Winding coils: how many turns = how many feet? - Page 1
Forum section: electronics / projects

author: iXod
author_rank: Frequent Contributor
timestamp: November 04, 2021, 07:45:16 pm
content: For a given spool, how can I calculate the number of turns I’ll need to wind a desired number of feet (or meters, if you prefer)?I’ve searched but come up with inductance calculators which I don’t need.Thanks.

author: TimFox
author_rank: Super Contributor
timestamp: November 04, 2021, 07:55:10 pm
content: The circumference of a circle is (pi) x (diameter). That is the length of wire for one turn, with a specified diameter (strictly, measured to the center of the wire cross-section).Are you talking about multi-layer coils or some other geometry?

author: iXod
author_rank: Frequent Contributor
timestamp: November 04, 2021, 08:12:36 pm
content: Thanks for your reply.Yeah, need to calculate turns on multi-layer winding. Spool is 0.625 inch wide, radius from 0.5 to 1.6 inches.It’s simple to calculate for fixed radius, but surely somebody will have made a calculator for multiple layers which will mean changing radius?Thanks.

author: pqass
author_rank: Super Contributor
timestamp: November 04, 2021, 08:21:28 pm
content: Try this on-line calculator:https://www.daycounter.com/Calculators/Coil-Physical-Properties-Calculator.phtmlChange the various dimensions until you get the "Total Length of Wire in Coil" desired.

author: Benta
author_rank: Super Contributor
timestamp: November 04, 2021, 08:22:35 pm
content: Quite frankly, you can do that calculation on a paper napkin in a minute.Are you trying to squeeze a couple of cents out a design to want this kind of accuracy?

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 04, 2021, 11:25:17 pm
content: It's simple, but not that trivial.You need to know the length of an helix.https://en.wikipedia.org/wiki/HelixTo sum it up... given: D the helix diameter (spool), p the pitch (the spacing between turns) and N the number of turns, the length L is:L = N*sqrt(p^2 + (pi * D)^2)You can rearrange it to get N:N = L / sqrt(p^2 + (pi * D)^2)An example:L = 10 m, p = 0.2 mm and D = 10 mm => N ~ 318.3Note that you can consider the pitch to be the diameter of the wire if the wire is wound very tightly. But you should add some tolerance for realistic results.And, this formula is for a single-layer winding, of course. If there are several layers, you can go from there, but you'll have to take the diameter of the wire into account for the helix diameter of each layer - a bit more involved, since each layer will have a slightly greater diameter.Also note that if the pitch is small compared to the diameter of the spool/coil, you can approximate this to just N = L / (pi * D), as TimFox suggested. Now the greater the number of turns, and the larger the error. Same for the number of layers and taking that into account as I suggested above - you may be able to neglect this if the wire diameter is small compared to the spool diameter and the number of layers isn't too large. But I gave you the exact formula anyway...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 04, 2021, 11:49:12 pm
content: Just take the average diameter of the spool and multiply by pi and N. Add a fudge factor as needed -- for example the pitch as mentioned above.Tim

author: Benta
author_rank: Super Contributor
timestamp: November 04, 2021, 11:52:23 pm
content: Quote from: SiliconWizard on November 04, 2021, 11:25:17 pmRight... if you're autistic.Or, you could just take the "packing" of the coil.Example:10 mm wide bobbin, 100 turns of 0.2 mm wire = two layers of wire = 0.4 mm thickness.On top of that: 1000 turns of 0.1 mm wire = 10 layers of wire = 1.0 mm thicknessOn top of that: 20 turns of 0.5 mm wire = 0.5 mm thickness.Add to that Kapton tape in between layers (if needed).Now, computing the wire lengths using 2pi x r and the average radius for each winding is no big deal. Rocket science is not needed, just the paper napkin.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 05, 2021, 01:01:30 am
content: Quote from: Benta on November 04, 2021, 11:52:23 pmAnd you're not?Was my understanding that was more the rule than the exception here...Tim

author: SiliconWizard
author_rank: Super Contributor
timestamp: November 05, 2021, 01:24:40 am
content: Apparently, people do not like answers to questions here.Giving exact answers doesn't hurt. At least you'll know when you can neglect parameters, and when you can't. If you're just considering a coil as a stack of circles, you're not doing yourself a favor. Now as I said, if parameters allow, that can be a good enough approximation. You just need to know when it is if you don't want to just be plain wrong.And it'll be useful for people wondering about it, dealing with wider wire, smaller coil diameter, larger pitch, etc.

author: amyk
author_rank: Super Contributor
timestamp: November 05, 2021, 03:23:26 am
content: One-word hint: volume.

author: sleemanj
author_rank: Super Contributor
timestamp: November 05, 2021, 04:52:33 am
content: Don't you usually care about how many turns it is, and the length is just whatever it takes, not the other way around? If you were just storing/measuring wire, then I suppose counting turns is useful, or if you want to see "how much wire will I need for X turns".Here's a crude rough approximation spreadsheet...https://docs.google.com/spreadsheets/d/17iXqOhW13DjspQEJFTOt1xya3Pm9ldq_JvZLL39r7Rc/edit?usp=sharing( File > Make A Copy, or File > Download )If you have a length and want to know how many turns, just look down the Total Length (F) to find the next highest length, and adjust the Turns cell (D) in that row until the total length is closer.If you have a number of turns and want to know how long that is, just look down the Total Turns column (G) to find the next highest turns, and adjust the Turns (D) on that row to make Total Turns match what you need then read off the Total Length.I just checked with some random 20AG wire and a spool for 2 and half layers and it was pretty close.Obviously doesn't take into account all the things that it needs to in order to be accurate, but it's at least ball-park.

author: jonpaul
author_rank: Super Contributor
timestamp: November 05, 2021, 09:52:26 am
content: Bonjour a tous, just seeing this thread now.A few comments please;1/ We check a wire spool by weight, to see how much wire is on a stock spool.The wire manufactures give the loaded and tare spool weights for any wire gauge and insulation.If you go by weight of the wire you can look up the length on a spool.2/ The wire length on a wound transformer or inductor on a "spool" (bobbin) depends on many factors:Bobbin ID, OD, interlayer insulation, packing, winding method (eg random, perfect layer, universal).The volume is calculated by use of OD-ID (radial build) and winding length but modified by the packing factor, always less than 1.00 and often 0.6..0.3!3/ Magnet wire Insulation can be single ...quad build and the wire can be "served" with an outer layer eg silk, Kynar, or Teflon.Thus the diameter is not just the bare Cu gauge of the conductor, you must also include the insulation.4/ We have used heavy gauge wires that are not round, we have used #4-# AWG 8 rectangular and square. These can be perfect lay wound (not easy!) for a greater Cu packing factor,5/ The excellent Software Magnetic Designer (now free) has estimations for common bobbins and wire types for a prefect layer.http://www.intusoft.com/mag.htmJust the ramblings of an old retired EE.Bon Chance,Jon

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 05, 2021, 01:52:03 pm
content: Len_mean_turn (Lmt) was a necessary parameter in my experience with power electronics, both for losses and for BOM.A 2 D axisymetrical FEM model of a cylindrical winding will give length & volume of conductor in each turn, and losses, taking account of current density distribution. The manufacturing tolerance and skew factor and leadout lengths should still be applied manually.That approach is also valid for square windings if corrections are made for area and circumference.In the old days, where maximum losses were in contract, and often the conductor was a custom run of extrusion, Lmt was important.After design, we would plaster the prototype winding and core with thermocouples into an old L&N pen recorder,and use "wooden mirror" meters to measure the losses.These days I still use LMT in the small hobby windings for radio and inverters.Hand winding a toriod is easier if the conductor is cut to length before starting.I usually use bifilar or quadrifilar windings laying flat on the coating, and prepare the start leadout first.

author: bson
author_rank: Unknown
timestamp: November 07, 2021, 01:42:19 am
content: https://www.engineeringtoolbox.com/circles-within-rectangle-d_1905.htmlIf you're just winding them as tight as possible, the triangular pattern is the one of interest. But if course it's more complicated than this because the winding direction will alternate between layers (L to R, then R to L, L to R and so forth) and you will cross the wire underneath once per turn.But the easiest is probably just to measure out the prerequisite number of feet of wire and wind it up.

author: HighVoltage
author_rank: Super Contributor
timestamp: November 07, 2021, 11:29:11 am
content: I have come across a neat little program to make a quick calculation for coils and inductance.The program is called: "Coil Maestro"Enclosed is the small program in a ZIP file.Download the file from the attachment and rename it from "Coil_Maestro.txt" to "Coil_Maestro.zip" and unpack.