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Analog Angle Blog

Litz Wire & Other Component Cleverness

The other day, while browsing through an engineering magazine (one of my favorite activities for learning via serendipity), I saw an ad from a vendor of litz wire.

I thought, “There's a term I hadn’t seen in many years.” In this case, it was an ad from New England Wire, although there are other vendors, of course.

If you're not familiar with it, litz wire is not named after a person or a place. It's short for Litzendraht , the German term for braided, stranded, or woven wire. It's a very clever solution to the problems and inefficiencies caused by the skin effect — as the frequency of the current that a wire carries increases, the current tends to go to the outside of the wire.

The faster the electronics zip back and forth, the more they repel one another. As they try to stay farther away from one another, the effective resistance of the wire increases, and so do the losses. More and more of the wire's cross-section is rendered ineffective. Due to the skin effect, you are paying for copper and associated physical bulk, yet it is doing you little good and is actually counterproductive to effective signal conduction and component performance. The problem is most apparent and debilitating in components with lots of wire (think inductors or transformers).

Litz wire is effective at minimizing the skin effect up to about 500kHz. Above that, it is less effective for various reasons.

To make litz wire, the vendors abandon the single strand of wire. Instead, they use multiple strands of finer wires. With conventional stranded wire, the strands are meant to provide more bending flexibility, but thin strands alone won't reduce the skin effect. There are two additional characteristics of litz wire: The individual strands are insulated from one another, and they are woven together in various patterns to manage the electromagnetic fields of the wires at higher frequencies (different patterns can provide variations in performance characteristics). Thus the overall cable is composed of many fine, insulated wires in a precise braid.

I can only imagine what the production facility must look like. Since it must handle hair-thin wires that have very little strength, the pulling tension must be kept low, yet the braiding machine must also do its magic as the wires are pulled through the system. How do these machines keep the strands from breaking? How do they keep the whole thing from become an impossible tangle? Those production machines must be fairly impressive, and the people who designed them must be pretty clever.

Whether or not you have ever heard of litz wire or used a component made of it, you've undoubtedly run into other components that were cleverly designed to overcome or work around some physical limitations. There are capacitors, for example, in which the internal layers are fabricated in a way that reduces parasitic inductance or lowers equivalent series resistance.

These are just two examples, of course. Are there any other such design workarounds that caught your attention when you first saw them?

35 comments on “Litz Wire & Other Component Cleverness

  1. Brad Albing
    May 7, 2013

    As a kid, I would tinker with AM radios (like everyone else who reads these comments, it seems). I recall trying to fix the loop antennas on the back of these from time to time. These were the version wound on a piece of masonite or pressboard on the back of the table radio, not the ferrite loopstick.

    Initially, I didn't understand what the deal was with the funny wire they used. And, as if removing insulation from a piece of enamel coated wire wasn't tough enough, here was a bunch of enamel coated wires.

  2. Davidled
    May 7, 2013

    Cable for high frequency might need Center conductor made of silver plate, Dielectric, shield of silver plate, and copper braid.  Time delay should be met to a certain value, and RF leakage and bend radius should be measured. Cable has been tested to more than the required number with no electrical or mechanical degradation. I think that Nano tech based Litz Wire can be used for this type requirement.

  3. pmoyle111
    May 8, 2013

    Yes, I remember the same thing. I also remember tearing open some caps that unrolled wiht tinfoil and what looked like wax paper. I didn't realize at that time that that was a high quality cap.

  4. steveyoth
    May 8, 2013

      I think this article is very timely.  We at Panasonic are seeing an increase in the use of Litz Wire in Cellphone Inductive Chargers, a relatively new technology.  You throw your cellphone on a charging mat that has flat coils of the Litz Wire which inductively transfter power to the cellphone.  It's also called Wireless Charging.  The flat coils usually use Litz Wire for improved efficiency.  The frequency used is between 100kHz and 205 kHz, per the new Qi Standard of the WPC (Wireless Power Consortium).

  5. DEREK.KOONCE
    May 8, 2013

    Sounds like a very nice application of Litz – high frequency tesla-based chargers.

  6. Guru of Grounding
    May 8, 2013

    While we don't use Litz wire to make our transformers – they're aimed at very-high-performance audio applications – we do use some very fine wire. To reach our low-frequency design goals (typically -3 dB at <1 Hz to >100 kHz), some of our designs involve 10,000 or so turns of #46 AWG wire (that's thinner than a human hair). I've always had a great admiration for the folks who designed our Swiss-made winding machines that can wind this fine wire onto bobbins at speeds of several thousand RPM, ramping up and down in speed flawlessly without breaking the wire!

    When I was a kid of 8 or 9, I remember taking many AM radios apart. I found Litz wire in the loop antennas and inside the IF transformers. I found soldering to Litz wire almost as frustrating as trying (the operative word) to solder the “tinsel” wire used for headphones. Today, that long-flex-life application normally uses high-strand-count oxygen-free-copper. – Bill Whitlock, president & chief engineer, Jensen Transformers, http://www.jensen-transformers.com

  7. Guru of Grounding
    May 8, 2013

    I think you'll find that “wire” of any kind is pretty useless at 30 GHz!  Perhaps you meant 30 MHz?

  8. Bill_Jaffa
    May 8, 2013

    I guess Litz wire is the opposite of so-called “monster” audio cables: OFHC heavy gauge, stranded, but not insulated per strand.

  9. Davidled
    May 8, 2013

    “Nano tech based Litz Wire” that I mean is that particle structure of wire can be modified and changed by Nano technology. Now, Nano impacts all areas including medical industry. I guess that it could be possible in the future.  So, wire will be transformed to optical fiber. Referec tech is “Iron man 3.”

  10. Spanners
    May 8, 2013

    Perhaps the better definition of the “skin effect” is that  the e-m energy field which surounds the wire, penetrates the conductor less as the frequency rises due to the wires  self inductance

     In any circuit from DC   to light It is always the external e-m fields we are interested in as this carries the actual power or information . This field is only guided by the wire – hence the formation of conductors to transmission lines as the frequency rises to the point where  external the field only touches the surface as in a wave guide – the ultimate being fibre optic where the em wave is guided by the boundary between  two materials of different refractive indexes

  11. Guru of Grounding
    May 8, 2013

    I'll spare you my rant about “Monster Cable” and all the pseudo-science used to promote it and rigged tests offered as proof of its alleged superiority. Those who are tempted to believe any of it should visit http://www.nousaine.com/nousaine_tech_articles.html.

    I've long researched the origins of oxygen-free copper wire and, as best I can determine, it was developed for the US Navy during WWII to solve the problem of copper wires breaking from repeated spooling when dragging sub-detecting magnetometers behind ships. The wires were breaking from well-known “work-hardening” from the repeated flexing. It was found that, if the wire was annealed in an oxygen-free gas, this work hardening was drastically reduced. So oxygen-free copper is a good solution for cables (like headphone cables) that see a lot of flexing. And, nowadays, almost all wire is oxygen-free anyway. But audio cable marketing folks have used pseudo-science to ascribe new, and often ridiculous, attributes to OFC wire.

  12. Bill_Jaffa
    May 8, 2013

    I hear you on the presumed virtues of monster cable, and how it has been marketing to the unwary or to those “pseudo-sophisticated” audio folks. I was just using it as a complementary vision to that litz wire!

  13. Brad Albing
    May 8, 2013

    I suppose even at those not especially high frequencies, litz wire would prove effective (i.e., improve Q so improve efficiency).

  14. Brad Albing
    May 8, 2013

    Sounds (pun intended) like a good blog topic.

  15. Brad Albing
    May 8, 2013

    46AWG is even finer than that which is used in guitar pickups, if memory serves.

  16. David Ashton
    May 8, 2013

    “Tinsel” wire – now you're taking me back!   When I was a callow youth of around 18-19, I worked on military radios in Rhodesia (now Zimbabwe).  Occasionally we would need to reterminate a tinsel curly cord for a microphone or handset.  We would strip a bit of stranded wire and use one strand of that to bind the tinsel strands togehter before soldering them.  I got pretty good at it, but try as I would, I could never quite get as neat a result as the pre-made cables from the manufacturers.   It's very good for flexible cords though, we very rarely had to fix them, mostly we had to reterminate them with different plugs for different equipment – we did a lot of “making do” and adapting in those days.

  17. David Ashton
    May 8, 2013

    Sorry, the above comment is replying to @Guru of Groundings remarks about tinsel wire above.

  18. Brad Albing
    May 8, 2013

    Yet again the germ of a new topic for a blog.

  19. Brad Albing
    May 8, 2013

    Sometimes the comments don't nest quite right. Yep – I used that same technique as a kid to repair or terminate telephone handset cords. If you soldered just quick enough, it worked pretty well.

  20. RB3200
    May 8, 2013

    When I started reading this article, I was at first thinking it was about solder wick. Where I grew up (in the Netherlands) we called this “soldeer litze”.

    From the practical year of my BSEE education, I remember getting a tour of a coil winding facility in a Philips component factory, where they made many kinds of coils and transformers from various diameters of enamel-insulated copper wire. Those winding machines worked at high RPMs and I remember being told that they occasionally used to have some minor accidents with the prototype coils being launched through the factory, when they were spinning out of control.

  21. Guru of Grounding
    May 8, 2013

    I've seen that happen in our factory once or twice! When the bobbins are small, it's a very entertaining event 😉

  22. alzie
    May 8, 2013

    Since the current is in the skin of the wire,

    increasing the surface area for the total diameter

    will lower the AC resistance at any frequency.

    In the process of designing a reasonably sized (4' sq) portable HF resonant loop antenna,

    the radiation resistance is much lower than the AC resistance of plain wire, thus

    the efficiency is low.

    Eg. any thing that can be done to lower the AC resistance of the wire

    will improve the efficiency.

    My 1st version is using 3 strands of 12ga wire braided together.

    This equivalent to 1/4″ refrigerator tubing, but flexible.

    Its not too bad, but real litz wire will improve it greatly.

    Next version.

    Lits wire isnt cheap, but its worth it.

     

  23. eafpres
    May 8, 2013

    Hi steveyoth.  I'm curious about the efficiency improvement of inductive charging by using litz wire.  The frequency is relatively low, and the power is relatively low.  However, since the power transfer is basically from a primary coil to a secondary coil (as in a transformer) and the alignment may be imperfect and the core is non-ideal (i.e., plastic, air, etc. instead of a nice core) the efficiency of power transfer may not be so great anyway.  

    Are you saying there are issues with resistive losses in the coil of the primary (or base side) of a Qi charger?  And these losses are significant compared to the efficiency of the system inherently?

  24. eafpres
    May 8, 2013

    Hi Derek–Why do you refer to the inductive charging system as “tesla-based” chargers?  The system is basically a transformer, and Tesla did not invent the transformer.

  25. steveyoth
    May 8, 2013

    Hello, eafpres. 

      I'm not an expert on Litz wire.  Litz wire is required in the Qi standard.  For example, I have TI's bq500210EVM-689 Evaluation Module (Transmitter) with the bq500213EVM-725 (Receiver) which I tested out just last week.  See http://www.ti.com/tool/bq500210evm-689.  The funny-looking wire that makes up the transmitting coil in the picture is Litz wire.  I got the rated 5W power transfer with about 7W input for about 71% efficiency.  (And I didn't have to center the receiver very carefully above the transmitting coil either.)  I figure that's pretty good.  My guess is that the Litz wire helps to boost the efficiency of the system.     

      TI has a listing of suggested transmitter coils at http://www.ti.com/lit/an/slua649d/slua649d.pdf.  The transmitting coil in the evaluation module is an Elytone ETW-00296A.  See it here: http://www.elytone.com.tw/upload/productfile/fr2zce1k20121011164651.pdf.

      The Qi Standard requires the “A1” type of charger to have a primary coil using no. 20 AWG type 2 Litz wire having 105 strands of no. 40 AWG wire.  Other types of chargers require different types of Litz wire.  Its all explained in the specification that you can download at http://www.wirelesspowerconsortium.com/downloads/wireless-power-specification-part-1.html.

      – Steve

  26. Netcrawl
    May 9, 2013

    Yes ,you're right Tesla didn't invent transformer but they're pretty good in solar battery chargers, the on ewe use in our hybrid cars. 

  27. Netcrawl
    May 9, 2013

    @steve thanks for the links, I think Litz wire is perfectly fit for high frequency applications where the skin effect is more pronounced and proximity effect can be an even more severe problem, I'm not familiar with Qi standard, your links would probably rpovide some great help.

  28. Brad Albing
    May 9, 2013

    Eafpres – while Tesla didn't invent the transformer, I recall he did some demos that he set up more as parlor tricks – light a light bulb with no apparent connection made to it (except for a transformer secondary winding). Very impressive to the Luddites of his day.

  29. Brad Albing
    May 13, 2013

    I also recall doing that with the [waxed] paper + al-foil devices. These were not especially high quality capacitors, but were plentiful in the old radios and TVs of my youth.

  30. TheMeasurementBlues
    May 16, 2013

    We have a blog about why skin effect occurs at The Connecting Edge.

    See The Skin Gets Ever Thinner.

  31. Brad Albing
    May 16, 2013

    Drat – and I was just planning on writing a blog about the skin effect. Hmm – need to think of another topic.

  32. DEREK.KOONCE
    May 16, 2013

    I was thinking the same thought. Skin effect can affect copper usage – although based on current and frequency of power operation.

  33. amrutah
    May 20, 2013

    compared to using a co-axial cable having a CAT5 cable for connecting the 100BT line is much better due to the efficiency.  But how difficult is it to model the transmission involving multi strands?

  34. Brad Albing
    May 21, 2013

    Amrutah – do you mean, how difficult is it to model multi-strand litz wire when used at high-frequencies as part of a transmission line? Hmm… not sure how well various simulators do in this regard. Or more generally, how well do most simulation sofwares do with skin-effect? Maybe someone else can speak to that.

  35. amrutah
    May 22, 2013

    Brad, Exactly, that's what I meant.

      A single line Tx is itself very difficult to model (whether to use a distributed or lumped model) and then we have the Z0 (characteristic impedance) and the delay of the Line.  If its multi-stranded line how do we model it?

      

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