Magnet Wire-Making Inductors Q

[Chuck H]

I'm going to attemt building an attenuator. Inductors have become very expensive. Does it make any sense to roll my own? And...

I'm going to use the Aiken active load design for starters. Any idea how many feet of, say, 16 or 18 ga. magnet wire it would take to wind a 25mH air core inductor? I'm not looking for a formula. But an approximation would help so I know how much magnet wire to buy.

Thanks

Chuck

[cbarrow7625]

Chuck,

That will be one BIG inductor! For values like that you will more often see steel core inductors. A 25 mH air core inductor made from 16 ga.will probably end up the size of a decent-sized torroidal transformer.

The large wire gauge is good to get a low DC resistance but not at all necessary for power handling in this design. You can probably get away with 20 gauge for power handling. You don't really need a high Q either since it gets damped by the parallel resistor anyway.

You'd be better off overall winding it on a metal core (old transformer core) if you can. It will save you a lot of size and wire. At only 100 watts, I don't think you will have too much trouble with reaching saturation of the core (really the only reason to use an air core).

Sorry, i know this doesn't answer your question directly other than to say it will be a good amount of wire. What are your options for buying the wire? How big/small are the available spools? I could probably give you a ballpark estimate if it might be enough if there was a photo of the spool sizes available.

Chris

[mooreamps]

I have a question ?
Why do you want to use a wire gauge "that big" ?
What do you calculate as you max target current load ?


-g

[Chuck H]

Chris, I don't know enough about the nature of inductors to be sure if it was OK to use an iron core. So I picked an air core design because thats what is typically used in speaker xovers and attenuators. The little I do know warns against it because an iron core inductor will change impedance @ frequency X under condition X, and then it would no longer be doing what I need it to do. A simple solid core wouldn't seem too hard to saturate. But if you think it won't then you must be basing it on something. And I have nothing. So I'm inclined to believe you. A core sure would help reduce the size.

Gary, The wire gauge is not based on current handling but reducing DCR. As you know if you've ever measured a speakers DCR, the resistance is usually lower than it's rated impedance. So if I'm trying to emulate a speaker it just made sense to try and keep DCR down as much as possible. Plus, since the design I'm working on uses a resistive load to vary the levels between the speaker and the reactive dummy loads, it seemed to me that keeping DCR as low as possible would keep as much current as possible in the reacive loads instead of the resistive divider. Probably am excersize in futility since some of the more popular attenuators just use wire wound inductive resistors and seem to sound fine for it.

Thanks guys, But I guess I'm still not sure what I'm going to do.

Chuck

[Enzo]

Speaker crossovers use iron core inductors all the time.

Parts Express has a selection of both types for example, though not up to 25mHy

[loudthud]

Parts Express had a good selection of inductors intended for crossovers. They spec size, resistance, and wire size, with and without cores so you can get a good idea of what will meet your requirements. Won't tell you how much wire you need but maybe you could work backwards knowing the resistance and wire size.

[Steve Conner]

A quick google for "air core inductor formula" should make you happy.

Also, remember that you're building a power soak here. The object of the game is to waste power. So there's probably no point in building/buying a large, low-loss inductor, only to connect a resistor in series or shunted across it. The same argument goes for enhancing the Q factor with an iron core, and then throwing that Q away again.

If you want to save money, you should study your power soak design to see if there are any resistances that could be folded into the DCR of your inductors. You can replace an expensive high-Q inductor and an expensive power resistor with a single crappy inductor that runs hot

[Chuck H]

Thanks guys.

Enzo, I haven't seen cored inductors in xovers, but i haven't seen everything either. I'd much prefere to use an iron cored inductor that won't be as big as a football. 25mH is big, I know. You just series inductors and sum their values, right? Not the best way to reduce DCR, but it's not a perfect world.

Mr. thud, I have been to Parts Express. Best place I have found for this so far. Working backwards is my MO

I found this. Lots of good info if anyone is interested. I GET about a third of it.

http://info.ee.surrey.ac.uk/Workshop/advice/coils/#turn

Chuck