Winding a multi-tap inductor

[ren_zokuken01]

So I need reference inductances (to be tried on different circuits). So I decided I'd make a multi-tap inductor. Now, it's gonna have different layers of winding... But, I'm not sure how to pick the proper cross area thickness of the magnetic core. I need to make sure it creates a large enough magnetic field to reach all layers of winding.

These are the rough parameters for the multi-tap inductor:

Magnetic core:
Material: Electrical steel, so μ = 0.005 (But can be swapped with anything else. For now it's for storing magnetic flux from a pulsed DC)
Core dimensions (for now): 40mm L; 5mm D

Winding:
12 layers of winding, the longest having ~90 turns (So this sticks out in comparison to the other layers. It's just easier than tapping onto the middle of a winding)

Minimum current that all winding will be covered by the field:
1 mA (I know, it's very small...)

AWG of wire:
22

So, how do I determine the right dimension of the magnetic core? For now it's 40mm long and 5mm in diameter, which I know, is too long to properly concentrate the magnetic field, but it's for other considerations for now.

This is a real question.

 

[chemelec]

What Frequency are you operating at?

What is This........Electrical Steel?
If Low Frequency, it should be Silicon Steel as used in Iron Core Transformers.
If High Frequency it should be Ferrite Material.

Minimum Current, 1 mA.
More Important, What is Maximum Current?

22 AWG is typically good for 900 mA.
40 AWG is typically good for 14 mA.

For What Inductance Values?

NEED MORE INFO From You!

 

[dr pepper]

As chem says the freq is important, silicon steel presuming thats what your using has a freq limit, and you mention pulses so freq could be high.

Whenever I design or choose an inductor out the junkbox saturation current is an important consideration, and the cross sectional area is one of the most important fators affecting saturation, the higher the csa the lower the core flux density is, and the greater the current the inductor can handle (as a rule).
You can use the universal transformer equation to calculate flux density within the core, silicon steel from memory can withstand 1 Tesla, ferrites aroun 0.1 to 0.3 Tesla.

The closeness of the windings to the core affects coupling, leakage inductance and other parasitics, this can get complicated, keep the windings as close as you can.

The guage of the wire affects the current carrying capacity of the inductor, there are diffrent rules of thumb, on ebeing current density, if you keep density less than 4a/mm2 you should be ok at freqs below 300kc.

 

[ren_zokuken01]

What I'm putting together is just an variable Inductor, basically. Just a tool, as I think now and in the future I would be playing around with inductors. So having all the working range as wide as possible and making it as general purpose as I can (hence having the intricate tapping and swappable and screwable core) is a must. Though I realize that parameters for the range I want this to work, do not work well with each other.

Oh, a maximum rating of 3A should be good enough for me.

 

[ren_zokuken01]

Dr pepper, there are still concepts that I have to read about. It takes me at least an hour to digest an average length Wikipedia page, so I haven't gotten to it. But, I think "flux density" is the property that I was referring to.

 

[dr pepper]

OK then.

If your putting togther a reference device then go for ferrite, you can buy tranformer kits made by ferroxcube, just becuase they are transformer kits doesnt mean to say you cant make inductors with them.
The ETD range is one of the most usefull for me, there are guidelines as to what size core you need depending on the power level in the datasheets.
What inductance taps do you want, and at what frequency and current, I might be able to give you a couple of pointers.

If your working with inductors, esp if your new to them then build yourself an inductor test bench, you can make one from a standard cmos chip and a fet, inductance, saturation current and leakage inductance for transformers can be visualised on a 'scope, not necessarily brilliantly accurate, but an image says a lot of words.

 

[ren_zokuken01]

If your working with inductors, esp if your new to them then build yourself an inductor test bench, you can make one from a standard cmos chip and a fet, inductance, saturation current and leakage inductance for transformers can be visualised on a 'scope, not necessarily brilliantly accurate, but an image says a lot of words.

Sounds like a lot of work for just constructing a tool for my workbench meant to build/test more tools. Also, this is a little bit more intricate as I decided that I have the capability to take the different taps and make them series or secondary wiring, etc. So I have to be careful that the parameters hit the sweet spots.

 

[chemelec]

What Range of Inductance or you attempting to get?
In the "uh" or "mh" Range?

Up to 3 Amps, and up to What Frequency?

 

[tomizett]

I think you might be mistaken here:

...
Minimum current that all winding will be covered by the field:
1 mA (I know, it's very small...)
....

Although the field gets weaker as you move out from the core, I don't think it has an "edge" which can be considderd to include or exclude some of the turns - otherwise you'd see the inductance dropping off at very low currents.

You can run into trouble with high frequency transfomers, though, if you make your winding layer thicker than the "skin thickness" - effectively meaning that some of your winding is shielded by its neighbouring layers and does not see the magnetic field.

Hope I'm right here - I should add that my understanding of magnetics is still pretty patchy.

Dr Pepper, do you have a link to any plans or discussion of the test setup you mention? Such a thing would be useful, and a welcome addition to the list of testgear that I'll never get around to building.

[edited for typos]

 

[dr pepper]

Hi Tommi,

Yes here it is, I rattle on a bit about saturation as just about every budding smps designer gets 'got' by it, its half way down the page, which is a good read:

https://www.dos4ever.com/flyback/flyback.html

 

[tomizett]

Thanks - and yes, I've been "got" by saturation, too.