Litz wire

Litz wire. I’m using it on the primaries of the new plasma drivers.
What do we need to know.

Choosing proper size:
Use simple table to choose individual wire diameter based on frequency.
10-20kHz = 33 AWG = 0.18 mm
20-50kHz = 36 AWG = 0.127 mm
50-100kHz = 38 AWG = 0.1 mm
Buy bundles of these wires with suitable coating.
Good tables and calcs available here: http://www.litzwire.com/index.htm
Also here: http://www.mwswire.com/litzsearch2.asp
Pity none of its in mm 🙁
Look up wire guages here: http://www.bulkwire.com/wiregauge.asp

Coatings:
Need a coating to deal with temp and voltage breakdown.
Formvar – good to 7kv and 105 C
polyester-imide – similar but can go to 180 C and abrasion resistant.

Soldering:
Hard to get the coating off.
Can be removed using several methods. these work:

  • with a solder pot set to 433 C. dip slowly for 8 seconds – drag across and lift to remove coating. Skim pot with cardboard to remove excess coating.
  • with NaOH bath – then washed with water and detergent to neutralise
  • stripped by mechanical abrasion with dremel and SS brush. Flip over to do other side. Clamp first to strain relieve.
  • by burning with butane torch. Clamp above strip point with metal heatsink to avoid heat strip creeping back.
    Easy coating – heat in lamp, dip in alcohol.

General info:
The r-f resistance of the coil includes the direct current resistance (ohmic resistance of the copper), and is greater than it because of skin effect and because of losses that occur in the coil materials which are located in the rapidly alternating magnetic field of the coil. The skin effect is due to the tendency of the high-frequency alternating current to flow through a comparatively thin shell of the outside surface of the wire rather than uniformly through the whole cross-section area of the copper. Since this thin outside shell has comparatively little cross-sectional, it naturally offers a higher resistance to the flow of current than would be the case if the entire cross-section area were being used. The net result on the resistance offered by the wire to the flow of current through it is precisely the same as though a smaller wire were used.

What “Litz” Wire Is

Now the d-c resistance of a coil of winding can be decreased by using larger wire (but such increase is usually restricted by limitations regarding the allowable physical size of the coil). The “skin effect” of a coil winding may be decreased by increasing the total surface area of the wire. In order to increase the surface area without increasing the cross-section of the copper used, the conductor is sub-divided into as many individual strands as is economical and practical, insulating each strand individually from the next by a thin enamel coating. Commercial wire in this form is known as “Litzendraht” (commonly called “Litz” wire). Litz wire consists of many strands of fine wire, each stand individually insulated with enamel, and the group of wires covered with some protective textile insulation such as cotton or silk (although sometimes enamel, paper or other covering is used over the group of wires).

Because the total sum of the cross-section areas of the conducting outer shells of the many individual small conductors in litz wire is greater than would be the cross-sectional area of the larger, thin conducting outer shell of a single solid wire composed of the same total amount of copper, the litz wire has a smaller skin-effect (lower “skin resistance”) than the solid wire. For this reason, it has greater high-frequency conducting efficiency and is used in r-f coil windings where higher Qs are desired.

Why Individual Strands Of Litz Wire Should Be Twisted

“Skin-effect” Losses in the conductors of which r-f and I-f coils are wound have dictated the use of Litz wire wherever economically possible. However “skin-effect” goes a step further and requires that conductor not only be subdivided into multiplicity of individually insulated strands but that these strands be arranged in such a manner that each occupies a place on the surface of the conductor an equal per cent of the time so that the total r-f current will divide equally among the many strands and thereby give the lowest effective r-f resistance. Originally, the strands in Litz wire were braided in a certain manner to get this effect. Because of price, however, modern Litz wire as used in radio receivers is merely twisted so as to bring the different strands to to the surface at different points, giving a result approaching that of braided Litz, but at far less expense. Where Litz wire is made without twisting (that is, with parallel strands) the results are inferior to twisted Litz on two counts: (1) the losses are consistently higher than for twisted Litz; (2) coils made of it exhibit greater variations in resistance than coils made from twisted Litz.

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