Re: Drum Core Stray Magnetic Field

>> ...snip....

> > stick two on a vector analyser and see what they do when mounted like > you intend? > > some what related: >
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> > -Lasse

Or, a Network Analyzer and note the S21 term.

There is a widget called an H-Field PRobe used to be made by HP. Calibrated thingy, looks like a doorstop with an SMA connector on the back to connect to your SA. It's shielded, calibrated, and only measures magnetic field. Usually used for 'sniffing' fields emanating from PCB traces, but could be used here.

Reply to
RobertMacy
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CD00268020.pdf

Buy a ferrite core. Put both windings on it and make a transformer and put aside all this "local coupling" nonsense, especially if you are trying to rely on it for some aspect of circuit operation.

Same core equals guaranteed coupling. Use a single turn if all you want is a sense line.

Reply to
DecadentLinuxUserNumeroUno

Yes... grab it from the side. I would use an air core for the grabber though. No core needed on the guy you are trying to grab stray fields with.

Then, you can make it small, and turn it 90 degrees and place it right up next to the exposed winding faces.

Reply to
DecadentLinuxUserNumeroUno

Is that the near-field approximation? Far-field is 1/d^3.

I was playing with the idea of mounting a pair of surface-mount drum cores on opposite sides of a PC board, axially coupled, to make a high voltage isolated coupler. But the customer went away so I didn't develop the idea. Side-by-side should work, too.

Things like this are easier to try than to analyze.

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

Beats me. Formulas like that show up for simple cases. Although I think that was for axial rings, not sideways, so it may well be steeper.

As for the far field, come on John, you have whiteboard to scratch it out on. :-)

Axial is probably worse, especially with big copper pads between them. The end caps tend to shield, and pads don't help.

Wouldn't be too bad with rod cores, but those are big and through hole.

RF spring coils are probably quite good, side by side. Or at funny angles...

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The left hand coils are 4MHz, double tuned and critically coupled. Remember fields go through angles as they spew off the end of a solenoid, so the near right angle between the coils is correct with the (admittedly hard to see) offset axes.

The coupling in this orientation was better than end to end (coil forms too long) or side by side (not enough k, increased C).

I've taken apart FM and TV tuner boxes that have their coils arranged in stacks of turns, so they look like one continuous solenoid but made from separate segments of wire. This allows for taps, secondaries, input/output coupling, etc. with reasonable k's (> 0.1).

And frequently bent over in goofy ways and gooped up with beeswax, to adjust the tuning (without spending money on varicaps) then stabilize it mechanically.

Well, the geometry would be pretty easy to set up in FEMM, I expect. But yeah, it's mostly one of those "suck it and see" situations, whether simulated or tested. Any analytical formula will be crude at best (hence my disclaimer).

Tim

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Reply to
Tim Williams

We'd have to punch a hole in the ground plane, for sure. But no surface creepage to wory about, just 62 mils of epoxy-glass.

RF is easy: just resonate both coils. Wideband, specifically pulses, gets trickier.

Wideband, with ferrites involved, could be bench tested in ~~1% of the time it would take to set up a sim. Or maybe 0.1%

I got some drum core samples last week, for a different use. Maybe I'll try a coupled pair. Too bad that customer went away; they piled on too much crap to make a deal possible.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

That is pretty much the same I am trying out

Other idea is spiral turns, adding a custom ferrite cylinder to reduce the gap, but I do not think that will add much gain, since running in resonance would combat the gap

Cheers

Klaus

Reply to
Klaus Kragelund

I am currently laying out a PCB with a bunch of test circuits, and I'm going to include some "transformers" just for fun... spiral traces on parallel PCB layers, some shielded and some not. No ferrites.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

One of my coworkers tried it a year or so back. He tried a ferrite core around the transformer, too. FR4 was too lossy to make it worthwhile, for power anyway.

Reply to
krw

I did it once with spiral turns, two layer, got 70% efficiency (at tuned frequency)

But, I am looking another way, since the spiral turns requires 4 layer board and blind vias or some special way to get the inner spiral end out to the rest of the circuit

It's fun. Kind of strange it is not seen often (I have never seen it in a product). It way cheaper than any other method, except for isolation via FR4 capacitances

Cheers

Klaus

Reply to
Klaus Kragelund

like a wire link? Just give the inner ends the required clearances.

NT

Reply to
meow2222

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Reply to
Wim Ton

That defeats the purpose. Required creepages are from 8mm to 12mm on outer layers depending on which standard and working voltage is relevant, so the spiral would need to be very large

Cheers

Klaus

Reply to
Klaus Kragelund

On Sun, 01 Mar 2015 04:11:00 -0700, Klaus Kragelund wrote:

Doesn't require blind vias.

I used the 'spiral' PCB coil in a Medical Product. Cheapest way to make a robust 100 turn coil was to use 6 layer 32 mil PCB cut into 2 inch round discs. 100 Turns was easy with good layout and judicious crossovers, only requires a few vias, work it out. The Q was awful as expected, but the circuit did NOT rely upon Q to operate.

One PCB coil was Tx, the other Rx. Current in TX created a magnetic field that you could see in Rx and uniquely identify. Based upon the principle that magnetic fields drop off at a known rate, it was possible to determine the distance between a set of Tx and Rx discs. Plus, magnetic fields go right through the human body so...with four discs placed judiciously against the skin around the lungs, you could observe several distances. TxA to RxA, TxA to RxB, TxB to RxA, and TxB to RxB. See how by adding more discs you get a permutation of measurements and really make it possible to reconstruct lung volume [breathing] in real time? The discs being very flat fit comfortably into pockets in a flexible vest. The vest was used for monitoring a patence's health in situ. My disc system design replaced iron core 'lumps' used in a demo prototype. The PCB discs were more accurate, had lower noise floor, and were much more comfortable, easier to sleep on, than having little 3/8 to 1/2 inch lumps caused by using expensive iron core coils. The system could measure distances easily anywhere from 6 inch to 28 inch distances. How accurate? At 8 inch separateion the measured noise floor was 1 mil, yes, 1 mil rms noise MEASURED. Not bad for a 'cheap' system. Plus it operated below 10kHz, so no 'pesky' regulatory issues and even operated to spec while sitting in front of an OLD magnetic deflection monitor at a PC.

Each Tx/Rx PAIR required around 5 mA total from a 3.3 Vdc supply, so was VERY battery friendly for portable operation. Plus, I got to 'piggy-back' all the logic/control into the existing TI's MSP430, so logic was 'free' and the cost of all the extra hardware was negligible except for the $0.25 PCB coil.

I've got a couple of PCB discs around here, and may even have a sample gerber file. Do you want pics? Let me know and if your email address appears correctly as hotmail.com; I'll send you what I've got laying around here.

Robert

Reply to
RobertMacy

rote:

top/bottom

n't be so

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ld give

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educe the gap, but I do not think that will add much gain, since running in resonance would combat the gap

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

r layers depending on which standard and working voltage is relevant, so th e spiral would need to be very large

a spiral on the top and bottom layer each with an smd 0R resistor to get ou t from the center?, requires mounting components on both side, but apart from that

-Lasse

Reply to
Lasse Langwadt Christensen

On Sunday, March 1, 2015 at 5:40:21 PM UTC+1, Lasse Langwadt Christensen wr ote:

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

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Yes, that's pretty much the same as the SMD bridge (ultra larger 0R SMD). A standard SMD 0R is going to have big pads to brigde a decent number of spi ral tracks, but yes it's a more standard solution, one which the production guys should have no problems with

Cheers

Klaus

Reply to
Klaus Kragelund

Your solution sounds really ingenious. If you have time, then please send to my hotmail ;-)

Thanks

Klaus

Reply to
Klaus Kragelund

Magnetic fields are insidious, once made they're hard to get rid of. They go EVERYWHERE! I've got cheap broadband magnetic receivers that can tap a phone line. undetectably, from 3 ft away.

Again, you NEED to get a copy of FREE femm 4.2 and do some 'engineering' to replace preconceived notions that cause you to say things like the spiral has "to be very large".

For example, it is EASY to communicate between spiral coils at a separation distance equal to the diameter. At separations of 2 times diameter, you have to be more careful in the design. And, at 3 times diameter of separation, since only around 1% of the field makes it out that far, designs are doable, but just must be careful on your design.

You'd be surprised just how far magnetic fields can emanate. I've gotten Communication System Designs based upon magnetic fields to work at two miles separation using realistic power, for monitoring 'down-hole' well drilling.

Reply to
RobertMacy

will do, may take a day, or so.

Reply to
RobertMacy

I did download it a couple of months ago (due to one of your great posts)

Right now I have another project running, and the spiral experiments are do ne on my own time and have pushed off the femm tool due to the learning cur ve. But I guess I should get started, will probably pay off in the long run (the design cycle time is very short compared to ordering and testing PCBs )

That sounds interesting. The previous experiment I did, I had a 9 turn spir al, 10mm dia, seperated 1.6mm. So that does have pretty good coupling. I kn ew that allready, these guys has done a lot of experiments:

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Cheers

Klaus

Reply to
Klaus Kragelund

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