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Old 6th December 2002, 06:24 PM   #1
Jeff R is offline Jeff R  United States
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Default Different way to wind inductors?

Like, I am sure, a lot of you, winding one's own inductor has a lot of appeal. With the cost of an audio grade inductor being quite high (more than the speaker in many cases), and with an inductor being nothing more (in concept, at least) than some wire rolled on a spool, it would seem to be an easy thing for the DIY'er to do.

However, it seems few of us are winding our own inductors. I think there are two primary reasons for this: many of us don't have accurate inductance meters and, well, it is plain painful to wind inductors. Let's look at these issues in detail.

1. The cost you could save from winding your own inductors may very well more than pay for a decent inductance meter (or, alternately, a test setup from which to accurately measure inductance). Even if you have a meter or bridge, however, it is still difficult to determine the exact inductance as you are wiring. To measure as you go, you would have to pierce the wire's insulation - not a good thing. If you cut the wire too soon and too short, you are hosed -- splices in an inductor can't be good! I would hate to wind a 1 mH inductor with 10 AWG wire and find I ended up at .9 mH! On the other hand, I would hate to further tear up my hands and wind on a couple of extra layers of wire and find out I have a 2 mH inductor.


2. While winding a piece of wire on a form is simple in theory, it is very hard on the hands. Large diameter wire does not like to be tighly wrapped around a small core, and it is difficult to keep the windings evenly spaced and layered. While it can be done by hand, it is slow and painful. Some use a slow-turning lathe or a hand crank to help, but you still need to use your hands to keep tension and to route the wire. No wonder people buy inductors!

But, there may be another, much easier way. As any of us who have done some electrical wiring about the house knows, most every hardware store sells electrical wiring in boxes of 25 feet, maybe less. This wiring is spooled up and is one nice, big inductor! I don't have an L meter handy and have never tried to measure the inductrance of a box of house wiring, but I am sure a 25 foot lengh of wiring produces a lot more inductance than most any crossover network needs - and it only costs a few dollars.

A roll of 14-2 house wire has three 14 AWG wires. If we connect the ends together in parallel, this is equal to about a 9 AWG wire, I believe. So, why not take a spool of this this common house wiring, measure the inductance, then start clipping it back to get to the desired inductance?

The size of the spool is on the order of maybe 4" inside diamter to 8" outside diameter, but I don't think that using large diameter air core inductors is bad compared to smaller diameter, tightly wound inductors and, in fact, might be better as there would be fewer layers.

If anyone tries this, you might want to rough cut the wire to about 10% over value, then use wire ties or tape to start tightening up the coil, as it will be pretty loosely bound in the box (meaning the inductance could shift). As you continue to trim, you would continue to tighten up the coil so that when you are done trimming, you have a tight coil or wire that you can firmly secure to prevent the coil from later expanding.

Any reasons why this would not work?
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Old 6th December 2002, 06:40 PM   #2
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When I need a big inductor, I just use a roll of MWS
magnet wire, whatever gauge you like.
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Old 6th December 2002, 08:00 PM   #3
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Default Romex Inductors

Wow. This make it possible to build that sub-subwoofer xover that I've been dreaming of!
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Old 6th December 2002, 09:18 PM   #4
tiroth is offline tiroth  United States
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The wire in a box is a good idea, but 25' is not very much. Using this

http://www.oz.net/~coilgun/mark2/inductorsim.htm

I couldn't get more than 0.2mH out of any configuration of 9 meters of 14 gauge wire. Now, I didn't try very hard, but I doubt this will be a suitable technique for larger valued inductors. I like the idea of using prewound magnet wire bobbins better.
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Old 6th December 2002, 09:41 PM   #5
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Default Oops.

Mea Culpa. That's what I get for making a joke without doing the math!
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Old 7th December 2002, 08:56 AM   #6
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Default better to waste 3m than 15m

I had the same dilemma when winding some coils the other night..... when to stop..... it is difficult to keep accurate count, so when I reach approx the right no of layers, I wind on an extra layer and a half or so...., then gingerly cut the wire (with teeth gripped)....

doing this my first initial inductance was 0.47 (wanted 0.33), the next night I was more daring and got to 0.37 initially..... better to waste a few metres than 15metres, plus the bits left over will be useful for something else - like nice thick hardwiring pieces etc etc, surely if you really need to the leftovers could be joined with solder/heatshrink and wound up anyway......

it certainly does work out cheap thats for sure, I made 4 0.33mH coils from 2mm wire for A$26..... web prices for same using 1.2mm wire were A$20 each..... nice saving for me.... work well too.
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Old 7th December 2002, 10:22 AM   #7
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I once brought swg18 (largest size avaiable) from Dick Smith Electonics, 100 grams in New Zealand. Without unrolling it, it measured exactely 2mH on the bobbin it was sold on, which was perfect for what I wanted.

Adrian.
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Old 7th December 2002, 02:07 PM   #8
Jeff R is offline Jeff R  United States
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Well, it looks like my house wiring idea won't fly. I forgot that as wire guage goes down and coil diameter goes up, the inductance goes down rapidly.

Nelson's idea, vouched for by kiwi, does seem like a great idea. Thanks. Mag wire is relatively cheap, I think.

I simply hate the idea of spending $$ on a high end audio crossover inductor that is little more than a spool of magnet wire.

Jeff R
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Old 7th December 2002, 05:35 PM   #9
grataku is offline grataku  United States
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I think we are oversimplifying the problem a little too much,
inductors are usually wound and then dipped in varnish or fastened together in some fashion, inductor will mechanically resonate just like caps if you ever used a large size cap to AC couple an amp you'll know what I am talking about.
You can build a variac with a plastic bucket full with salty water and a couple of cu plates, but do you really want to do that??
If you are just putzing around and experimenting that's fine, if you on the other hand want to build a semi-permanent setup you don't have to spend a fortune on audio grade inductors but you should try and do things a little more appropriately. By the way, I am pretty sure inductors can be calculated before hand.
Look up the formulas in some EE book.
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Old 8th December 2002, 07:06 AM   #10
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Default look these up

http://www.oz.net/~coilgun/mark2/inductorsim.htm

http://www.colomar.com/Shavano/inductor_info.html

quite accurate from what I have rolled up.... I use some 50mm PVC pipe as a core, with two scraps of mdf with 50mm holes in as sides, makes a form adjustable for height, just slide the sides along (I fix them with screws) and wind away......

I'm not sure the varnish would make a great difference, as long as the coil is tightly held in place by something, I use cable ties, about 10 per coil. Would make tweaking at a later stage less of a nightmare......

I picked up magnet wire in Brisbane (Australia) for A$13.20 a kg last week (2mm wire)
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