# How many turns On the Coil

1. Nov 12, 2012

### sssuneth

Hello there

Its a air core coil I am not too sure how to calculate the
number of turns I have to wind.

Specs are below

Height : 4mm
Outside D: 9.5mm
Inside D :7mm
Coil wire thickness : 40 AWG / 0.08mm
Coil resistance : 55 Ohms
Coil weight : 0.7g

Important figures highlighted. I have made a detachable bobbin.U sing self bonding wire.
coil is air -core.

How can I calculate the number of turns that i would require to make this coil

Thank you
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2. Nov 12, 2012

### trini

Turns = turns per layer * number of layers

turns per layer = height / wire thickness = 4 mm/0.08 mm = 50

number of layers = (OD-ID)/(2*wire thickness) = (9.5-7)/(2*0.08 ) = 31.25 = 31 layers

therefore, Turns = 31 * 50 = 1550

3. Nov 12, 2012

### trini

oh btw that's assuming rectangular packing, if its triangular packing then divide the result by 0.78

4. Nov 13, 2012

### Carl Pugh

Having been in the transformer business, I'm very pessimistic.
Phelphs dodge gives for #40 Bondeze wire a nominal diameter of 0.0037 inches.
This works out to a diameter of 0.0939 mm.

When hand winding you usually get maybe 60% fill.

#40 wire is THIN, so is extremely difficult to work with.

Good Luck

5. Nov 13, 2012

### trini

Carl is right, AWG 40 breaks incredibly easily. 60% is also a more practical number, so multiply the result I originally gave you by 0.6 (or if using triangular packing, which I suspect is what it would come down to, divide by 0.78 then multiply by 0.6)

6. Nov 14, 2012

### Enthalpy

You need 15.4m of wire to achieve the resistance. 0.08mm needs care.

With a setup that rotates the coil former you can still make a perfectly compact coil of 0.08mm by hand but only with training and good hands.

Then each layer packs nearly 50 turns, I estimate 47 (again under good conditions...) so the first layer packs 1.0m and the latest 1.4m with 1.2m as a mean, so you need 13 layers.

I could never pack properly successive layers in the same direction (wire returns makes a mess), nor did I see a coil made this way, so each layer takes >0.08mm and the 13 perfect layers fit well in OD=9.5 ID=4

BUT

you'd go crazy before finishing the 13 layers. With said training and good hands you achieve 4 layers or 50 turns with this tiny wire, then you throw all through the window.

Next solution: pack the wire disordered, taking advantage of the available 50% filling factor. For 650 turns of tiny diameter you DO need to rotate the coil former or you'll break the wire. At 50% filling you can't make much of a mess, the winding must already look good.