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Litz wire strand number best for primary winding on flybacks

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Lightening Rod

Thu Jun 12 2014, 10:09AM

Registered Member #2674 Joined: Fri Feb 05 2010, 12:35AM
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Posts: 11

I have noticed that Litz wire works better than solid copper wire when making flyback primaries for plasma drivers. What is the ideal number of strands and gage wire for the primary winding on a flyback 20-100Khz frequencies?

Mads Barnkob

Thu Jun 12 2014, 02:23PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

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.

Lightening Rod

Thu Jun 12 2014, 03:09PM

Registered Member #2674 Joined: Fri Feb 05 2010, 12:35AM
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Posts: 11

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.

Thomas W

Thu Jun 12 2014, 03:42PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
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Posts: 1276

Lightening Rod wrote ...

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.

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.

Sulaiman

Thu Jun 12 2014, 05:07PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

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

Ash Small

Thu Jun 12 2014, 05:45PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

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.

IamSmooth

Fri Jun 13 2014, 02:55AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

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.

Newton Brawn

Fri Jun 13 2014, 08:46PM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

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.

Ash Small

Sat Jun 14 2014, 01:01AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

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.

Newton Brawn

Sun Jun 15 2014, 03:00AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

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.

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