Re: Drum Core Stray Magnetic Field

Is ok. Here is the modeling technique, so you can do it yourself on the

330uH coil is:

First work at LOW frequency, Rdc and that Inductance (at 1kHz). Take the physical dimensions. Try to find the 'window' size of the coil. Take a Wire Gauge Table of diameters/Areas. Assume a Stacking Factor of anywhere from 0.5 to 0.7 For small coils the SF can be fairly good, even 0.7 Also, assume a slightly 'less' diameter caused by stretching the wire. The tension placed when using these automatic winders slightly thins the wire, shifting the whole manufacturing range to smaller diameter and slightly higher resistance

Given the Rdc, find the length that makes that resistance for each gauge of wire near your estimation of wire gauges. Then wrap each length around the core, any core, These open cores are so poor you can use perms anywhere from 100 to 2000 and get the SAME inductance. Adjust the coil window to accomodate. Check to see if you 'filled' the window. If your guess fills the window, makes the right inductance, and makes physical sense as it lays on the bobbin, you're done with the basics. You have N and wire gauge.

Now you'll find you get close to the inductance, but what threw me off for a while, until I realized the logistics of making a connection to the coil, was that there had to be a 'half' turn. once I applied an 'adjustment', in this case, ((N-0.5)/N)^2 times the inductance of 21 THEN I got better than 1 % accuracy to match both the Rdc and the L, and used the bobbin up completely. My guess is Bournes just takes a bobbin and fills it with varying sizes of wire, and that's what makes up the family of available parts.

The skin effect was almost enough to destroy the Q of the coil going up in frequency, but not quite. The Rwinding went from 0.15 to over 4.5 ohms. I didn't see an easy way to model the perm of the core to get 'something' at

1kHz down to AIR at SRF of 45MHz. [I tried making the core slightly conductive which almost works] but since the Q was so bad at 7.96MHz, I assumed that by 45MHz the core was completely gone, so I used the 900nH air core value to calculate the 13.4pF parallel capacitance. As I said, got the basics can work from there.

Anyway, that's the history of modeling the 4.7uH, 0.15ohm inductor. Now, you can do the same for your 330uH coil.

What really amazed me was how 'leaky' the fields were around that bobbin! To line up two cores that are only 2.5 mm tall by 3 mm diameter and separated by 2 mm of PCB and still get 0.084 coupling is pretty good. Or if NOT wanted, pretty bad. To prevent EMC problems, where that fields can so easily reach around your 'line filter'; I would NEVER use an open core. Well, unless you're willing to make your own shield, but why not just buy a closed core? I assumed NO conductive planes inside the PCB, because you wanted to have two cores communicate. You can go back and model the mounting WITH 1 oz copper layers. and see how much, at what frequency, gets blocked. You'll see the inductance drops, the Rwinding goes up and the coupling drops. You can write LUA script to do a series of analyses that will create a table of coupling values vs distance along the axis. You'll find you get a function that up close is proportional to inverse square of distance, then as you get further away its more like inverse cube of distance. Rule of thumb, at 3 diameters distance [or 3 of longest dimension] the fields are less than 1% of strongest field. Plus you can replace any complex structure with a simple magnetic dipole. In other words *if* you're far enough away, you can't tell anything about the magnetic structure, they all look alike.

Extra measurements? Try to characterize the part from low tohigh frequency Use whatever steps are easiest, but I prefer log steps, like, 1kHz, 3kHz

10kHz, etc finer or more coarse. At each data point characterize in terms of XL(f) and R(f) Later you can curve fit to smooth the data using octave, FREE Matlab clone. Then overlay plots using the FREE femm at the same frequencies. Working back and forth, get VERY close.
Reply to
RobertMacy
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Very nice explanation, thanks.

I will have to find a quiet spot, no children, and give it a try

Cheers

Klaus

Reply to
Klaus Kragelund

The first tests:

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test.xlsx These are done on the SDR0503 When the SDR0302 arrives, I will do more test Notice that the coupling gets a lot better at lower frequency. I would have thought that the results would have been good at 200kHz, since thats still way off the SRF of 7MHz

Side to side and top to top are good, bottom to bottom (mounted on each side of 1.6mm PCB) is not as good, but not way off

Cheers

Klaus

Reply to
Klaus Kragelund

Bad news! I can't get ANY model to fit the data sheet! within the constraints of physically realizable components.!! I drew the expected structure, filled the window with 38 AWG wire, wire is getting small already, got the 330uH, but got 3.8 ohm resistance! too high and there's no room for 'fatter' wire! Consdiering this component is 'early' in their product offering you got me what's going on! For higher values of inductors they have to use smaller wire. I know credit card size modems contain telco transformer that use 54 Awg wire. maybe they just go right on up.

If you get one of these parts, measure the 'lid' thickness, the heighth of the coil window, and the heighth of the bottom 'lid'. They should add to 3 mm. If you have a microscope, see if you can discern the wire gauge.

You got me, I don't know. Maybe the 'cut' into this bobbin is huge. But even with that, trade-offs stop achieving much.

FWIW, years ago during tenure in the Aerospace Industry, we tested the reliability of little transformers that used little wire. Simple temp cycling test. Ten cycles, full range temperature cycle over twice a day, test took five days. If the wire was too small the rransformers failed. Conclusion: Don't use ANY transformer with wire size smaller than 44 Awg. The expansion/contraction of the temp cycling and the physical structures having different rates of expansion, literally broke the wires.

Reply to
RobertMacy

bad link. did you put the whole line inside ?

Reply to
RobertMacy

Try cut-n-paste of the entire line into your browser. Worked that way for me.

Reply to
John S

Old excel format:

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test.xls

Cheers

Klaus

Reply to
Klaus Kragelund

Maybe it's a high perm core?

I will measure what you asked for and post back

Cheers

Klaus

Reply to
Klaus Kragelund

Klaus,

Please do it next time with angle brackets:

--

-TV
Reply to
Tauno Voipio

Or write it up in plain English. I hate spreadsheets.

--

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 

jlarkin att highlandtechnology dott com 
http://www.highlandtechnology.com
Reply to
John Larkin

Got it !! Thanks

I saw that 'brackets' made NO difference to someone's requesting you use brackets. But this should work for others:

When you get your sample, please count the turns on that top layer.

Reply to
RobertMacy

Forgot to ask for turns count on top layer. I didn't try infinite perm, perhaps I should. But I started with perm of 200 and got around 320uH then went to perm of 2000 and then the inductance only went up to 350uH. Remember from air core to perm core is only 5.4 on the SDR0403-4R7 An open core 'rod's effective permeability is based upon the length dieamter ratio NOT the perm.

Last page of Fairite catalog, circa 70's shows an excellent plot of effective perrm vs length to diameter ratios AND permeability. The curves asymptotically limit to a function of length to diameter, independent of high perm.

Anybody have a copy of that page? I can't find mine! not long ago I think someone posted it here, I got a copy, and can't find where I put it!! Just because I did not name it correctly. Or, put it in the right folder.

Reply to
RobertMacy

I searched a while and found it in the 13th edition, page 24:

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Reply to
Klaus Kragelund

Thanks for posting! ...but that's not the page, although has the information.

Reply to
RobertMacy

The back page of a 70's print catalog? You might try Snelling's 'Soft Ferrites' for a reliable reference. I believe he worked for Mullard/Philips.

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RL

Reply to
legg

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