Calc. # turns to get coil resistence

It wasn't homework.

John

I wrote some software a while back that estimated the number of turns (or length of wire) providing you knew other critical bits.

The basic requirement was you needed to know what diameter wire it was, and the physical dimensions of the bobbin, including the diameter of the core of the bobbin.

The software made the assumption you could neatly roll a layer of wire to fit the width of the bobbin, calculate that, then do it again on the second layer, which was now assumed to sit on top of the first layer. Repeat layers till you get to the outer diameter, or whatever will fit on the bobbin.

Even with the assumptions, it was close enough to be usable.

I used it mainly to determine how many *turns* I could get onto a bobbin using a particular wire type. Great when I needed a certain turns ratio, but had no idea determining the amount of wire the bobbin can hold.

Unfortunately, this was back in the DOS days, I've lost it since then, and I've moved into IT so I don't need to re-write it. So can't help.

It's certainly not hard though, nothing that early basic high school maths can't sort out. Even if you have to factor in resistance.

Read the subject line, John.

It did not need to be, but then again, the coil diameter was about a metre. I wanted a coil that would provide an 8 Ohm resistive load to an audio amp in worst case of very low frequency driving [although I planned to drive it with 1mS spikes ]

I should add that I knew what approximate inductance I needed and worked out the number of turns, estimated the length of wire needed and then chose its diameter based on resistance per metre. So maybe not quite Larkins method, more a kind of inverse. But because resistance was more important than the inductance I used that as the primary measure.

Why? #28 AWG is 64.9 ohms/1000 ft. So do the arithmetic. Wind that many feet on the coilform and let it decide how many turns get wound. Make it enough longer than your calculation, to provide for a couple of flyleads.

Have Fun! Rich

Maybe Phils d*****ad is so small, he can't find it?

I think JF has in mind the "Larkin method", in which JL specifies that you ignore the number of turns.

As to # of turns, I suppose that might be useful information if you have a winding machine with a turns counter, and were winding onto a big form like the OP is using, wanted to wind 12 ohms worth of wire, and didn't want to measure the length up front. That's a helluva lot of suposin'. :-)

A client of mine produces wire. All of that information, and more, is important to them, and measured. Turns count, form dimensions, length, weight, resistance/ft, material, tensile strength, wire diameter and on and on. But AFAIK, their final product is always so many feet of specified wire, not so many ohms of it. :-)

Ed

From a wire table figure out the length of wire needed. Then figure out the volume of the wire, but instead of using the actual cross sectional area ( r^2 pi ) use D^2. This will be the volume of the wire plus the spaces between wires. Then figure what the dimensions of the coil on the bobbin with this much volume. That will let you figure the number of layers of wire. Calculate the number of turns per layer ( length of coil divided by diameter of wire ). Multiply number of turns per layer times layers.

Regard this as a starting point. Without perfect close packed winding, the real answer will be fewer turns.

Dan

Close, but you need the wire's EXTERIOR diameter (which will vary according to the insulation applied, is NOT the same for all '28 AWG' copper wire), and you need the packing fraction (the ratio of the wire to wire-plus-gap cross section areas). Then you know how much of the winding throat is filled with wire (this gets you the necessary info on how high the radius gets as you wind the bobbin fatter and fatter...).

Practically, the bobbin manufacturer tells you in a table how many turns of wire fit, for a variety of gauges. It's worth noting that wire is stretched slightly when it is wrapped around a form (maybe stretched a lot if the wire is thick and the form is small diameter), so a measured resistance before winding is a lower limit on the after-winding value.

Packing factor Ku of 0.6 (ratio of _copper_ cross-sectional area to bobbin fill cross-sectional area) is one rule of thumb for transformer/inductor design (Billings).

I was just trying to show Glen a way to attack the problem. He said he was not sure of the math formula. The general idea was to get wire length, convert this to volume , convert that to turns on the bobbin.

To me it is pretty obvious that wire diameter meant actual wire diameter, but maybe I should have emphasized that point. I did say to regard this as a starting point assuming perfect winding. He did not say how accurate an answer he needed , how he was planning on doing the winding , or even why he wanted to know the number of turns.

Dan

Well, if he wants an equation he can start by calculating the overall height of one circle sitting between two of the same diameter in a triangular formation.

Bwahahahahahaha!

Fields, You sure have Larkin gasping and grasping!

What a maroon he is! ...Jim Thompson

surely that would only work where successive layers were all wound the same (left to right or right to left) way, and that doesn't happen.

Lets not get all melodramatic and teary-eyed! :)

Jamie