# Help winding my own inductor?

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20 millihenry, 1 amp, 25 KHz. Pot core, I presume. Don't know enough about these to know if I need a core with a gap or not. Willing to learn...

I have a 20 mH inductor (off the shelf) but is getting hot with the current I'm passing through it.

Where do I start? References and advice welcome.

Thanks,

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If your 20mH inductor is getting hot, then (obviously) you are forcing more current thru it than it is rated for...and you are very likely saturating it, meaning the actual inductance may be 1uH or less. First, with a given core configuration (with or without a gap), you determine the effective pernmeability in amps per turns squared; then calculate the number of turns needed for the inductance you would like to have. The ampere turns gives you the flux density, and from the effective BH curve, determine if it is saturated. Increase the core area and/or the gap and re-calculate; repeat as needed. Then use wire of a size as follows: cross-sectional in circular mils = milliamps of current rating for the inductor. That wire size, and number of turns can be used to calculate if it will fit in the available space. Tis not symple.....

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A practical inductor that you describe will need a gap to store the energy of the inductor without magnetic saturation (either pot or EE core) or a distributed gap (powdered metal toroid) and also the wire window to carry the wire size that will handle the current for those turns. The easy to wind solution (filling a bobbin) would be a pot core or some other variant of an EE core. 20 millihenries at 1 amp would need a substantial core size.

The Fair-rite catalog has a design guide for their core types that is pretty applicable to any brand.

Getting your hands on a selection of cores and bobbins about the right size is the hard part. I have been collecting them for 20 years (most recently shopping on ebay), just to have the parts to make prototypes.

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That's a mother big inductor. Are you sure you meant 20mH

Aw.Ae = L.Irms.Ipk/Bpk.J.Kcu.Kw

L is the value of your inductor in Humphreys Irms is the rms current you want it to carry in amps Ipk is the peak current you expect it to suffer from in amps Bpk is the maximum flux you want the core material to hack in Teslas J is the operating current density of your wire. 4E6 amps/meter^2 for a nominal 30C temp rise. Kcu is the utilisation factor of your wire.... 0.7 for round copper. Kw is how well it fills the available winding area. 1 for a single winding.

Aw is the winding area Ae is the effective area of the core.

Aw.Ae = 20mH x 1 x 1/0.3 x 4E6 x 0.7 x 1 Aw.Ae = 23.8E-9

Square root it

Aw = Ae = 154mm^2

Ooooh maybe it's not so big.

Find a pot core that sort of does that. And they won't...... Bugger

Number of turns

N = LIpk/BpkAe N = 20mH x 1/0.3 x 154E-6 N = 433

Just make sure that when you wind it you lock the door and take the phone off the hook. Begin 1.... 2..... 3.......30.... 80...... 333. 'How many sugars do you want in your tea?' Arghhhhhh, bastard.......

1.....2......3.....28

Size of wire.... err

154E-6/433 = 0.35565mm^2

Square rooted.... 0.596mm diameter, enammelellmeleded heavy insulation... AWG 24 or 25 if you am an American.

Al value is L/N^2 or 106nH per root turd. Now that's just got to be an almost standard set. N27 or 3C80 or some such.

DNA

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On Fri, 19 Dec 2003 2:36:06 -0800, John Popelish wrote (in message ):

Use maybe 78 material from this catalog? It is recommended for applications

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On Fri, 19 Dec 2003 5:33:54 -0800, Genome wrote (in message ):

Yes, I'm sure. I measured it with reliable L meter, at 25 KHz.

Henries?

Don't understand what "And they won't" means... Can't use a pot core for this application?

Hmm... the one I've got now is about that wire size. Is it overheating because it's core is saturating? Maybe I can just put it in another pot core? Or is the wire size the *only* thing that determines whether or not it gets hot?

(Yes, 'Merican.)

Where would I find these "standard" pot core sets? A mail order supplier, maybe? Suggestions?

Thanks,

• posted

I read in sci.electronics.design that Genome wrote (in ) about 'Help winding my own inductor?', on Fri, 19 Dec 2003:

I hope that anyone who thought Genome was an idiot is now firmly disabused. Although he still couldn't resist misspelling 'turn'.

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On Fri, 19 Dec 2003 6:27:16 -0800, John Woodgate wrote (in message ):

Being a non-pro, I was wondering about that term :-) Thought maybe I'd slept through some important topic in electronics lecture...

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Nah, our Genome merely has a mild case of whatever James Joyce suffered from. I say a "mild" case because one can usually figure out what Genome's on about in a single reading. ;>)

Mark L. Fergerson

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Oh, when you're halfway through winding a large toroid you start thinking of them in those terms.

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Amidon

has been the standard place to buy this stuff for amatuer radio operators, and they have a reasonable selection of cores. They seem to list the fair-rite numbers in their ferrite cores (they also have iron powder cores).

Most of the posters are on track as far as I can tell, here's my summary and two bits:

- Design for the inductance you need _at_ your maximum current. The effective inductance of your coil will go down as the core saturates, take that into account.

- If you put a constant voltage on your core you can watch the current and see if it's saturating: a constant inductance will be indicated by a constant slope as the current increases. As the core starts to saturate the current will curve upward slightly. If the current _really_ curves upward before you've reached your design current you've got problems.

- If you're making a core for a switching power supply you probably don't have to worry about gapping it -- gapping the core reduces the inductance variation with variations in current by limiting the overall reluctance of the core. If you need controlled inductance for some reason, run away.

- The winding will exhibit loss due to current x voltage drop. The core itself will also show a loss due to hysteresis. The core loss is proportional to the coil voltage x frequency (actually the core flux x frequency). You need to size both winding and core, but if you use a material that's recommended for your frequency and application you'll probably get the right size core.

- One final note: if you do this again for much higher frequencies (100-500kHz), make homeade litz wire to reduce skin effect: calculate the wire cross section you need, and make it up with three to five strands of smaller wire, loosely wound. This will bring the effective AC resistance of your wire down closer to the DC resistance.

applications

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I read in sci.electronics.design that Tim Wescott wrote (in ) about 'Help winding my own inductor?', on Fri, 19 Dec 2003:

Yes. I bought a demister antenna kit from Maplin that required a small ferrite toroid to be wound with 1.6 mm wire. In two words - im possible.

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I wish I knew. If you can't find any supplier, I may be able to scrounge 1 or two core sets and bobbins from my personal stash to get you started. This is the easiest component to make yourself that can compete with a commercial product in quality, but it is very hard to find the parts in low quantity. And that is a real shame.

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The first turn probably wasn't so bad.

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Yeahhh I was laughing my ass off while reading his answer. That was a riot!!! I'll bet he's a good one to have at a party...

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True enough, but the rest of those turds were just plain nasty!!

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Using a program we have developed over the years (which will do coils and core selections, plus many other things as well). I plugged in your numbers to come up with a core size. Take a look at it at

As you can see, we could have used a little bit more information. If you would like us to change something, let us know and we will enter in the new data (and display it for you). This program will also give you all the formulas used.

The core used is "Magnetics" molypermalloy powder core (MPP core).

-- Building user friendly software for electronics.

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Not impossible What you need is a shuttle. If the hole is real small you can use a needle as a shuttle. If it is larger then you can take a wire coat hanger and cut a section off and with a riffle file, file notches in a section of it and wrap your wire around the notches in the wire (Or a metal or wood rod) that will keep your wire neat and nice while you pass it around the core. I have wound many of them

As an alternet use a pot core if applicable.

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MagInc use to sample me all the time. They are pretty good about that. But of course the stuff I hand wound often wound up in production.

"John > > Use maybe 78 material from this catalog? It is recommended for applications

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I read in sci.electronics.design that bushbadee wrote (in ) about 'Help winding my own inductor?', on Sat, 20 Dec 2003:

The 1.6 mm wire work-hardens after a few turns and then it would be easier to wind the coat hanger round the core.

If I send you the core and wire, will you wind it for me?

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