Magnetic field of a solenoid

Sorry...

Well don't really know what highly sub-optimal windings is - and lily painting?

Yep, I have done that before. The power is limited and inductances in the loop must be low.

Will follow that remark - I may be stuck to often in front of the computer doing simulations.

Thanks

Klaus

Actually the project is splitting into two. One to transfer power and one to transfer a signal. In the application of only transmitting signal I could just run with a low duty-cycle to ged rid of the ISM band concerns. When transferring power I need 5V and 20mA

Yeah, that could be a problem. It would be a shame to be caught by a certain EMC requirement in a small region of the world

Thanks, will check that out.

I found an earlier measurement I did using the traces of the PCB and adding a ferrite core (like two planer transformers - with only one core segment):

This one measured a self-oscillation frequency of 35MHz. So I pulled the frequency down by adding a 470pF cap on coil, that brought it down to 5MHz. That will allow for the use of a modest frequency circuit (CMOS 4000 logic)

Guess, I will have to try out some different setups to get some more insights into what is the best way to go.

Thanks

Klaus

It could possibly be done across the same transformer if cost is a concern. For example power at 27.12MHz and the signal at 6.78MHz so the switcher noise and harmonics don't interfere. Usually FM or digital is best. It depends a little on where it's marketed. I believe 6.78MHz isn't ISM exempted in as many countries as the other frequencies. Then there is also 40.68MHz. Not sure how much ERP is allowed there since I have not used that one for power stuff in a while.

Another nifty method for slower digital data streams is load modulation where the secondary is puls-loaded before the buffer cap. This is then sensed on the primary side. For example, a slow changing analog value could be V/F converted or come from a uC that way. Then it switches a FET and load resistor. On the other side you'd need a highpass and use NRZ reception so it won't react to "real" load changes.

Yep. Sometimes it can border on the bizarre. Once we had a regulatory roadblock placed in the way. Small country, I believe the former Tchechoslovakia. They insisted that a guy from their regulatory agency had to first come out and inspect everything. To our surprise he was actually a nice guy.

Can work. However, if you have the coil itself plus a 5% cap determine the oscillation frequency you can't reliably stay in an ISM range and then the federales come swooping in. If in the end it's all in a metal enclosure that might not be a problem though.

When getting into inductive power transfer just keep in mind that simulations can be frustrating there. It's usually best to build stuff on the lab bench right away.

Lily painting is a American expression saying "to try to perfect something that's already good enough".

I don't think that'll get you 5V/20mA with any reasonable effort in this situation. Unless you go to 2.45GHz where there is an ISM band for microwave ovens ....

I've always wondered whether someone makes a USB soldering iron ;-)

Mine has the male on the charger, female on the toothbrush. No problems that way around.

Google on "contactless power transfer" to find volumes of papers and manufacturers data on this subject.

As long as the bathroom is properly vented it should be ok. But if not and people take hot showers in there it might become a health concern (condensation).

"lily painting" there is a homily about "painting the lily and gilding fine gold", in essence talking about wasting time optimizing things that do not need optimization and where the efforts perhaps even mar the result. It is another instantiation of the "big stones first" principle, a.k.a., go after the tallest tent poles first (and a gazillion other variants). So the suggestion is to determine the key factors and sort them by magnitude. (Sometimes also discussed as finding the bottlenecks) But the key element is sorting them by just how much they limit the desired behavior. Then don't waste any time on any except the first one or two, and only when those are addressed as fully as possible do you move on to others -- typically by redoing the list based on the modified design. Cross-coupling between two solenoids, whether end to end or side by side is fairly poor -- thus such windings are "sub optimal", i.e., less than optimal, for power transfer. Taking something so sub-optimal and optimizing it (with resonance) rather than fixing the problem (poor coupling) was what I was referring to, I just used phrasing that was an attempt to be jocular and not sound directly critical. I'm sorry if it became obtuse as a result. But since that is the most limiting factor in the first design idea, that is the one to address.

Going to a cored transformer with two half-cores separated by an "air-gap" (this being the plastic, and since the mu of the plastic is expected to be mu-nought, just as would be for air, hence use of standard "air-gap" terminology) will give extreme improvements in primary to secondary coupling, which probably obviates any need for resonance (at least in an attempt to improve power transfer, resonance may still be useful for other reasons). So, use a proper geometry and a core (if possible) and make the winding goemetry much closer to optimal as the first step, that was the recommendation.

As for calculating -- there are times it is indispensible. You don't want to "cut and try" when peoples' lives are at risk or when it costs multi-mega$$ just to try it. But for small projects, the expense of time and effort to try something and then adjust it a few times is often far lower than the time required for calculations of sufficient accuracy to be useful. More powerful computers and pre-written software start shifting the cross-over point, but SPICE is pretty awful at things where parasitics are important, and you'd need a field solver for this problem also, so "cut and try" looks better than the computational alternatives. MHOO

I hope this is clearer now, please feel free to ask away on anything that isn't or new issues raised. And the "lily-painting and gilding fine gold" thing sometimes even gets referred to as "lily-gliding" -- so I would recommend you avoid gilding any lilies ;-)

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The supply may run in many 100k/year, so I would at least have to have a large design margin in tests

Great, and thanks for the lengthly explanation :-)

Regards

Klaus

that

My someone is paranoid :-)

Nope. But married :-)))

Maybe by Darwin's Law, people who are too lazy to use a manual toothbrush deserve to die of deadly toothbrush fungus.

Cheers! Rich

Wow! Only 485 words to say, "If it ain't broke, don't fix it." ;-)

Cheers! Rich

I was a big skeptic regarding electric brushes. Until I used one. It does clean a whole lot better. Also, since then the dentist doesn't find any serious mineral buildups near the gum line anymore.

A long time ago we had a brainstorm session at a med ultrasound company, to come up with product ideas. The sky was the limit. So we gathered in little groups and ours had on its list, you guessed it, an ultrasonic toothbrush. Laughing was prohibited but the guys were rolling on the floor holding their bellies. Needless to say we left the stage like wet dogs. A few years later Philips built one and still makes oodles of money. We could all be on our own islands now :-(

The capacitive coupling method would only come into play if only a signal and not power were to be transferred

Regards

Klaus

True. Signals are pretty easy to get across if it doesn't have to transfer anything with the bandwidth and dynamic range of a Mozart concerto. And even then there is 40.68MHz. For mundane stuff I like to mux the signal into the power path to save cost.

What's the input voltage range and available primary current on this product? I think there should be a way to do that power transfer on ISM and save the core. At the quantities you were writing about it looks like the core adds too much cost. Not just the ferrite but it's a manual operation to place and pot them in. Unless it's in the tens of million units where you could build a custom robot for that.

OK -- although power can be transferred that way, B field coupling is typically more effective for power transfer at macro-scales. Vibration or ultrasound can also be used for both power and signal transfer through solids, and photo coupling through trnsparent or translucent materials.

I've trimmed sci.physics as we are really talking engineering and design issues, not issues of fundamental physics here.

"Rich Grise, Plainclothes Hippie" wrote in message news: snipped-for-privacy@doubleclick.net...

Very few people who use manual toothbrushes do as good a job brushing as those who use electric brushes. Your argument is kinda like saying that people who are too lazy to write things out long-hand rather than a word processor deserve to get carpal tunnel syndrome. :-)

That's when you're supposed to look around the room and see if there are enough people *not* laughing to run off and for, your own company. :-)