litz wire braid layout geometry effects? - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: April 10, 2025, 08:59:58 am
content: If you look at various kinds of litz wire, you can find many different construction techniques, similar to rope.You can twist all the strands together, or you can make several smaller structures twisted together and then twist them together, or you can even interleave it like braid in a more complex fashion.Are these techniques used to offer different electrical performance, or is it purely a mechanical consideration to facilitate a particular bend radius , durability for flexing and or tensile strength?The main adjusting parameter I understand for litz is the strand count, i.e. how many carrier wires you use for a particular end gauge. Is the construction technique treated as a nonideality that has to be considered to some ideal form that might have bad mechanical properties (I assume just twisting all of them together is not robust but electrically the best)

author: uer166
author_rank: Super Contributor
timestamp: April 10, 2025, 10:12:30 pm
content: For Litz to work correctly the geometry matters: ideally every single strand would go through the same amount of magnetic field (both self induced and external: the former is what colloquially causes "skin effect", the latter "proximity effect", they're both the same thing fundamentally) to cancel out proximity effects.If for example you don't twist Litz and wind an inductor through it, you'll have very large circulating current in a loop that goes around the outside strands, and returns on the inside strands, completely negating the Litz-ness of it. I don't know what the particular geometries exist and their trade-offs are, but they all have the exact same goal overall.

author: coppercone2
author_rank: Super Contributor
timestamp: April 10, 2025, 10:15:27 pm
content: Ok, that makes sense that the basic twist is important. I guess the problem might be if you can get the correct amount of twists per unit of distance? So you might be able to control it more if you break it up into smaller ropesI was thinking along the lines of a transmission line, with a transformer you have very nonlinear fieldsBut for a strait run, untwisted litz sounds better? so long you terminate it to twisted litz on the coil end far away enough from the transformer where the fields are weak?Like if it was a 50 meter run to a industrial induction heater. Like a twin-lead run

author: uer166
author_rank: Super Contributor
timestamp: April 10, 2025, 10:59:18 pm
content: Quote from: coppercone2 on April 10, 2025, 10:15:27 pmI don't think a "straight run" exists in the physical world: where is your return current? If it's like one giant coax where the Litz is down the middle, then perhaps the fields will be symmetrical, but I can't see other geometries that achieve the same

author: coppercone2
author_rank: Super Contributor
timestamp: April 10, 2025, 11:01:15 pm
content: Hmm I guess that makes sense for a practical wire runthey do have this thoughhttps://www.newenglandwire.com/product/coax-style-litz-wire/

author: coppercone2
author_rank: Super Contributor
timestamp: April 10, 2025, 11:14:04 pm
content: https://ryz.ece.illinois.edu/pdf/compel2014.pdfI like how they have the integral which looks like "lol"and they also wrote "pp"pp = lol lolthey have a untwisted pp but unfortunately their next equation does not say "twisted pp". I wonder if the author caught on.I never thought this might be so fun

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 11, 2025, 11:25:25 am
content: Quote from: coppercone2 on April 10, 2025, 10:15:27 pmIn theory, I believe that litz wires with no twisting and perfectly paralleled strands would have the same AC resistance as solid wire (assuming no gaps between the strands). If that sounds counterintuitive, consider that with a solid wire at high frequencies, the skin effect forces current to the outside of the wire, but the direction of current flow would still be entirely along the length of the wire, so separating the conductor into small strands would not disturb this current distribution. Even if you twisted the entire bundle of conductors, it likely would not have a significant effect. You need a bundling scheme which forces strands to change their depth within the bundle in order to actually get any improvement in AC resistance.So yes, the bundling scheme of the strands matters immensely, especially as operating frequency increases. This is the "secret sauce" which manufacturers guard carefully.

author: coppercone2
author_rank: Super Contributor
timestamp: April 11, 2025, 11:38:50 am
content: You mean if there is too many wires or something?Because normal stranded wire has a lower AC resistance then solid core (t3slacoil measured it, its relevant to industrial setups... like 14 strand already has benefits at some frequency ranges over a solid core) and they definitely do sell simple litz that is just twisted without being bundled... I have a bunch of it I pulled out of the trash a while back and recoiled when someone made a big mess. It was being used for prototyping cheap wireless power chargersI did kind of get a hint at the idea that if its a thicker wire, you should wind it with a separator in the center to improve the twistI am hoping someone on the forum happens to have a shit load of different kinds of litz wire to make a measurement, or perhaps experience from LW/MW radio engineers or induction heater / wireless charger engineersits a very confusing topicI also have some big chokes I got from the trash long long ago that were wound with simple litz (maybe 5 or 6 strand) wound on a stack torroids to make someones old design pass EMC on low frequencies, but they changed the electronics by increasing frequency to get rid of it IIRC, the EMC place designed it for them after they went to the lab or something, and it got replaced with a off the shelf bobbin ferrite inductor

author: mtwieg
author_rank: Frequent Contributor
timestamp: April 11, 2025, 01:27:20 pm
content: Quote from: coppercone2 on April 11, 2025, 11:38:50 amI've heard this anecdote, but have always observed the opposite.QuoteCompanies and engineers are free to refer to whatever wire as Litz wire.

author: uer166
author_rank: Super Contributor
timestamp: April 11, 2025, 08:56:02 pm
content: Quote from: mtwieg on April 11, 2025, 11:25:25 amThis sounds correct to me as well. The counter intuitive missing piece here is that the ends of the Litz are all shorted together at either end, so the circulating currents are free to enter one Litz strand and exiting the other, so you'll get a "global" kind of a skin effect.You would need to alternate the strands' locations to balance the fields to see any sort of improvement.

author: coppercone2
author_rank: Super Contributor
timestamp: April 12, 2025, 01:31:47 am
content: lol that sounds like a hell of a project, a hot copper block with like 100 holes to carefully rearrange wires with tweezers, so for every test you have to warm up both sides to reconnect it like a switch board

author: coppercone2
author_rank: Super Contributor
timestamp: April 12, 2025, 03:11:37 am
content: maybe you can just put a 10x10 grid of female pins into a block of copper and then solder every single litz wire to a male pin?btw if someone could find test data (not more simulations) it would be nice to read. I know they don't want to sit down and wind things but they also have grad students, it all feels like work from home science