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Old 12th February 2007, 02:40 PM   #1
Tenson is offline Tenson  United Kingdom
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Default Winding inductors - tips?

Hi,

Can I ask for people to post their tips on winding inductors? I just got some 1mm wire and started winding but it is pretty tricky as I'm sure you know! I have been trying to wrap it around a little bottle but the wire always wants to spring back off.

What do you do?

Cheers,
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Old 12th February 2007, 02:51 PM   #2
Zen Mod is offline Zen Mod  Serbia
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google for winding machines,you'll see lots of pictures;
anyway-you need former with side plates;

fixed former is if you leave inductor on it......
temp former is if sides can be dismantled,and you can pull inner core out
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Old 12th February 2007, 03:20 PM   #3
KISS is offline KISS  United States
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We wind our own cylindrical tantalum heaters at work for bottles. The form is a threaded piece of aluminum with a hole to grab the wire. OD is about and inch and the guage is about 18-20 AWG. This is then placed in a similar threaded piece of boron nitride with a boron nitride egg-shell like cylinder retainer.

With us, we just use the lathe on a very slow speed to wind the circular form.

Try using a screw. You might be able to approximate a hand winding jig by getting an adjustible clutch to keep tension on the wire and use a shaft collar to hold the start piece in place. Sources for parts is www.smallparts.com. Their part number is MSCA-4. I've used this clutch for other projects.

I picture a vertical plate in a vise. Put a two pice shaft collar on it with 2 holes drilled in it. Then a short piece of 1/4 shaft. the chutch. A small piece of 1/4 sharft. A shaft coupler. A drill chuck. 1/4 hex to chuck that's easily findable in a hardware store. Then another piece of 1/4" shaft and finally a Knob mounted at the end to turn. This can be made without any special equipment, Just a saw and drill and file. or a Dremel tool.

FWIW:
I used to scrap transformers when I was a kid and I used to use a rotating drill bit as a mandril for theunwound wire. Held core with screwdriver and wound the copper on a drill. You reverse twist it and it comes off.
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Old 12th February 2007, 07:35 PM   #4
Tenson is offline Tenson  United Kingdom
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Hi,

Thanks for the suggestions. I managed to wind onto an old solder reel. But, I seem to have very high DC resistance. Around 1.3 ohms, my meter tells me! This just wont do, it completely messes up the crossover and can not be compensated for elsewhere.

Any ideas why this might be? The calculator suggested around 0.4ohms since I am aiming for around 600-700uH. I didn't manage to wind it perfectly, I got on to the 3rd layer and then all the new turns started to 'sink' in to the previous ones and it just got worse as I went on. I did my best and got the approximate number of layers and turns then did a few extra so I could shorten it after doing a measurement of the speaker to get the right value. I didn't put so much extra on that it should be over twice the DC resistance though! I am using 1mm wire.

Any ideas whats up? I'm tempted to just buy some coils from Falcon Components but I'd like to learn to make my own.

Also, sorry for a question that I know has been answered elsewhere but I did a quick search and didn't find a direct answer... how do I know when iron dust core inductors are suitable or not? I know that they are just as good as long as they don't saturate, but how do I know when they would saturate? I want this for baffle step correction so it would take a fair bit of power, being on the woofer I guess.

Really appreciate the help.

Cheers
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Old 12th February 2007, 10:53 PM   #5
KISS is offline KISS  United States
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Yep. You can't measure resistances that small using a standard ohmmeter. You either need a 4-wire ohmeter or you need to do it the hard way.

Connect a power supply in series with a resistor and to the coil you need to measure. Measure the voltage across the fixed resistor which will give you the current and the voltage across the coil.

Or you can use 2 multimeters with a series resistor and a voltmeter. One multimter is set for current and the other for voltage. In this case, you don't need to know the value of the resistor.

In esscense, you need to measure the current through the coil and the voltage across the coil. This will give you the resistance that your after. There are lead and contact resistances that nned to be removed. This is done with the Kelvin technique. R is then V/I.

The multimeter is not a zero resistance ammeter, so you need two.
If you don't have 2, you need a good precision resistor. Your voltmeter will generally use a different resistor depending on range.

I'll bet that if you measure the resistance when the probes are touching, you'll get a large value somewhere between 0.5 and 1 ohm.

You have to balance the wattage of the resistor, avalable power supply and regulation and voltmeter resolution and accuracy to get the right value. You might me able to use an LM317 in current regulator configuration. You can use your ammeter to measure the current and then use this current source to measure the coil.

I hope all of this makes sense.
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Old 12th February 2007, 11:36 PM   #6
Magura is offline Magura  Denmark
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You can see how I usually wind inductors here:

www.briangt.com/gallery/magura

Magura
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Old 13th February 2007, 12:23 AM   #7
Tenson is offline Tenson  United Kingdom
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Hi,

Kiss, I think I understand what you are saying but frankly it sounds like a pain in the **** and I would rather buy them than go to all that trouble.

Actually I take it back, my meter tells me 0.8ohms and if I press the probes together directly I get 0.4ohms. So does that mean the coil has a resistance of 0.4ohms? (sounds about right)

Either way it didn't do what I wanted of it, I guess maybe the inductance was too high and I should have soldered the terminations rather than using crock clips (which I think is why I got 1.3 ohms before). I'll give it another shot tomorrow with soldering it.
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Old 13th February 2007, 01:30 AM   #8
KISS is offline KISS  United States
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Sort of.

That 0.4 could be 0.3 or 0.5 because specs are usually +- 1 digit and similarly for your 0.8. If you said it was 0.800 and 0.400 then resistance would be 0.800-0.400 or 0.400, but as you noticed the croc. clips addded contact resistance.

Take a look here:

http://www.powerstandards.com/4terminal.htm


Here is a much better explanation:

http://www.cirris.com/testing/resistance/fourwire.html

Beware:
Sometimes zero doesn't exist and sometimes 1.5 E 18 is equivelent to 2 E 18.
You can buy something called a "0 ohm resistor" but it's not. It's just a wire in a resistor body. Paper has a resistance and I can measure it at work and it depends on moisture content. A wire in air generates a current because it's moving in the earth's magnetic field.
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Old 13th February 2007, 11:41 AM   #9
AMV8 is offline AMV8  United Kingdom
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Tension

I wind my own inductors for crossovers. I have wound both air cored and ferrite cored.

I have found it essential to keep tension tight and to pack the rows tightly against one another.

I have never had a problem with unusual resistence readings. I have measured resistance with an analogue meter or a bridge - I have never used a digital meter on a coil - not sure if that aviods the problems kiss mentions but it has worked ok for me.

I have drawn graphs of coil turns against coil inductance for air cored and ferrite cored and the results seem consistant so I believe all is well. If it helps I can give you the info from my graphs.

Don
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Old 13th February 2007, 05:04 PM   #10
Tenson is offline Tenson  United Kingdom
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I'm sure it was just the crock clips messing up the DC resistance as when I soldered it, it has worked more as intended. However, I have actually decided to change the approach to my Xover so I need a different value now anyway.

I am going to chicken out and buy some, sorry guys!

Can someone tell me though, do inductors for baffle step correction need to handle lots of power? I will have a 7 ohm resistor and a 5.3mH inductor in parallel with each other and in series with the woofer. Then another 7 ohm resistor in parallel with the woofer, you know just the usual BSC network. Since the inductor is a large value I want to go for a small wire gauge if it doesn't need to handle lots of power.
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