Re: Drum Core Stray Magnetic Field

If you cant spare the extra size, dispense with a separate pad and solder a rigid bridge onto the end of the spiral coil.

The bridge to coil gap only sees the V across the winding, so I wouldnt expect hipot creepage distances to come into it.

You can also let solder stick to the spiral tracks to reduce ESR.

NT

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to reduce the gap, but I do not think that will add much gain, since runnin g in resonance would combat the gap

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layer board and blind vias or some special way to get the inner spiral end out to the rest of the circuit

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A standard SMD 0R is going to have big pads to brigde a decent number of s piral tracks, but yes it's a more standard solution, one which the producti on guys should have no problems with

I guess you could also make the coils on separate small pcbs and surface mo unt them like that many modules

-Lasse

I'm not sure exactly what you mean. The SMD bridge "sees" only low voltage, but still needs to keep the creepage and clearance from the secondary side (the bridge is used to avoid the via, and the problems that causes)

Yes. Generally that will only reduce the ESR by 15% (if my memory serves me well) and create problems with possible solder bridges/shorts)

On Sunday, March 1, 2015 at 9:19:17 PM UTC+1, Lasse Langwadt Christensen wr ote:

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mount them like that many modules

Yes. My thought was to mount one on a base PCB like an normal SMD inductor. Then have the PCB milled 1.2mm down from the other side, mount the other c oil on a add-on PCB, flip that over and mount this. So the coils are 0.4mm apart on a 1.6mm PCB

But, I am not sure if the board manufactor can mill a non-through hole...

Cheers

Klaus

Use a wire link but instead of mounting it thru hole, mount it sm. Either its raised or insulated to prevent shorting. With no through holes, clearance isnt an issue introduced by the wire.

I've seen preformed flat wire bridges used.

Better suited to wide tracks than fine ones, where one can mask off the outer edges of the tracks. Bridging is normally prevented by patterning the solder mask. But I guarantee nothing!

NT

manufacturer GMR Magnetic Field Sensor:

NVE Corporation (800) GMR-7141 (800) 467-7141

Only thing to keep in mind is that the BMR sensor has a 'built-in' noise floor you can't get around. For example 9k ohm sensor has the johnson noise of 9k ohms, and then you're stuck. Also, a bit tricky to make really, really linear. But DOES have the advantage of being a 'point' sensor, unlike the broader coil.

I think NVE has some good AppNotes and tutorials.

To shorten the learning curve. WHICH SMD power inductor? URL to datasheet, please.

I'll draw up a model that shows you the field around one. That should shorten the learning curve a bit, much faster to work with an existing model and poke around rather than try to start from scratch!

Also, will look at modeling your 9 turn spiral. Using femm and lua script it's easy to create table of induced voltage vs distance etc.

Will check again, but could NOT gain access to my google gmail account. Google may have upgraded their gmail to the point I can no longer even access incomding emails. arrrggg! wouldn't be so bad but there's no gain for any of the 'improvements' they've done so far.

I haven't spend much time selecting one yet, but I have a kit from bourns, so could start with the SDR0302-4R7ML:

That would simply be great. Then some good whiskey is going your way :-)

I could reverse engineer the inductor, number of turns, Al value, RDC etc. Would that be of benefit?

Hope it works out for you

Thanks

Klaus

A pair of those, on opposite sides of a board, would be interesting as a HV isolator. No vias to worry about.

An LED on the top and a photodiode on the other side might work, too. Shine the light through the FR4, through a hole in the ground plane.

Less than that. The ferrite plate shields flux in the axial direction, which is pretty weak to begin with (helical solenoids coupled end to end have k < 0.1 or so for winding length and end-to-end gap comparable to the diameter). So expect the coupling to be pretty bad.

If you could get "drum" type inductors that are mounted perpendicular to the board, it would look better, at least as long as the height between plates is greater than the PCB thickness. Dogbone or wirewound chip type parts may not be too bad, given enough length (say, 1206 and up), but they lack the width that a drum style would have (which would be like having a horizontal axis >= 1818 wirewound).

Hmm, ferrite chips placed off the ends of a "drive" inductor could act as pole pieces, improving coupling (making an "H" pattern, in the top-down view). It would be automated-assembly-friendly...maybe. The necessities of small pads and close part-to-part spacing won't go over too well, though.

Tim

On a sunny day (Mon, 2 Mar 2015 18:04:56 -0600) it happened "Tim Williams" wrote in :

'////' = coil

same on bottom but flipped vertically

Old rotating transformtr. At least the field lines would sort of link.

That's nice.

I tried to find numbers for the transparency of the prepeg, but found nothing.

Cheers

Klaus

thanks.

Hey, your on! Yes, please reverse Engineer to get N and wire Awg. You might be able to do with 'examination' WITHOUT destroying, eh?

I'll duplicate your efforts here by trying to replicatee the spec from a model.

Will do two, one with NO plane and one with 1 oz. copper footprint that goes under the inductor. The eddy currents play havoc with the field.

I forgot. What frequency are you planning on communicating with?

Apparently false alarm. back to 'normal' later in the day. WHEW!

Found the gerbers for the spiral coil, but can't find the 'reader' I use. sigh. Have a 'bad' pdf document that shows ALL the masks. Bad in that even at 300% you can barely discern what's going on. The good news is that you can definitely see VERY FEW vias. And figure out how to do such a layout yourself.

Robert

Having an old 2-side board and a mini-maglight handy, I'd guess around

I only had the SDR0503, 330uH, in stock, so will try that tomorrow and post back here:

I don't know yet, I will try different parallel capacitors for the tank and sweep the frequency

Cheers

Klaus

Klaus,

I sent you some modeling of that 4.7uH part It appears there are three layers of 7 turns each layer of 34 Awg wire total of 20.5 turns, half turn to make the connections to the base. Surprisingly the fields go EVERYWHERE! Due to the short little profile even a perm of 200 in the core yields around 5.4 effective perm.

I could NOT get the high frequency operation easily The low Q at 7.96MHz implies a drop to 2.7uH, or such so quit. But appears that when the core loses ALL its perm; you still have the AIR core left which provides around

Regarding coupling between two: One ontop a board, one on the bottom couple ataround -.084 which doesn't mean much drop between them. putting to the side willdrop, 'lifting' off the PCB will drop the signal but pretty much a lo goes through

yes, take a look at the top layer and tell me if you can measure the Awg AND count the number of turns in the layer.

Robert

Always, happens workon one, and it's NOT the one! Still much applies, hope you got my models for the 4.7uH

I coupled two inductors on opposie sides of a 62 mil PCB to find out that the 4.7uH has K of around 0.084, which is fairly good.

That sounds promishing. I had the SDR0503 in stock, but I will order the SDR0302 4.7uH, so I will measure on the same as you simulated. But, it means it will take a couple of days before I report back.

Please let me know if there are any other special measurements that would be prudent, then I will add those

Cheers

Klaus

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

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

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