Lits wire vs thick magnet wire - Page 1
Forum section: electronics / projects

author: blueskull
author_rank: Unknown
timestamp: February 22, 2015, 12:14:16 pm
content: Hi,I'm trying to build a 13.56 MHz coil, about 1.2 uH.I need high power handling capability, and low loss.To reduce core loss, I'm planning to use an acrylic toroidal core, which theoretically has no loss, it is essentially not different than an air core.Another loss factor is copper loss. At such high frequency, which wire could be better giving the same outer diameter? Litz wire or magnet wire?PS. Magnet wire suffers from skin effect, which is commonly known. But Litz wire suffers from proximity effect, which is not quite well known.Anyone has any experience with proximity effect? Any ideas are welcomed.

author: Pjotr
author_rank: Frequent Contributor
timestamp: February 22, 2015, 12:37:42 pm
content: Depends also of the thickness of the strands. At 400 kHz 30 um strands have substantial lower AC resistance than 50 um. But above a few MHz litz wire has no advantage any more over solid wire: Use as thick solid wire as you can afford. Seems strange, but because all the current runs in a thin layer at the circumference you will have the circumference as large as possible.

author: Neilm
author_rank: Super Contributor
timestamp: February 22, 2015, 04:21:39 pm
content: I do know that high frequency induction heaters use copper pipe for coils. It also allows the designers to keep the coil cool by passing water through the pipes.

author: Pjotr
author_rank: Frequent Contributor
timestamp: February 22, 2015, 07:13:48 pm
content: Quote from: blueskull on February 22, 2015, 04:14:04 pmWhy do you need isolated strip if you have an isolated former? Thickness doesn't matter much at this frequency. A good source for thin copper foil are foil wounded coils used in loudspeaker cross over filters I found. The foil is 70um thick. If you want to isolate small strips of bare copper foil, use the standard yellow 3M polyester tape. The tape you see used in almost any SMPS transformer.

author: tautech
author_rank: Super Contributor
timestamp: February 22, 2015, 07:30:34 pm
content: Quote from: blueskull on February 22, 2015, 04:14:04 pmCopper strap is often used in arc welders, altough I have not seen either enamaled of less than 10mm wide.Normally it is paper wrapped.Because of the cost of copper lately, flat Ali is sometimes used now.Is that a option for you?

author: MrAl
author_rank: Super Contributor
timestamp: February 22, 2015, 08:28:00 pm
content: Hello there,Lets try to clear up some of the mysteries about the skin effect, first.The simplest way to look at skin effect in a round cross section wire is that at some fixed frequency the wire, which resembles a rod, only conducts in the outer shell which is closer to the surface, and much less near the very axial center of the rod. This is a simplified view but is accurate enough to use for now.This "outer shell" would look like a pipe, which is hollow in the center as all pipes are, and so the conduction is limited to the metal of this pipe which might be much thinner than the rod. Because it is much thinner, the AC resistance is higher than the DC resistance. A side effect is that the metal (usually copper) that occupies the center of the rod is mostly wasted because it doesnt conduct much.This in no way suggests that a larger diameter wire somehow has more AC resistance than a smaller diameter wire, because at a fixed frequency as we let the diameter increase the area involved in the major part of the conduction also increases, and thus lowers the AC resistance. So a larger diameter wire always has less DC and less AC resistance than a smaller diameter. So using a larger diameter wire will always increase the Q of the coil.What is wasted however is the copper bulk that occupies the central part of the wire, and also the bobbin or window space on the core. Sometimes this doesnt matter, because it is a small RF coil and even with larger wire it's still quite small. But with a core there is the window space to consider, where the larger wire takes up more of the window and so less turns can fit onto the core.Now using the simplified theory of the skin depth, if we use a wire of radius equal to the skin depth, there is less copper wasted because there is significant conduction in most parts of the cross section of the wire. This means the average current density is going to be higher on average, and since the window area is limited we usually want the maximum current density, unless of course we have room to spare. So enter in the Litz wire, which will have smaller strands of wire to make up a larger wire. If all the radii of all the strands are equal in size to the skin depth, then the wire should conduct more than a larger diameter wire, and thus result in a higher Q, and take up less room in the window area of the core.We just talked about this in depth on another site, so i have a quick example here.At 100kHz, an AWG #26 gauge wire would be considered to conduct the AC current almost the same as the DC current (although a more accurate calculation yields about a 25 percent decrease in conduction).At 100kHz, an AWG #22 gauge wire (going down 4 wire sizes) would waste more copper and take up more room, but conduct twice as well for AC as the #26 wire.The skin depth for copper wire can be approximated from this simple formula:d=1361/(6641*pi*sqrt(f))where f is in Hertz and d is in meters.A quick example for f=100kHz is d=0.21 millimeters.Since f is in the denominator and we take the square root, this means a frequency 100 times greater will have skin depth sqrt(100)=10 times less, so at 10Mhz the skin depth would be 0.021 millimeters.At 15MHz the skin depth would be about 0.017 millimeters, and twice this is 0.034mm, and that's about half the diameter of an AWG #40 gauge wire.When the Litz wire is wound it is wound in a way so as to reduce the proximity effect, so i would think the conduction would still be better for Litz wire, provided you cant use a larger diameter wire.We could estimate the various comparative Q for inductors made with different size wires.

author: MrAl
author_rank: Super Contributor
timestamp: February 22, 2015, 09:05:36 pm
content: Quote from: AcHmed99 on February 22, 2015, 08:29:40 pmHello,If you read post #13 you can estimate the copper loss.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 24, 2015, 12:36:59 am
content: 1kW power handling will require a fairly large core, but it's quite likely a #2 mix would do well. Probably a 2-3" toroid. You can find them at Amidon, or probably there's a more local supplier you're familiar with, or able to find, given your location.If minimal size is not a concern, use an adequate sized air core coil. Q over 200 should be easily obtained, in which case you need only size the coil and wire so that it will dissipate 5W easily (assuming that, by "power handling of 1kW", you mean a reactive power of 1kVA, since you have not otherwise mentioned how much voltage, current or power this exact coil needs to handle). hamwaves.com/antennas/inductance.html is an excellent calculator for straight solenoid type coils.Tim

author: MrAl
author_rank: Super Contributor
timestamp: February 25, 2015, 05:55:58 pm
content: Hi,One of the problems with using Litz wire is that it may not do that much at frequencies that are higher than a certain limit, where Litz wire with strands made of wire that are thin enough for that frequencies can not be found. If we cant find a Litz wire with wires of a small enough diameter, then we will still be using wire with a radius that is larger than the skin depth so we'll still be wasting copper and using up more window area.As i noted previously, at 15MHz the skin depth is very shallow, and even a AWG #40 gauge wire isnt really good enough. I dont know if anyone makes a Litz wire with internal wires smaller than that.Of course this assumes we are after maximum Q.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 25, 2015, 06:45:27 pm
content: I've heard of strands down around #46, maybe #48. But capacitance between strands becomes a big problem too.Tim

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 25, 2015, 09:36:15 pm
content: Wouldn't the hams and the military do this out of silver plated copper tubing?

author: MrAl
author_rank: Super Contributor
timestamp: February 25, 2015, 11:49:44 pm
content: Quote from: blueskull on February 25, 2015, 09:09:22 pmHi,All this over a 1uH coil? That's kinda funny. I guess you want it to be small?If this was a regular coil with regular wire you could spread the turns slightly to reduce capacitance.If you had some way to test this coil when it is done that would be good too.