Re: Litz wire strand number best for primary winding on flybacks
Mads Barnkob, Thu Jun 12 2014, 02:23PM

Calculate the skin depth at your drive frequency, chose a wire size with a little overhead and add enough strings to carry the current you are pushing in the primary coil.

Re: Litz wire strand number best for primary winding on flybacks
Lightening Rod, Thu Jun 12 2014, 03:09PM

This is too complex for me. Can you just provide a ball park number of strands and gage? The flyback runs at around 40 KHZ and draws 1.5 amps when tuned for the plasma sphere.

Re: Litz wire strand number best for primary winding on flybacks
Thomas W, Thu Jun 12 2014, 03:42PM


Asking someone else to do the work for you is not allowed on the forum....

try do it yourself....

Google skin deph calculator... oh look!


now you have a skin deph (325.9778um or 0.326mm)

if your wire is thicker then 2 x that number, you add another wire to it.

infact. you don't even need to do that.

look here:


look at the 22AWG wire.

this is just fine for what you want, it says the maximum frequency for skin deph is 42kHz, above what you are using, so its good.
add 2 of these wires and it will be fine. and to be honest, 1 of these wires is fine as its 7A for chassis wiring and your not having multiple laters for this transformer.

Re: Litz wire strand number best for primary winding on flybacks
Sulaiman, Thu Jun 12 2014, 05:07PM

I have small quantities of Litz wire for various purposes that I buy from eBay.
Your choice may be limited to what is cheap/available rather than a theoretical evaluation.

I have bought this from this seller, adequate for your purpose.

46 awg is an overkill, this guy in USA has a good choice of Litz, probably 44 awg is more economical.

In air you can use up to 10A per sq. mm.
in a transformer I'd use about 1 sq. mm total for your transformer.
It seems a lot of copper but;
- the cooling from inside the transformer is poor
- 1.5 A average dc would be up to 5A peak in flyback mode
- having to rewind will be a nuisance and cost more than being a little conservative initially

Re: Litz wire strand number best for primary winding on flybacks
Ash Small, Thu Jun 12 2014, 05:45PM

You can make your own Litz, but you need to plat it (plait it?), not just twist it together. and it's best to have more than the calculator says, as the wires inside don't conduct as well as the wires on the outside (if it's platted all the wires are sometimes on the outside, and sometimes on the inside of the 'bundle'.

I think I read somewhere that you need to add ~20% extra.

(You obviously need insulated wire, but enammeled insulation is fine.)

If the wire is thick enough, you only need one strand, but only the outer skin will conduct. This is ok if you have sufficient space, and large enough wire.

Re: Litz wire strand number best for primary winding on flybacks
IamSmooth, Fri Jun 13 2014, 02:55AM

Look at my induction heater tutorial where I show a picture of what I made:

I a few types: one with 26 strands and another was a 64 strand bundle to carry 50A of current. It is the current you need to consider as mentioned above. At high frequencies the current flows on the skin, resulting in heating. Use a skin effect calculator to determine the depth your current will travel. Pick a wire gauge that is close in size to the skin-effect depth. You know the max bundle current this wire will carry. Now figure out how many strands you need to get to your max current you need. As your gauge size gets smaller, the skin approaches the actual thickness of the wire.

Re: Litz wire strand number best for primary winding on flybacks
Newton Brawn, Fri Jun 13 2014, 08:46PM

How about use thin copper foil instead of litz wire ?

A thin copper foil of 0.2 to 0.6 mm thick for 42kH or a bit less satisfies the requirements of 42 kH skin effects.

If the foil is 10mm wide and 0.2mm thick the cross section will be 2mm^2 that is fine for 20amper. (following the data of 10amper /mm^2)

For continous work the transformer coil design may be based on 3amper per mm^2, so the foil 0.2 x 10mm may carrier 6 amper.

How many turns and wath size of wire are you using now ?

Some transformers use copper foil windings.

Re: Litz wire strand number best for primary winding on flybacks
Ash Small, Sat Jun 14 2014, 01:01AM

Following on from Newton's comment, if you don't want to pay for copper foil, use aluminium foil. You'll need more, due to the increased resistance, but it will be cheaper. there are several ways to insulate the turns from each other, a layer of polythene or paper will probably suffice. You'll probably find enough of both in the kitchen or wherever for free.

If need be, fold it length wise enough times to achieve the thickness required, if it's too thin. Fold the ends at 45 degrees to bring the 'ends' out at 90 degrees.

I'm considering making a coil this way myself.

Re: Litz wire strand number best for primary winding on flybacks
Newton Brawn, Sun Jun 15 2014, 03:00AM

I have used aluminum foil to wind pulse transformers.
The coil was made with 0.3mm thick aluminun foil, 12mm wide for 10 amper load.
Aluminun was bought at construction supply ( Home Depot) for roof flashing.
I also use craft paper 0.1mm, 18mm wide ( liquor store package bag) for insulation between turns ....
after tested and satisfied with the performace the coil was immersed in a alkid vernish, dryed, and re-assembled in a flyback core.
It is a battery charge.