What are the tricks for winding a tight air-core inductor? - Page 1
Forum section: electronics / beginners

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 13, 2022, 05:28:29 pm
content: For a project, I need some inductors around 10 uH to 33 uH. Because they're used in a pulse-forming network, the peak current is extremely high, up to 500 A but just one microsecond.Due to core saturation, I think that an air-core inductor is the only acceptable choice, so I decided to pick up this lost art and wind my own. Now the prototype has successfully generated the waveform I need.But I found it's difficult to wind a professional-looking, tight inductor. If you look at an inductor from classic shortwave radio (see attachments), you can see the winding is tight and evenly spaced, and they're often mechanically stable without additional support. What are the tricks for winding a good inductor coil?In my attempt, I tried winding my roll of enameled copper wire on the body of a RF torque wrench or a whiteboard marker, 10 uH was around 40 turns. If I don't push the wire, the spacing would be too large. But when I tried to push the coil, it creates a strong mechanical stress. When the coil is eventually released, the windings suddenly spread out and jump like a spring (the mechanical version of inductive kickback?). The lengthening significantly reduces the inductance value. After it's soldered onto the board, I have to squeeze it significantly while watching a meter to tune its value, the final result is a completely deformed coil. The shape and size of the final inductor is unpredictable.

author: Benta
author_rank: Super Contributor
timestamp: August 13, 2022, 07:14:02 pm
content: How about using a bobbin and isolated copper wire? That'll also prevent microphonics and other mechanical influences (vibration, shocks).

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 13, 2022, 07:28:23 pm
content: +1 to Benta.Hold the bobbin rigidly on a mandrel held in a vice or similar.Tape or solder the start leadout so the turns are wound with tension, by hand.A better way is on a lathe, but by hand is ok too.

author: radiolistener
author_rank: Super Contributor
timestamp: August 14, 2022, 04:05:23 am
content: 1. to get the best Q: use the same length and diameter for the coil2. to avoid parasitic resonances: keep fixed step distance between neighbor windings of the coil3. to avoid deformation: use coil frame from good material. Good material should have minimum losses at working frequency.You can use microwave oven for quick and dirty test of material for coil frame. Just put piece of that material into microwave oven and turn on at full power for a minute. Good material should stay cold. More heat means more worse material.

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 14, 2022, 05:11:42 am
content: I just learned that there are Material #0 toroid cores for sale, basically just a polymer core. I think I'll try this solution instead. It needs more turns, but it comes as a standardized part, good for PCB use. Parasitic capacitance and Q are not critical for this circuit.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 14, 2022, 06:34:35 am
content: Nice radio!The coil in the second pic has more personality, I like that one more. The ones in the first pic are too perfect, have no soul. If it has to be uniform, maybe use a piece of plastic pipe as mechanical support, a drill to shape the coil, or cut a piece of PET bottle and roll the plastic sheet into a support cylinder.For the high current coils, a donuts magnetic will keep the field in the core, while an air coil will radiate a lot of RF parasites, especially with high current and sharp pulses.

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 14, 2022, 07:02:55 am
content: Yes, a closed magnetic path in a toroid core is definitely a plus for EMI/EMC... Though I don't mind going back to a solenoid if I can find a more suitable option. The purpose of this entire device is to generate a pulse for immunity testing, so the DUT would also radiate even if the generator does not, so I'm not particularly concerned about inductor radiation (though one can argue a large solenoid inductor is a more effective radiator).Also an observation during my search for parts - off-the-shelf (not custom) air-core power inductors are surprisingly still being manufactured, by some "boutique" Hi-Fi audio vendors for loudspeaker crossover networks. Due to well-known reasons, Hi-Fi parts are often overpriced. But in this case, $6 for a high-quality power air-core inductor is pretty reasonable to me. Unfortunately, though they have what I need, my local distributor doesn't sell the uncommon low-inductance coils, otherwise I'll just get a premade one.

author: RoGeorge
author_rank: Super Contributor
timestamp: August 14, 2022, 07:22:17 am
content: Another thing for microseconds pulses, that would mean the frequencies in the coil are in the MHz range and higher, where the skin effect might become important. If high Q factor is needed, a coil made with isolated strands of wires (instead of single wire solid copper) would be better.

author: jonpaul
author_rank: Super Contributor
timestamp: August 14, 2022, 12:13:36 pm
content: Bonjour à tous1. high current PFN pulse forming network are very well studied since 1940s for wwii radar.Yes aircore is the gold standard. At 500A most magnetic matériels will saturate2. Use a fiberglass or other tube, wind single layer, hold temporary with tape, apply varnish, let dry or bake out, remove tape.3. Audio Speakers voice coils use a Bondease insulated magnet wire.With curing heat the adjacent turns of Bondease wires will bond to each other.4. Depending upon the ambient temperature, airflow, duty cycle, ohmic and skin effet losses, minimum wire guage is determined.5. in 1980s our 4 kV 500..1000 A aircore MRI test coils were ~ 1m dia, 1m l, wire was #8 rect, HPTZ on a custom fiberglass cage frame.6. The pulse causes mechanical forces that tend to disassemble the coil. They need to be well fixed by varnish or encapsulation.Just the ramblings of an old retired EEJon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2022, 05:32:51 pm
content: Mind that Q factor is worse for tight spacing, than for pitch ~ 2 * wire dia. Worse still for multilayer, where the current crowding occurs between layers (particularly the inner ones), and as mentioned, worse at high frequencies compared to using litz cable -- but Q also generally goes as sqrt(F), so you might not care if the frequency content is high enough (solid wire is fine for >= 10MHz).Aspect ratio (coil length / diameter) also matters, best Q being close to 1.For point of reference, with litz cable, it's possible to make coils with Q factor of 2000 or more, at medium frequencies (100s kHz to few MHz). Single layer, even spaced, solid wire coils will have Q in the 100 ballpark (give or take say 3x), in the same frequency range. Tight, multilayer coils can be down in the 10-30 range. Probably, one of these ranges will be adequate for your application, thus determining the geometry.I guess magnetic materials could still be used, 500A isn't completely insane; but it will require quite a wide air gap, even for single-turn inductors. So the result will look more like a string of beads, than a winding as such. And rather than solid (ring) beads, they would be split for air gap, or even just flat plates arranged either side of the conductor. Preferably, using powdered iron (for the higher Bsat).Alternately, a shielding box could be arranged around a solenoid coil, which roughly doubles its inductance while shielding the surroundings -- maybe helpful if you need to pack things together tightly. The flux density outside of a coil is lower than in the center (where a core would otherwise go), so this can be reasonable even with ferrite; though it may have to be fairly thick still.There's also machinable powdered iron / composite materials, such as Fluxtrol.The main application for core materials here, would be if your pulse rep rate / duty cycle is high enough that the air-core inductor is getting too hot. You can either increase its size (wider and longer, thicker wire, fewer turns), use Litz, or add core pieces. Can also use tubing and circulate cooling water -- this is the only reasonable solution for 500A continuous, as for induction heating coils.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: August 14, 2022, 07:26:40 pm
content: RebonjourThis giant 1980s is huge but I think similar specs to the OP requirement.1.2 mHy 500A 5 kV MRI pulse inductor.Dimensions ~ .85 m dia 1 m lWeight ! 75 kGFabricated many 100s over some years.HUGE winding machine, as big as a car...Your feedback?Bon Soiree!Jon

author: mag_therm
author_rank: Frequent Contributor
timestamp: August 14, 2022, 08:07:10 pm
content: Hi Tim and Jonpaul, comparing Induction heating with the present case, coils need low copper R series for best efficiency.Freq is low and dimensions are large, however mostly, the dimensional scaling is linear considering an empty coil with Ur=1So:Efficiency rolls down when length < 3* diameter.Any non mag shielding closer than about 1.5 dia increases Rseries and reduces jwL hence reducing Q_empty.Multilayer coils are rarely used due to the inner layers being in the longitudinal H component.The current density distribution in turn cross section shows highest density at the inner radius with a fall- off based on reference depth in copper.Turn space factor is kept high and ranges up to about 0.8 to approach current sheet if voltage and refractory resistivity permit.If designing Nico's coil at 1 Mhz, I would just follow the above , with a mini version using enameled wire. However the pulse duty is needed to get the size.I might do a 2D when I get home from Ozzie and check the effect of L/D ratio etc.

author: niconiconi
author_rank: Frequent Contributor
timestamp: August 14, 2022, 09:00:04 pm
content: Quote from: jonpaul on August 14, 2022, 07:26:40 pmThis is impressive but overkill for my purpose. My peak current looks high, but the total energy involved is extremely low: 3 joules. Repetition rate is equally low, once per minute. Similarly, efficiency is something of no concern for this simple experiment device. A "shortwave radio" style air-core coil is sufficient. (An experienced forum member in the field of EMI/EMC can even guess exactly the generator type that I'm building here, given all my previous hints, but I'll save the details for a full article later).

author: coppercone2
author_rank: Super Contributor
timestamp: August 15, 2022, 01:04:29 am
content: Well not every inductor has a bobbin, if you take apart a TV tuner or something you will see alot of free standing inductors.Not sure if its done for a cost or performance reason.

author: niconiconi
author_rank: Frequent Contributor
timestamp: September 15, 2022, 06:02:02 am
content: I'm glad to report that I've successfully made the needed air-core inductors for 500-amp, microsecond pulse generation. I finally decided to use Material #0 (= plastic) toroid cores. The part number is T130-0 by Amidon Inc. Its advantage over a homebrew solenoid includes mechanical stability, standardized part for reproducibility, and a closed path to reduce EMI. Only the inductor at the top is the pulse-shaping inductor, the two additional ones at the bottom are di/dt limiting inductors to protect the thyristor (an air core is unnecessary since they don't handle the actual high current pulse, they'll be replaced with a T130-1 core in the next revision).The actual circuit I'm building is a 1.2/50-8/20 μs impulse generator for testing surge protection circuitry. My breadboard prototype has successfully generated the required 1000 V / 500 A surges, and its waveforms conform to IEC 61000-4-5 Combination Wave Generator's specification.Open-Circuit Voltage:- 1.02 kV, Front time: 1.42 μs, Duration: 58.90 μsShort-Circuit Current:- 496 A, Front time: 7.93 μs, Duration: 22.27 μsThe next step is transferring the design to a real circuit board. When the project is completed, I'll publish all the design files along with a complete article to describe its operation and uses. For now, you can just read Wikipedia, I'm the author of the "IEC 61000-4-5" article. The old version didn't have any details, now it has been updated with all the information I've learned during the construction of this prototype.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 15, 2022, 06:15:38 am
content: Aha, nice.Anyone else looking to make a circuit like that, might also find these of interest...https://www.allelectronics.com/item/mkp-204/20.0-uf-400-vdc-polypropylene-capacitor/1.html(At least in the US; YMMV.)Tim

author: jonpaul
author_rank: Super Contributor
timestamp: September 15, 2022, 06:18:15 am
content: Rebonjour, bravo pour l'effort. But your breadboard has neither the voltage nor current capabilityes required.We were consulting for decades in power electronics, HV and compliance/EMI/transient protection. We built similar test fixt in 1980s..1990s for testing of transient protection eg from Furman Sound, Monster, Tripp-Lite.1/ The toroid's have very close spacing of each ends of wdg,2/ solenoids are much better topology than toroid's for HV pulse inductors.3/ Fine wire has high resistance, skin effect, use much thicker wire eg #18..16 AWG4/ The large film caps have way too much series L and too low a pulse i rating.5/ Tiny film resistors will be subject to change in R or blowouts.6/ PCB may not be a preferred way to build. We used FR4/G10 0.125 thk fiberglass perfboard ("Vectorboard/VeroBoard) and Teflon standoffs.Bon chance!Jon

author: niconiconi
author_rank: Frequent Contributor
timestamp: September 15, 2022, 06:35:10 am
content: Quote from: jonpaul on September 15, 2022, 06:18:15 amThe repetition rate of this generator is once per minute, discharging just a 6 uF capacitor, the overall energy involved is very low. I don't expect the current handling capability to be the problem for the board, inductors, wiring or the thyristor.My most concerning problem is the pulse capacitor. The latest version, IEC 61000-4-5 Edition 3 requires a mandatory 18 uF AC coupling capacitor. It means every time the generator is fired, the AC coupling caps will experience enormous stresses, the stress on the AC-coupling caps is even higher than the storage caps. Properly rated pulse capacitors will be extremely expensive. Thus, I've omitted it for this simplified build. For testing unpowered protection devices, AC-coupling or not makes little difference. Another idea to reduce costs is to simply use a few normal capacitors in parallel - this circuit is built just for some personal experiments (or a quick pre-compliance test, perhaps), it's not meant for production in an EMC test lab working from 9-to-5. So even a lifetime of a few thousands pulses is perhaps acceptable.QuoteThis impulse generator, by design, requires a low-Q inductor. It uses a 0.7 Ω resistor in series with the inductor to kill its Q. High wiring resistance is not a problem. But it's true that dissipating the heat into the resistor is always better than dissipating it into the inductor's ESR. Though I did a quick calculation, at once per minute, the average power is pretty low and I think the wires and cores can definitely handle that.

author: jonpaul
author_rank: Super Contributor
timestamp: September 15, 2022, 06:52:07 am
content: We used a PRF 1/sec for easier manipulation of the DUT.The surge V and I are affected by coil design.Photo is typical HV solenoid we manufactured and used in such transient pulse gen.Note perfect layer, Wind square wire, fiberglass tubeJon

author: RoGeorge
author_rank: Super Contributor
timestamp: September 15, 2022, 10:51:13 am
content: Why the square wire?

author: jonpaul
author_rank: Super Contributor
timestamp: September 15, 2022, 03:11:20 pm
content: RoGeorge, this arc lamp series injection transformer generated 35 kV to ignite a 1 kW Xénon lampAfter ignition the current can be 10s..100 A.A square wire has more Cu cross section and holds the shape better than round.The 1 M coil previous photo used #4 recto HPTZ, Belden, either square or rect is special order.Jon

author: coppercone2
author_rank: Super Contributor
timestamp: September 15, 2022, 03:41:22 pm
content: why is the rod inductor better then a torroid for pulse generation?I am wondering just because I conceived that it could be possible to wind a air coil helical torroid inductor a while back but no one here seemed to know why it would be made. Also wondering about inductors wound on aluminum or copper (less then 1 relative permeability). I never saw one made. I thought it might be good for a LISN if you could make a big enough one for the 250uH but its some what absurd.Square wire might get more popular now I think because EVs.

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 15, 2022, 04:40:20 pm
content: I did a few core-less toroids , big water cooled ones.The OD, ID and turn diameter have ratios. I can't find the recipe in my old book now.They greatly reduce stray flux problems compared to air cored solenoids.One way to do it, a solenoid is wound on a round or square mandrel, then the turns are curled around another mandrel.And if the inductance calc is wrong, a complete rebuild usually, so you get dis-honorable mention by the winding department in the morning meeting.

author: coppercone2
author_rank: Super Contributor
timestamp: September 15, 2022, 05:09:15 pm
content: Well if the stray flux is better (the only benefit I know), then why is it worse for this pulse gen?I always figured it would be more stable if the field does not happen to go through the chassis, stuff on the chassis, etc. And you could revise the chassis/system without worrying about the magnetic field. For instance when I built my 50uH LISN prototype with 12 AWG wire air cores, the impedance dropped by 10uH when I put it in the steel box (1 foot long seperated segment coil inside of a ~2x1 foot steel box out of 10AWG steel (made from old storm cover for a basement, it warped to shit from welding and the project is stuck unless I want to do alot of flame straitening).And if the inrush current is bigger for a torroid, is it not better for discharging for a pulse also? Could the problem be that the dIdT is too big for the control circuit to handle, where the rod provides some current limiting?

author: niconiconi
author_rank: Frequent Contributor
timestamp: September 15, 2022, 05:22:47 pm
content: Quote from: jonpaul on September 15, 2022, 06:18:15 amThis prototype used Metallized Polypropylene (MKP) capacitors, the capacitor of choice for pulsed power applications. The pulsed current ratings of these capacitors range between a few hundreds to a few thousands amps. The ESL is not really a big problem for this particular impulse generator, since it's a pretty slow impulse, the frequency content is negligible above 2 MHz. For IEC 61000-4-4 Electrical Fast Transient tests (which I also plan to build in the future), ESL will be a big problem.Quote from: T3sl4co1l on September 15, 2022, 06:15:38 amI already investigated the available pulse capacitors on the market. The original goal was a 1500 V pulse (now already reduced to 1 kV), I looked at 1600 and 1700 V capacitors. There are three major series: Kemet R75 & R76 series, Vishay MKP385 series, and Cornell-Dubilier 940C & 941C series (the last one also has a cult following by Tesla coil builders), 941C is currently the cheapest, 1600 V, 1.5 µF for just $11. There are also cheaper alternatives, usually sold as "snubber capacitors" for suppressing transient voltages for IGBTs, and thus also has a high surge rating and use the same MKP technology, a few models can be found at LCSC at $5 or so.The downsides of using proper pulse capacitors are enormous size and high price. I ordered some 941C and I found their size is unreasonably huge for a perfboard, so I used some cheaper 1.2 kV MKP capacitors from China. Without AC coupling, the whole circuit only requires 4 capacitors, so even huge and expensive capacitors are still acceptable. The 4 huge CDE ones cost $44, with a large but still manageable board space. But if 18 uF AC coupling is used, the circuit will be really enormous with 10 huge MKP capacitors just for output coupling, and costs over $100, and be the single most expensive item in this circuit.Since the goal of this circuit is to make this test easy and accessible for other experimenters, long-term reliability or AC coupling can be sacrificed, as I mentioned previously. I plan to do some lifetime testing of "improper" capacitors and see long they can survive.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 15, 2022, 06:10:47 pm
content: Sounds about right. And the above part, probably isn't rated anywhere near enough pulse to be considered reliable, but yeah, same idea, who cares. In this case, they're oversized and under-volts, so, series combinations will do.Inductor type doesn't matter based on given constraints (i.e., not enough of them!). Toroid has lower external field. Solenoid is easier to make, and, I don't know how Q compares. But Q isn't very interesting for pulsed operation anyway. DC current ratings are irrelevant (in contrast to arc starter application!). The surge network needs a lot of resistance already, so the relatively thin wire isn't a big deal (as long as it doesn't fuse, lol).I was going to do a EFT generator, eventually. Figure C0G chip capacitors, SiC MOSFETs, and enough in parallel (about 8x) plus a transformer to match back to 50 ohms at enough voltage. Making a good enough quality TLT isn't going to be easy, but feels like the better option compared to a cascode switch.Or just get a hydrogen thyratron and slam-dunk it that way.Tim

author: tsat
author_rank: Newbie
timestamp: April 14, 2023, 03:05:10 pm
content: Hi, I have recently purchased a multitester for EFT/Surge tests thanks to a national call that support the development of small companies by funding with the 50% of equipment cost.Even so was not easy to buy it so I didn't purchased any coupling/decoupling networks. I purchased a used EFT capacitive coupling clamp for EFT and I am thinking to build some CDNs for surges.To start I would like to build a decoupling network for I/O signals (figure 9, EN 61000-4-5) so does anybody knows what kind of chokes I need to use ? Are there any COTS available or shall I make one by myself ?Unfortunately even if I have access to CDNs these are completely potted so there is no way to check the components.Then I need to built the coupling part so a 0.5uF suitable to be stressed up to 5 kV. Any ideas?I am not very familiar with high voltage circuits that is why I am seeking help, I am an EMC/Wireless test engineer.BR,Stelios - Greece

author: jonpaul
author_rank: Super Contributor
timestamp: April 14, 2023, 03:10:57 pm
content: In 1980s for arc lamp series inj ignitors We got HPTZ square or rect wire from Belden, Phelps Dodge etc.Mandrill wound on a LARGE winding machine like Micafil or Adama Maxwell.For the fastest high current pulse switch, The DNA, AEC, LLAN used triggered spark gaps , like the EGG ceramic types.Best pulse caps were from Maxwell,Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 14, 2023, 05:54:30 pm
content: Quote from: tsat on April 14, 2023, 03:05:10 pmJust has to pass the calibration steps (following section).Typical values are going to be in the 100s uH, and rated... more than a few amperes peak, of course. Recommend running a simulation to get a feel for the waveforms, current and voltage magnitudes, and what ratings you'll need. Likely need either air-cored or some big powder cores (or ferrite if you like, but it's probably going to be rather big, or maybe even laminated iron). Inductors will most likely be larger for the higher impedance tests (smaller C and added resistance).Tim

author: tsat
author_rank: Newbie
timestamp: April 14, 2023, 08:58:00 pm
content: Thank very much Tim & Jon. I will give a chance to myself.Stelios

author: jonpaul
author_rank: Super Contributor
timestamp: April 15, 2023, 06:36:28 pm
content: CopperCone!>>why is the rod inductor better then a torroid for pulse generation?>>EXCELLET question.In HV, pulse and other apps the insulation and volrag egradient are critical.A toroid is wosrt as every turn is clost to the core, so S><F voltage must be withstood by S turn and F turn to core insulation.A solenoid air cosr is best for capacitance, SFR and insulationnext best is rd core solenpoid, SINGLE layer perfect lay.See Nikola TESLA nes 1889..1900, ALL of his HV coils were air core, perfect lay solenoids.With a rod core, a heavy insulating tube is easuly slid on before the cois is applied.Finally a solenoid is MUCH easier to eind than an toroid.We designed and manufactured may HMI, Xenon and other arc lamp ignitors in 1970s..1900 like thisHV lamp series injection ignitors use that design to this day.Cinema lights to 12/18/14 KW with ignitors 35..75 KV are by IREM, BAG Turgi, Ianairo, ARRI!An ABSOULTELY FANTASTIC DAY to all!Jon

author: coppercone2
author_rank: Super Contributor
timestamp: April 15, 2023, 08:31:20 pm
content: I don't get the S><F thing you wrote.Do you mean that the wire on the inside of the doughnut is basically disfavorably positioned compared to wire on a cylinder that is uniform ? Like I get that on a rod core, the end is far from the start, so it is highly isolated, but in a torroid the end is near the start where there is dielectric problems?And that the wire wrapped around the interior of the doughnut is closer to each other then the wire on the other side since its like folded. But I assume with a proper core - spacing - gauge ratio the problem is mitigated, but I guess every slice is offset kind of from the ideal and seems imbalanced. The bending of the solenoid makes me think of a cusp related problem. Gradient on the insulation or gradient of insulation materials might help optimize to make it stronger to breakdown in the middle but not have so much capacitors on the edges. Like a semi air core torroid built around something like a chirstmas star acrylic ornament to have dielectric resistance in the middle but not the edges.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 15, 2023, 09:39:24 pm
content: I think: Start, Finish, and everywhere inbetween. Cores are notably conductive, so when you need say 10kV isolation, it's a big deal.Toroids probably aren't much better than solenoids even when air core, where pulse applications are concerned; unless you need a compact construction with shielding to nearby stuff, in which case the self-shielding nature of the toroid is of course helpful. But you still can't close it very tightly (start to finish distance) because of insulation still, which leaves the gap open, emitting a field equivalent to a solenoid the size of the gap.It's also possible to make a solenoid with a "shingled" arrangement, i.e. wind 10 turns forward, 7 turns backwards, repeat ad nauseum; this gets the inductance and density of a multilayer inductor, but with ~full length voltage handling, and a fraction of the capacitance of a multilayer winding. Alternately, "pie winding", making blocks or banks at a time, and stacking those up to the same effect.Tim

author: jonpaul
author_rank: Super Contributor
timestamp: April 16, 2023, 12:37:58 am
content: Solenoid .....even voltage gradient over the legntheg 10 kV pulse 10TSTART (S) = 0 turns/V1/1kV2/2kV....10 10kVToriod: S and F are adjacent so highes tstress and EVERY turn is clost to core.j