Magnetic field of a solenoid

Hello Klaus,

I'd suggest pot core halves, assuming the available diameter in that area is 10mm or more. 2mm isn't much of a gap, should not be a big deal to bridge.

The classical way to transfer energy over a gap is a series resonant converter. This basically "notches out" most of the leakage inductance and results in a good net energy yield on the other side. How much energy to do have to get across?

Like with all things there is a caveat: The regulatory guys. With a series resonant converter you usually cannot stay within an ISM band like 13.56MHz because it's only a few ten kHz wide. So it'll have to be a rather low frequency.

Calculations are difficult. Could be done with lots of data from the core mfg but it's better to test it in the lab and provide a really healthy net energy margin.

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Regards, Joerg

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How about 60 watts net power transfer at 7 meters separation with 40% efficiency?

It's been done to a fare-thee-well over a couple of meters separation,

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I actually already did a test with two pottet cores. See pictures:

This one is wound:

(bad mobile phone picture)

That can transfer plenty of power, but it will be costly (the turns are not PCB traces, but a cobber wire)

I was looking for perhaps using a standard part like this:

At least for the primary side. The stray field would frindge around the top, but I can get the manufacturer to make it more like a straight rod to get an even field. Then the secondary side could be either a coil like that or an air core inductor (it has to be 90 degrees angled to the PCB to that easier)

Actually I would like to be able to calculate the field, since I may be using it in another project where the end goal is to transfer only a signal with very low frequency. So perhaps a Hall sensor could do in that app.

But close to the core it is the near field that decides the magnitude. So its difficult to calculate

Regards

Klaus

That sound almost Star-trek like.

I just need 100mW

Regards

Klaus

The MIT folks used about 400W from a 10 MHz vacuum tube oscillator to perform this feat. For your application, two coils on silicon steel or ferrite "C" cores should work as a simple transformer at line frequency (albeit with relatively high leakage inductance from the air gap). Dual resonant circuits are not necessary but could be used, at line or higher frequencies, to improve overall performance.

Bert

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I've done this with parallel ferrite rods, with a tuned coil on each. If the rods are several times longer than your 2 mm gap, and close to the barrier, coupling should be pretty good.

A pair of C-cores would be even better, but bulkier.

How much power are you trying to couple?

John

Hello Klaus,

If the barrier is only 2mm, the best solution would be probably a magnetic circuit made of the two half cores.

How to calculate: We assume that mu of the core material times gap width is much higher then the length of the magnetic path. The magnetic resistance of a piece of material is length/(mu * cross section). The coupling between the cores is the fraction of the primary flux which gets into the secondary. Part of the primary flux goes through the air missing the secondary, the other part crosses the gap to the secondary. The ratio of those parts depends on geometry, and it is approximately 2*gap width/distance between the poles (if distance is much higher then the gap). Thus the coupling is ~ (1 -

2*gap/distance).

I can derive more accurate estimates if mu and the geometry is known.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

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Hi Vladimir

That great info :-)

Do you by any chance have a reference for where you got this info (or was it derived from general textbook material?)

Thanks

Klaus

Take apart an electric toothbrush charger?

Don't overlook thinking about the plastic as part of a parallel plate capactor if fringing fields are an issue. You'd be doing electric field stuff rather than magnetic.

The electric toothbrush chargers I have seen rely on one part being

*inside* the other.

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the

(if

Do you by any chance have a reference for where you got this info (or was it derived from general textbook material?)

The main idea is from the general EE textbook. There is an artificial trick to calculate the static magnetic fields: consider the magnetic path as an electric circuit, where:

mu = conductivity B = voltage Flux = current

And apply Ohm and Kirchoff laws to it just like you do with electric circuits. Of course, you have to account for the "magnetic currents" in the air and the complex geometry of the "magnetic conductors". However it is pretty simple to derive the estimates which are good enough for the practical purposes.

Vladimir Vassilevsky DSP and Mixed Signal Consultant

What's the cost of the milling for the circuit boards and potting or riveting in the core halves over there? Since you only want to transfer

100mW you may be able to use this solution. I had a case with a core of about 10mm and three turns. The coupling was quite good but only in the tens of MHz, of course.

What is the voltage you need on the secondary side?

The winding and potting into the core halves would have to be contracted out. If that is not an option I think it would have to be 13.56MHz or

27.12MHz ISM in this case, with just trace inductors. At 27.12MHz you may even be able to get away without a core but with more (free...) turns on the board. However, in that case the regulatory folks would have to check whether such ISM band usage is legal in all the countries where the product is going to be marketed.

ISM requires a narrow tolerance clock source, either crystal driven or resonator driven. Personally I prefer resonators for anything that might get banged around a bit.

Should be in line and same orientation. But this would be a custom part.

Precise calculations would require a software like EESOF (Agilent) but my experience is that simulations are of limited value with air-coupled inductors. Too many variables. It may be better to measure and slap on a huge margin. For calculating a single coil scripts like this help:

If all this has to be small, cheap and transfer 100mW or more I'd look at higher frequencies. In the range below 100kHz things become large. For example, the cores for charging electric toothbrushes and stuff like that are almost a cubic inch in volume. Many of those operate around

50-70kHz.

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Regards, Joerg

http://www.analogconsultants.com

Not necessarily. On the Philips Sonicare they a butting up against each other, with plastic in between. The two ridges in the base are only there to prevent the toothbrush from falling over.

--
Regards, Joerg

http://www.analogconsultants.com

Does that run at line frequency, or is there electronics?

John

I held the analyzer to it and AFAIR it was running around 60kHz. At line frequency you wouldn't be able to achieve enough coupling.

Those toothbrushes work great, BTW.

--
Regards, Joerg

http://www.analogconsultants.com

1) Curve both solenoids into half-circles, then you have an air-core toroid with the plastic sheet separating the two halves. Better yet, take a toroid and a glass-cutter and score it and break it into two halves and use those as the cores. (ferrites will break like glass, use glass "cutting" techniques) 2) Don't cross-post unless absolutely necessary. It was not necessary for this question. 3) I hope you are using a full wave bridge on the output side and not some !@#$%^&* half-wave rectification scheme. Resonating the secondary is not necessary with a ferrite core toroid arrangement. I'm not sure it would be necessary with highly sub-optimal windings, but it is likely to involve you in lily-painting. Optimize the winding geometry and core, then paint your lilies if you absolutely feel you must. 4) You could also do this with capacitive transfer. (I should probably keep my mouth shut and not mention this, but ...) 5) Two right-cylindrical solenoids is an absolutely atrocious way to transfer energy. The coupling coefficient is negligible. 6) Calculate NOTHING -- build, measure, adjust. The calculations needed to get decent accuracy are just NOT owrth the time and effort.

YMMV - MHOO - HTH Kevin

one

Well yes. But I'm a great believer in changing the problem to make the solution easier :-)

They so work across a plastic barrier - just not a flat one. So deform the barrier!