Voice Coil Calculations Seem Not To Agree With Specs. Help!

[kelticwizard]

Having some problems figuring out which gauge wire went into winding this voice coil. I cannot seem to get the dc resistance and the other calculations to agree.

Below are listed the specs for the discontinued Peerless 831732 woofer.

The Bl is listed as 7.2 N/A, which is the same as 7.2 Tm, or Tesla-meters.

The magnetic flux density B is listed as 1.1 Tesla, and we divide Bl by B to get wire length, so that means that in the magnetic gap there is 6.45 meters, or 258 inches, of magnet wire.

I hope my conversion to inches, etc does not throw off anyone, but I find it almost impossible to do complex figuring in metric, so please bear with me. Or perhaps it would be better to do your own calcs in metric and tell me where I went wrong-feel free to do so in metric if you choose.

The magnetic gap is 6 mm, which is app 0.24 inches. The length of the voice coil is 17 mm, which is 0.67 inches. However, this is a 2 layer voice coil, so for wire winding purposes we can call the magnetic gap 0.48 inches and the length of the entire winding 1.3 inches. The diameter of the voice coil is 33 mm, which is approximately 1.3 inches, so the circumference of the voice coil will be 4.1 inches.

We divide the length of wire in the gap, (258 in), by 4.1 inches voice coil circumference and find there will be 63 turns in the 0.48 in gap.

To find the width of the magnet wire we need, we divide 0.48 in of the gap by 63 turns of wire and we arrive at a magnet wire width of 0.0076 inches. That corresponds to a polyurethane insulated, single covered magnet wire of 33 AWG.

So far, so good, but here is where the trouble starts. 33 AWG wire has a resistance of 0.206 ohms/ft, which equals 0.017 ohms/inch. That figure multiplied by 258 inches yields a resistance of 4.43 ohms for the wire in the gap. However, since the gap is only 0.48 in. and the total winding is 1.3 inches, the resistance for the whole voice coil is:

4.43 ohms * (1.3in / 0.48in) = 12.0 ohms.

And the DC resistance is listed as 6.0 ohms.

Where did I go wrong?

I tried similar calaculations for another discontinued Peerless product and the ohmage I got was also too high.

Any help would be appreciated. Thank you.

[pinkmouse]

First thought. Is it actually round wire? If it is flat/rectangular, you will have a bigger cross sectional area, therefore more copper and less resistance.

[Vikash]

I've just confirmed that your calcs and conversions are at least correct in a spreadsheet.

Look at it like this: 1/3 of the VC winding is in the gap which makes up L (VC length / Gap height). So if DCR is 6 ohms then DCR of L is 2ohms. Thus 6.55m of wire is 2ohms which is 0.3ohms per meter, or 92ohms per 1000ft - which translates to somewhere between 29-30AWG.

PS All this imperial malarky stinks. Peerless uses metric

[AndrewT]

Hi,
have a look at this MSexcel spreadsheet.
Change the required Re from 5r7 to 6r0.
If you change one of the diameters to 0.225mm you will see a 2layer coil comes out @ 17mm long.