skin effect

If I wind a trans with multiple wires in parallel I reduce skin effect by multiplying the skin surface area.

Do I still get this benefit if the individual wires are not insulated?, ie if Iwound with a single core of tri rated 24 x 0.2mm, or would the wire act as a single core with the strands not being isolated from each other.

If the wires are not insulated then they will tend to act as one wire and you will lose the benefits of the multiple strands.
That's why such (Litz) wire is varnished to provide insulation.

Ok, ta.

I didnt know litz was insulated.

I'll explain what I'm doing.
I have a ferrite ring N87 102mm dia, its part of a supply, 40khz unrectified rf into a resistive load at 150a (ok yes it has a filter to reduce rfi), for the primary I have used 15 0.91mm enamelled copper wires, and for the secondary I was hoping to be able to use some welding type earth wire which is 35mm2 multi strand.
Now I'm thinking of using some copper tape, I can get pure copper 0.6mm thick 40mm wide, 2 layers of this gives me an area of nearly 50mm2, if I insulate the 2 layers (working voltage is a few volts, peak about 50) then I'll be able to get a reasonable skin area.
The switches run at 40kc, there is quite a bit of extra before the core saturates so I could slow it down a fair bit, and I was thinking of push pull topography grounded switch, if I went forward then I'd free up core area to get more turns on and lower freq further.

Here's an explanation of Litz wire and how the wires are isolated.

Interesting to see a cooker hob using it.

Come to think of it I've seen the stuff before, the copper wasnt coated it was seperated by fibreglass, but I think it would have been for the flexibility rather than skin effect.

The Litz wire that I have used in the past for pot cores and small transformers was constructed of bundled enamelled copper wires wrapped with silk.

The way I understand skin effect it has to do with surface area of the metal. Assume you have 1 solid wire 1/8" diameter and a bundle of 10 wires that are also 1/8" diameter for the bunble. There is several times more surface area in the bundle of 10 insulated wires. A 1/2" diameter copper pipe has the same skin effect as a 1/2" diameter solid wire. I do not know if electrons will travel through the center of the hollow copper pipe? It seems to me they should unless both ends of the copper pipe are soldered shut.

You can make your own Litz wire with a roll of enamel coated copper wire. Today my project is to wind 3 ferrite choke coils with 40 turns of Litz wire using 50 strans of #26 enamel coated copper wire. It is 2" around the toroids, to get 40 turns I need a wire length of 40 x 2 = 80" long plus about 4 extra inches. Solder the tip ends together then put a small amount of twist in the 50 wires this makes wires all the the same length around the toroid other wise some of the wires will wad up near the end. I might need to varnish all 50 wires to glue them together then let them dry before winding the toroids.

Those 100 foot tall mega watt 300,000 volt power lines that run cross country are 1.5" diameter solid plastic covered with a layer of aluminum foil over the outside.

I built a power supply for a rail gun instead of using wires to connect 48 capacitors together I used metal straps 1/16" thick, 2" wide, 14" long. Caps are assembled in a square, 4 caps wide, 4 caps long, = 16 caps per square, 3 layers of caps, all layers connected with metal straps to reduce skin effect. Caps are connected in groups to keep all the connections short as possible. Discharge from the cap bank is at the phycical center of the metal straps not the tip ends.

I built a Tesla Coil using 3 layers of #24 enamel coated copper wire in parallel side by side on the secondary coil 11" diameter PVC pipe. 3 layers of wire in parallel reduces skin effect, it also reduces wire resistance. The output of the TC increased 10%. 132" sparks turned into 145" sparks while every thing else remains the same. I posted this on the Tesla Coil forum about 10 years ago.

Learning something new every day. Working in electronics 43 years never heard of Litz wire. Interesting reading.

gary350 said:

The way I understand skin effect it has to do with surface area of the metal. Assume you have 1 solid wire 1/8" diameter and a bundle of 10 wires that are also 1/8" diameter for the bunble. There is several times more surface area in the bundle of 10 insulated wires. A 1/2" diameter copper pipe has the same skin effect as a 1/2" diameter solid wire. I do not know if electrons will travel through the center of the hollow copper pipe? It seems to me they should unless both ends of the copper pipe are soldered shut.

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**broken link removed**

gary350 said:

Solder the tip ends together then put a small amount of twist in the 50 wires this makes wires all the the same length around the toroid other wise some of the wires will wad up near the end. I might need to varnish all 50 wires to glue them together then let them dry before winding the toroids.

Click to expand...

Unfortunately, this ignores one of the fundamental characteristics of Litz, and that is that the twisting is done in such a way that each conductor is on the surface of the bundle for an equal portion of the total length of the bundle. If you just take 50 wires and twist the group, then the wires in the centre will stay in the centre, and because of skin effect, the central wires won't be carrying any significant current. So, you're wasting a lot of copper.

Litz is normally twisted in small sub groups of about six wires each so that each wire in the sub group is on the surface. These sub groups are then twisted into larger and larger groups, in such a way that each conductor will be on the surface for part of the total length.

I ebayed litz and mainly got audiophoolery.
There was one seller listing 24 x 0.2, a 5 quid a meter, quite expensive.
I think I'll try the copper foil, 2 layers of 0.6mm, either that or a whole load of 0.91 enamelled wires, I have a 15kg drum of the stuff.

Tried here? https://www.hi-wire.co.uk/acatalog/litz_wire.html

I don't believe that skin effect is even present at 40khz. There are ferrite cores that snap onto large conductors. I used to use them on #4 wire.

with the attenuation of data type cable, the skin effect starts to become obvious at around 15 to 25 kHz and is reasonably fully developed by 100 kHz. BobW is correct.
True litz is expensive to make.
It is used in the manufacture of High Q inductances. In many cases, the Q is sufficient if a number of parallel insulated wires are use. I seem to remember something about the VLF radio stations that were built for radio communication with submarines. The High Q, high power transmitters were full of huge tank coils wound with thousands of plies of fine wire and properly 'litzed'.
I note many SMPS transformers are 'multi conductor' which simulate litz. I have one SMPS transformer which uses a small quantity of litz like wire. Its all to do with reducing power loss and temperature rise.
With some of the coaxial cables used by the military, the centre conductor was made of copper covered steel. This was a way of making a cable with a more or less constant attenuation up to say 1 mHz or so. RG59 is one that uses 'copperweld'. these cables were used for video transmission in Radar receivers. No equalisation was necessary.

rumpfy said:

With some of the coaxial cables used by the military, the centre conductor was made of copper covered steel. This was a way of making a cable with a more or less constant attenuation up to say 1 mHz or so. RG59 is one that uses 'copperweld'. these cables were used for video transmission in Radar receivers. No equalisation was necessary.

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You can buy RG59 (or most common cables) with several types of center conductors, 'copperweld' was used for when high tensile strength was needed (like when suspended on structures) not for the electrical properties that were generally poorer than pure copper conductors. The video I/O circuits were typically designed to compensate for frequency-dependent losses in a set type of cable at a set length.

Routing through my dozens of salvaged transfo's only a couple have litz wire.
My smps software calculates the effective skin as it calls it, and specifies it in mm2, I take it this is the effective conducting copper area, it goes down with freq on the same dia wire.

rumpfy said:

with the attenuation of data type cable, the skin effect starts to become obvious at around 15 to 25 kHz and is reasonably fully developed by 100 kHz. BobW is correct.
True litz is expensive to make.
It is used in the manufacture of High Q inductances. In many cases, the Q is sufficient if a number of parallel insulated wires are use. I seem to remember something about the VLF radio stations that were built for radio communication with submarines. The High Q, high power transmitters were full of huge tank coils wound with thousands of plies of fine wire and properly 'litzed'.
I note many SMPS transformers are 'multi conductor' which simulate litz. I have one SMPS transformer which uses a small quantity of litz like wire. Its all to do with reducing power loss and temperature rise.
With some of the coaxial cables used by the military, the centre conductor was made of copper covered steel. This was a way of making a cable with a more or less constant attenuation up to say 1 mHz or so. RG59 is one that uses 'copperweld'. these cables were used for video transmission in Radar receivers. No equalisation was necessary.

Click to expand...

Copper coated steel wire reminds me of the time my father bought 150 feet of #12 copper coated steel wire and wanted me to run it to the barn to hook up one 100 watt light bulb. The light bulb lite up dull orange and made less light than a tiny birthday candle. The salesman said, this is the newest wire technology, since electrons flow only on the surface of the wire only the outer surface needs to be copper plated. I checked the voltage with my meter, 120 vac at the house circuit box and about 60 vac at the barn. I double checked the connections at both ends and still had 60 vac at the barn. I replaced the copper plated steel wire with solid copper wire and the 100 watt light bulb worked perfect. I later wired in 3 more light bulbs and they all worked perfect. The year was 1968.

The 'RG' series of coaxial cables are described/specified in the US military specification "MIL-C-17.
However, with the spread of Television broadcasting after the 2nd world war, the cable industry and the TV antenna people adopted the RG nomenclature to describe the physical dimensions of antenna cable. The CATV industry too adopted RG nomenclature when it suited the particular cable.
The CATV industry developed many different materials, typically aluminium tapes and wires for the outer conductor. Many of these constructions were required in order to meet Surface Transfer Impedance specs. All this stuff came about with the explosion in communications and telecommunications and the need to minimise spurious noise and other extraneous emissions into the ether.
Most of the current cables for antenna applications use foamed dielectric materials and this allows a lower signal attenuation. These cables, if they are described as RG types, would likely not meet the RG requirements for withstand voltage or corona discharge.
One just has to be aware thats all.
Probably have gone a bit far from Doc's post; sorry about that; but cable is full of skin effect affects.

crutschow said:

If the wires are not insulated then they will tend to act as one wire and you will lose the benefits of the multiple strands.
That's why such (Litz) wire is varnished to provide insulation.

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What he said.