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Old 11th July 2007, 09:18 PM   #1
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Default Inductor Reductor

Hi all,

I did a search but only came up with inductor calculators, when you're starting from scratch. What I want to do is take an existing inductor and reduce its value by removing coils.

I'm pretty sure I've seen posts about this here before but I couldn't find any. Is there a calculator or equation to figure this out with?

Thanks much.
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Old 11th July 2007, 09:54 PM   #2
Svante is offline Svante  Sweden
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I think there are two approaches to this:

The first is to realize that the inductance is roughly proportional to the number of turns squared. So if you have an inductor of 2 mH and 100 turns, and you want to reduce it to 1.8 mH, you should remove turns so you have 100* sqrt(1.8/2) = 95 turns left.

The other approach is to get an inductance meter and just unwind turns until you reach the desired value.
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Old 11th July 2007, 10:04 PM   #3
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I have read suggestions that you use the from scratch calculator, with your existing value and again with your target value. Subtract the two and you have how many winds to alter your existing unit.

I can't vouch for the veracity of this method, though it was suggested to be fairly accurate.
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Old 11th July 2007, 10:25 PM   #4
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Quote:
Originally posted by AdamThorne
I have read suggestions that you use the from scratch calculator, with your existing value and again with your target value. Subtract the two and you have how many winds to alter your existing unit.

I can't vouch for the veracity of this method, though it was suggested to be fairly accurate.
Makes sense - hadn't thought of that. However, when I try to use the Lalena designer (http://www.lalena.com/audio/calculator/inductor/) it asks for coil diameter and coil length; I'm not sure exactly what they mean here, especially for an existing inductor. Would I put in the inner diameter (air core diameter) - that would remain fixed while the outer diameter would vary. As to coil length I'm lost on that one.

Svante thanks also - however I'm doing this for economics so buying a new instrument wouldn't work for me - it would probably make more sense to buy fresh coils. Also, the other method with the equation needs the number of turns in the existing inductor, and I'm not about to try to count those!
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Old 12th July 2007, 12:06 AM   #5
OzMikeH is offline OzMikeH  Australia
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I got an inductance/capacitance/temperature/multimeter for equivalent of $30 US. It's always handy to have a spare multimeter around. It came with batteries, carry case and probes, even the temperature one. It makes you wonder what the poor little old lady in China assembling these gets paid.

I measured the inductors first to check it's accuracy.
It's a good idea to measure and match your capacitors anyway. tolerance is often quite bad on caps.
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Old 12th July 2007, 07:21 PM   #6
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So.... OzMikeH would it kill you to part with the Brand and Model Number of the Digital MultiMeter? Maybe even a source or two?

$30 seems extremely cheap. I've been looking for a new meter, and for $30, I would have been happy to get one that simply measured AC milli-volts.

As far as taking windings off and testing it, remember that the wire is coated with an insulating lacquer. To test it you have to chip through the lacquer and if you test several times, there is a chance that one section of exposed wire will touch another section of exposed wire.

To count the existing number of turns, you simply count Rows and Columns. That is, you count how many coils deep (inner core to outside) then multiply that by how many coils high it is.

You could also try this coil calculator. It has diagrams that explain the terms. The one thing it doesn't allow for, unless I misunderstand what I see, is wire gauge. I'm assuming, based on the diagram, that when it says Diameter, it means the coil not the wire. Still it may be of some help.

http://www.66pacific.com/calculators/coil_calc.aspx

Be sure to select 'Multilayer Multirow Coil' in the second section, that will reset the parameters in the first section.

100 turns, 1 inch in overall diameter, .5 inch length (thickness), and .5 inch depth where depth equals overall diameter minus the core, was 160 microhenries.

Don't know if that helps, but there it is.

Steve/BlueWizard
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Old 12th July 2007, 07:45 PM   #7
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Quote:
Originally posted by BlueWizard
As far as taking windings off and testing it, remember that the wire is coated with an insulating lacquer. To test it you have to chip through the lacquer and if you test several times, there is a chance that one section of exposed wire will touch another section of exposed wire.
I think you'll find that as you unwind the coil, the portion of wire no longer on the coil, adds little or no inductance. Therefore you need not chip through any enamel, you just measure it from the original, now unwound end.
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Old 12th July 2007, 08:23 PM   #8
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Thanks Steve,

I'll take a shot at this one - I hadn't considered your easy way to count the # of coils but now of course it seems obvious. So Svante thanks for your original advice about counting the turns!

This doesn't need to be a precision job - I just want to "throw" some coils on the $10 Madisound Peerless 4 ohm sale woofers I'm using in my car - they are very smooth and extended and a crossover for them really won't be necessary, but I want to start sloping them a little at around 6k with about 0.1 mH coils - I've got 0.24 Solens now that I have no other immediate use for, so I'll be trimming them down quite a bit I think.

Looking at the diagram on the site, it appears to be a cross section, the blue box being the core of the coil. So coil "length" would actually be the top-to-bottom dimension, if I'm interpreting this correctly.

I just ran a couple of trial calculations and it appears I'm entering things properly, so over the weekend I'll sit down with the starting dimensions of my inductors and work my way backwards until I'm in the ballpark.

It just seems to me that a more useful calculator would just let you enter your target inductance and resistance, core diameter and gauge of wire, then spit out the length of wire you'd need and the number of required turns.

Somebody work on that will you?
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Old 12th July 2007, 08:47 PM   #9
Pano is offline Pano  United States
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Quote:
Originally posted by sdclc126
when I try to use the Lalena designer (http://www.lalena.com/audio/calculator/inductor/) it asks for coil diameter and coil length; I'm not sure exactly what they mean here
The diameter is the inside core - the hole. The length is what I would call "hight". It's how tall the coil is when you put the flat side on the table.

So measure the inside diameter and the hight of the coil and stick those into the calculator. You should come up with a very close idea of the inductor you already have. From there you can recalculate your new value.
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Old 12th July 2007, 09:06 PM   #10
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stumbled across three great sources-

Our good Friend at Elliot Sound Products give you the calculations and an explanation, and two links to on-line caculators specifically for Audio people

Elliot Sound Products-
http://sound.westhost.com/lr-passive.htm

Very informative... scroll or jump down to section 8 - Winding the Coils.

Here you will find these two links-

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

At this site, you simply enter you desired coil in milli-Henries, and it gives you a chart for various wire gauges and winding to achieve your goal.


http://www.coilgun.info/mark2/inductorsim.htm

This one is a little odd, it seems to allow you to adjust the core size and shape by moving sliders and lets you pick the wire gauge, but I see no way to pick the Inductance ...very odd. Though, in it's calculations it does give you the resulting inductance, so you can move the sliders around until you see the inductance you want. Still odd.

Hope that helps.

Steve/BlueWizard
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