaper. Or just
| buy a programmable clock signal generator based on PLL and
| then make it into a superhet (you can still have analog
| filtering and detection, but digital frequency synthesis so
| it can look more like a modern radio with frequency
| display).
|
| There are so many options and all are very cool to explore.
| hasheddan wrote:
| > There are so many options and all are very cool to
| explore.
|
| Very much agree, controlling a varicap via an MCU is a
| nice intermediate step of "digital tuning" without going
| all the way to SDR.
| jacquesm wrote:
| Yes, varicaps are good and there are even varicaps with
| very large capacitance swings exactly for this purpose.
|
| But it does require a very steady voltage and many more
| parts than just a single passive to not accidentally load
| the resonant circuit to the point that it becomes
| ineffective. You need to de-couple considerably to make
| this work, especially without injecting (phase) noise.
| And frequency stability and phase noise can be extremely
| annoying in particular applications.
| pkolaczk wrote:
| That's why you probably want to use it in an oscillator
| rather than a filter. Then some of those problems go
| away. Although, some new ones appear like having to add a
| mixer ... Some time ago I thought making a double
| superhet would be too complex but apparently it turned
| out quite a nice DYI project.
| jacquesm wrote:
| One of the problems I'm struggling with is that I'm
| trying to recover a signal with a known pattern from
| under the noise floor and every little bit helps.
| Varicaps will come into play once there is enough signal
| that the thermal noise and power supply influence would
| no longer drown out the signal. Interesting project but
| it is one of those where when you start you think 'how
| hard could this be?' only to find out that it is in fact
| pretty hard. We'll see if I can pull this off or not, I
| give it 10% chance at the moment.
|
| Oh, and superhet wouldn't work, that would destroy the
| valuable part of the signal.
| pkolaczk wrote:
| What do you mean it would destroy the valuable part of
| the signal?
|
| If the signal has a carrier (like in AM or FM) or some
| other regular pattern (e.g. digital signal) then the
| typical the way to recover it from under the noise floor
| is to use a narrow-band PLL.
|
| I'm experimenting with it right now and just found that a
| quartz/ceramic oscillator pulled by a varicap controlled
| by a PLL gives pretty good results - low phase noise,
| good noise rejection and ability to recover the carrier
| from a very noisy signal. But for that to work, the
| signal has to be shifted to a constant frequency through
| heterodyning first.
| JKCalhoun wrote:
| (Aside: I saw those "PCBite" doo-hickeys in your photos and
| said to myself, "What are these magical scepters?" Picked up
| a small set just now.)
| hasheddan wrote:
| You will not be disappointed! Well worth the investment.
| contingencies wrote:
| I'm currently going about it the other way, specifying custom
| magnetics to suppliers. Suffice it to say there are a lot more
| variables when you get in to manufactured designs, which may
| involve multiple coils, thermals, various component type
| options (SMD, THT), disparate wire types per coil, various
| footprint options, multiple core options, load modulation, etc.
| jacquesm wrote:
| I don't envy you at all. I'd ship a working prototype and say
| 'like this one' :) My ability to spec this would surely
| become an exercise in finding all of the ways in which I
| could mess it up.
| contingencies wrote:
| Learning at industrial scale is sometimes temporally
| cheaper because it allows you to conclusively exhaust a
| design path through efficient and spectacularly memorable
| failure. ;)
| jacquesm wrote:
| HN should have a 'comment of the day' section. I'd
| nominate this one.
| resters wrote:
| I thought it was going to be about making the powdered iron at
| home. Fun article nonetheless.
| jacquesm wrote:
| You can, but the results will be messy and sub-optimal compared
| to industrially produced cores.
|
| I've done it for an inductor for which there wasn't a ready
| made core. It left me with something that sort-of worked but
| the grade was much lower than even the poorest commercial
| grade. To improve on that I would have had to invest a lot of
| money.
|
| Here is an overview of the steps in commercial ferrite core
| production:
|
| https://www.powerelectronicstalks.com/2018/11/ferrite-core-m...
|
| Edit: Oh, and the way I did it: take a thermally resistant
| resin with low shrinkage on curing and load it with as much
| iron powder as it will take before it starts clumping, pour
| into mold, let it set and then press it out of the mold.
| fc417fc802 wrote:
| That was an interesting read. However it doesn't look like
| small scale DIY at a high quality level would be particularly
| expensive in terms of tooling? Only in terms of time and
| personal experience.
|
| Ball mills are trivial to DIY - electric motor, belt drive,
| couple pieces of copper tubing, drill some holes in a wooden
| frame, use large metal coffee cans, throw some ball bearings
| in.
|
| Calcination is also easy. 1000 C is not a difficult
| temperature to deal with. You can hit that using plain old
| nichrome wire. You will need to purchase a few ceramic bricks
| of course.
|
| The only slightly tricky step I'm seeing is sintering. The
| nickle-zinc option mentioned in the article will be
| substantially easier (lower temperature, oxygen atmosphere)
| but the required temperature still pushes up against the
| limits of nichrome wire. Given that propane can get you to
| the vicinity of 1900 C and tanks of that are sold at the
| grocery store (at least where I live) I'm going to say this
| step is also cheap and easy to DIY.
|
| The real issue is that you can't do this at home if you live
| in high density housing. You will need a shop or a small back
| yard.
| jacquesm wrote:
| I've done worse than that in my garage but I also know that
| if I burn it down (and the neighbors garage with it) that
| the insurance company will not look at me kindly.
|
| Regardless it is not a process that you're going to make
| work on your kitchen sink :)
| cyberax wrote:
| I wonder if you can do that in a microwave...
| jacquesm wrote:
| Hehe, well played.
| Aurornis wrote:
| > However it doesn't look like small scale DIY at a high
| quality level would be particularly expensive in terms of
| tooling?
|
| Ahh, the lure of a DIY project that makes you go "How hard
| could it be?"
|
| It's an all too familiar feeling for me.
| davidwritesbugs wrote:
| When I was a boy into electronics the one thing that always
| stymied me designing circuits was how to make an inductor of the
| right value. I can't recall these many decades why but I needed
| to make ones of a value and you couldn't specify "xuH" to a
| component supplier, like resistors or capacitors, but nor was
| there any resource to say "x turns of y gauge wire" etc. It was
| all a black art to a 12 year old in the early 70s.
| jacquesm wrote:
| Funny, I had the exact same experience as a kid in the 70's. Up
| to coils it all seemed pretty easy.
|
| So yes, coils (or, to be more correct, inductors, in Dutch we
| use 'spoelen' which is closer related to 'coils' so I tend to
| make that mistake all the time) are 'different' in that sense,
| as are the equations that govern them. And the theory is
| sufficiently complex that you have a hard time hitting the
| right value right off the bat if you put something together
| that you think will have a particular inductance unless you've
| done it many times before. Slight variations can make big
| differences. On the plus side: the values are critical, even
| so, there are usually plenty of ways to compensate if you got
| it wrong. One trick is to overwind and then to remove windings
| until you hit the right value. Another is to hook up a scope
| through a very high impedance probe and to couple your coil
| magnetically to an oscillator with known frequency. You can
| then tune for the required response without ever knowing the
| exact inductance that you're looking for.
|
| In the higher frequency domain (when you start using air coils
| of silver plated copper) you can usually achieve the same
| effect by slightly opening up or compressing the coil windings.
|
| Resistors are easy, capacitors are bit harder, coils are
| 'magic', but with a bit of practice that magic becomes ritual
| and ritual should be at least reproducible to the point that
| the part becomes a manageable quantity.
| whynotmaybe wrote:
| I know there's a scientific explanation for it but the fact that
| the inductance changes just by shifting the rod baffles me.
|
| This is the kind of stuff that still proves Arthur C Clarke " any
| sufficiently advanced technology is indistinguishable from
| magic".
| jacquesm wrote:
| If you place the coil asymmetrically you are getting into the
| 'end effect' of the ferrite rod, so some of the magnetic field
| lines will escape and that will marginally reduce your
| inductance. It's as if you cut off a few windings from your
| coil. So the coil exactly in the middle has maximum inductance,
| moving it off to the side will reduce it very slowly until you
| hit the end, after that it falls off a cliff.
|
| hth.
| greenbit wrote:
| Is whole-word dyslexia a thing? I thought I saw "Making Ferrite
| Inductor Cores", and got excited to learn DIY techniques for
| making custom shapes/blend cores. Still a good article, though.
| juancn wrote:
| I love the symmetry of capacitors and inductors. It's just so
| elegant. Inductors: store energy in a magnetic
| field, pass DC, block AC Capacitors: store energy in
| an electric field, pass AC, block DC
|
| I'm omitting a bunch of edge cases, but it feels so nice.
| alhirzel wrote:
| Don't forget about ELI the ICE man!
| newman314 wrote:
| You can use one of these to act as a local personal WWVB station
| amongst other things.
|
| See https://github.com/hzeller/txtempus and
| https://github.com/GOTO-GOSUB/Txtempus-Passive-Antenna
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