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I need the lightest power inductors possible. Do inductors having aluminum windings (or any light-weight winding) exist?

I would also consider hollow-core wiring to be used at very high frequencies, like 2 MHz. At those frequencies, it would make sense to have the metal where the current is going to flow anyway.

Mattman944 wrote:

So, your real goal is light-weight power (voltage) conversion. Operating at a very high frequency is a great way to minimize your inductor size. That is how they get the phone chargers to be so small. Look around for devices that already have solved this problem.

That's a good idea, Matt, but I've already done that. I've been taking apart all kinds of converters for years, so I agree with you -- try to learn from existing practice. But I have an extreme use case, and don't have a spare airplane to take apart.

In the question, "Why don't we use 7075 aluminum Ethernet and USB cables?" a comment by Harper - Reinstate Monica under an answer stated:

When you compare by weight, Al is twice as good a conductor as Cu.

So I know I'm not off-base here.

EDIT 1

Since I asked this question, I have found one Microwave Oven Transformer (MOT) where both windings were aluminum (obvious as I cut the secondary off). Also, 3 MOT primaries that I removed were all aluminum (the one pictured is about 790uH and <500mOhm).

Two views of one of the aluminum MOT primaries:

enter image description here

enter image description here

My thinking in including these pictures as evidence for question validity follows:

If an aluminum-wound power transformer exists, then that is close enough to suggest the existence of an aluminum-wound power inductor.

I know that this is a transformer winding, but in its air-core form, it is an inductor all by itself, and I believe, efficient enough to be considered a power inductor, even if it is air-core.

MicroservicesOnDDD
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    Assuming you mean aluminium **cored** inductors: the cores of inductors and transformers usually use a ferro-magnetic material to concentrate the magnetic flux produced by the coil of the inductor. Aluminium is not ferromagnetic. You may as well use an air core. – Transistor Aug 06 '22 at 21:05
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    So, your real goal is light-weight power (voltage) conversion. Operating at a very high frequency is a great way to minimize your inductor size. That is how they get the phone chargers to be so small. Look around for devices that already have solved this problem. – Mattman944 Aug 06 '22 at 21:18
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    Interesting. I have never heard of small aluminium wire like that. – Transistor Aug 06 '22 at 22:33
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    Bite the bullet and learn to solder SMD parts. 0805 is super easy. 0603 and 0402 aren't that difficult. 0102 can be done with practice. I'm talking about soldering by hand with a regular soldering iron. It isn't **that** difficult. I prefer SMD over through hole parts - just easier to work with all around. – JRE Aug 06 '22 at 22:40
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    Optimizing efficiency will allow using a lighter battery, more weight saving than changing the inductor wire – bobflux Aug 06 '22 at 23:42
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    when considering any form of optimisation, always work from a baseline measurement. ie: select a standard inductor that is suitable for your application. What is its weight? measure the amount of copper wire in it and estimate the weight. Recalculate for aluminium. Is there much saving to be had? Then factor in what might have to be a custom part as standard inductors with ally windings are not super common. – Kartman Aug 07 '22 at 00:03
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    The ferrite core of the inductor probably weighs an order or magnitude more than the copper windings around it. I think you're barking up the wrong tree... – brhans Aug 07 '22 at 03:12
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    When weight is a concern, aluminum is a better conductor than copper. So I think it makes sense to use aluminum but I have never seen an aluminum inductor. I have heard of but not seen aluminum wound motors and transformers (for aviation). – user57037 Aug 07 '22 at 03:39
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    @MicroservicesOnDDD For most components, I think they use aluminum due to cost, not weight and that really only becomes a factor for really big components with big coils that use lots of copper so I don't think you're likely to find a tiny aluminum inductor off the shelf. – DKNguyen Aug 07 '22 at 04:59
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    Optimising a power supply involves so many components and design decisions that fixating on one material in one tiny part of it is the wrong approach. Notwithstanding its weight, copper has things going for it, #1, 2 and #3 being that it's low resistance, solderable and available in commercial components. Better efficiency allows lighter batteries, higher frequency cuts reactive component weight everywhere, thinner substrates and SMD components takes weight off the whole assembly. Do a Pareto analysis of your design, and requirements for that matter, before preamturely optimising. – Neil_UK Aug 07 '22 at 05:29
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    Ok - so not using ferrite you're trying to save on core weight - but what else would you use that has a comparable permeability to ferrite for the core, but weighs any less? Taking the ferrite out of the core and leaving just air means you then need multiple orders of magnitude more windings to get the same inductance - you'd end up using even more weight & volume for the windings than you saved by removing the ferrite. – brhans Aug 07 '22 at 14:37
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    @MicroservicesOnDDD `how do I heat the air?` What air? What heat? What are you trying to do? I thought you wanted a lightweight inductor for a power supply? It may be worth setting up a spreadsheet to design an inductor to a specification of inductance, stored energy and losses, with variables for core material, winding material and dimensions. You'll rapidly find that ferrite or iron cores beat air-core for stored energy/weight by some margin. That's a coincidence, all other inductor users seem to have discovered that as well. Al/Cu comparison may be more nuanced. – Neil_UK Aug 07 '22 at 19:50
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    Did you measure the inductance of the 2" air-core inductor or calculate it? By my estimate you're off by about 10x. I'd expect something more in the 3-4uH range for what you've built. Now consider this [little SMD part](https://www.digikey.com/en/products/detail/taiyo-yuden/CB2518T4R7MR/2002802) for example - it's 2.5mm long and 1.8mm tall & wide, and weighs a tiny fraction of a gram. Or how about [this one](https://www.digikey.com/en/products/detail/inpaq-technology-co-ltd/WIP322512A-4R7MLDG/15222603) if you need a little more power - 3.2mm x 2.5mm x 1.2mm, and still a fraction of a gram. – brhans Aug 08 '22 at 00:58
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    @MicroservicesOnDDD OK, back right up, I didn't register that this was an electrically heated hot air balloon. That changes the system concept completely. What's the output of this converter going to be used for? Is it going to be mainly powering electronic loads (nav, camera etc) and any waste heat can be used? Or is it going to be 100% heating the air? – Neil_UK Aug 08 '22 at 04:09
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    @MicroservicesOnDDD What are your electronic loads, if any - power requirements, duty cycle requirements? What are your batteries - technology, voltages contemplated? Is your flight self launching from cold, or merely self sustaining once airborne? Duration? A higher voltage battery could use very light linear regulators to power any loads, and the well-known poor efficiency of them would not matter. – Neil_UK Aug 08 '22 at 04:43
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    @MicroservicesOnDDD I was waiting for you to say 18650s! They have a case. I would expect pouch cells to be lighter. Worth a survey of available cells anyway. – Neil_UK Aug 09 '22 at 04:51
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    @MicroservicesOnDDD what is a 'balancing' take off?? Is your balloon hot helium rather than hot air? How would it fly on one watt dissipation? What's the purpose of the fan? Note that ex Tesla cells have very low capacity, so you get cheap, but you also get poor energy per weight, you're trying to reduce weight, remember? – Neil_UK Aug 09 '22 at 05:20
  • Note the pictured item is from a *transformer*, not an inductor. The distinction being, an inductor stores energy, while a transformer transforms power. MOTs in particular are extremely cost optimized, very much at the expense of performance. Whether that makes it a good example here, isn't clear. – Tim Williams Jun 12 '23 at 02:12
  • @TimWilliams -- Moved comment reply into question. (Will delete this later.) – MicroservicesOnDDD Jun 12 '23 at 23:21

1 Answers1

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I'm not aware of aluminum being chosen for on-board inductors, but perhaps they are out there.

The gains from pushing to higher frequencies, are far greater than the gains from using different materials. Broadly speaking, inductor Q goes as sqrt(F), so you can shrink the inductor by maybe 30% by doubling the frequency -- although this reduces its surface area as well, so it'll get hotter, and the real reduction might be more like 15%. Or if you quadruple frequency, that might put it closer to 30%. Compare a 260kHz LM2671 to a 2MHz MP2457, along with all the support components required (there are savings in smaller capacitors, too!).

Whereas the savings from wire alone, are 40% for the wire in and of itself, but there's insulation, core material (an air-core coil will be MUCH heavier for the same specs at these frequencies!) and the whole rest of the build, to consider; all together, that 40% improvement might be diluted to a couple percent. It has no effect on core size, nor capacitors or anything else. And the component will be physically larger, which may be of some consequence to EMI (i.e., it's a larger antenna -- by maybe just a couple dB, but still, "strictly worse").

There may also be advantage in using other construction methods (e.g. litz cable versus solid wire), though this won't really be applicable at these frequencies or scales (but is very helpful in the 200W+ and 300kHz+ range, particularly for DCM/BCM operation, or resonant converters). And, not that this is exclusive to copper, but you will have an even harder time finding aluminum wire usable this way(!).

So, in short: the gains from material choice are nearly negligible, whereas there are much larger potential gains from just designing it better. Still, when you're spending millions of dollars putting something in space, say, you reach for even the highest-hanging fruit, so maybe these are used in aerospace -- but don't expect to find them off the shelf, at least not for any price you'll want to pay(?).

The other major use case, is anywhere low frequencies are stipulated -- military and aircraft often use 400Hz AC for example. You're stuck on miniaturization, you can't design your way out of that -- at least, not without a lot more work and compromise. In that case, it's a great choice. Such iron-core transformers and inductors also need quite a lot of wire, and there's no support circuitry, it's just a dumb part, so the gains aren't diluted as much.

Pushing flux density up is also a priority (minimize core weight/volume), which also means losses need to be kept minimal (and/or allowing higher operating temperatures -- higher class insulation), so I would expect amorphous/nanocrystalline materials are of interest here, or at least much finer laminations/stripwound cores of lower-loss alloys.

Tim Williams
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    @MicroservicesOnDDD Beats me. I did a 1W "Joule thief" back in the day, using a PBSS303NX, powdered iron inductor, and single AA cell, which runs up to 350kHz or so: https://www.seventransistorlabs.com/Images/JouleThief3.jpg (That's my website.) A few MHz should be reasonable with these or better transistors. I don't know what your goal is; this seems more of a discussion thread than a question and answer. [Edited by a moderator.] – Tim Williams Aug 07 '22 at 20:09
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    I always had the understanding that you use the converter, because, well, you can't suck the last bit out of a 1.5V battery, into a white LED, without some kind of converter. And the blocking oscillator is a good candidate, being simple, yet reasonable efficiency (I measured about 60%, at 1.2V input, and instead of the LED, using a schottky and cap to rectify DC output to measure). So I don't get the motivation here; 4xAA will run a single LED just fine with a CC driver (linear or buck as one might prefer!). – Tim Williams Aug 07 '22 at 20:42
  • It's about efficiency, brightness, and more complete usage of dead cells (or regular ones). A higher voltage Joule Thief is better because it takes the cells down to a lower voltage, stealing more joules. For a 5-AA 7.5V Joule Thief with a single series string of 6 white LED's (~18V) will bring all cells down to about 100mV instead of the more typical 500mV. Besides, the higher voltage JT's are more efficient. And adjustably brighter with a potentiometer. Loved your Joule Thief, by the way. – MicroservicesOnDDD Jun 12 '23 at 01:03