Magnetic field of a solenoid

[Joerg]

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!

Mine is flat, except for two little ridges to keep it from falling out
of the holder. If it wasn't flat and the toothbrush would reside in some
kind of cradle a lot of nasty gunk could accumulate in there caused by
the fact that it's often wet when you return it. I could imagine mold,
staph and so on.

 

[Klaus Kragelund]

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.

Sorry...

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.

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

4) You could also do this with capacitive transfer. (I should
probably keep my mouth shut and not mention this, but ...)

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

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.

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

Thanks

Klaus

 

[Klaus Kragelund]

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?

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

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.

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

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:

http://smirc.stanford.edu/spiralCalc.html

Thanks, will check that out.

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.

--

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):

http://www.microdesign.dk/tmp/FIG2.TIF

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

 

[Joerg]

Klaus Kragelund wrote:

[...]

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

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.

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

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.

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):

http://www.microdesign.dk/tmp/FIG2.TIF

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)

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.

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

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.

 

[Joerg]

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

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

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

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 ....

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

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

 

[CWatters]

Mine is flat, except for two little ridges to keep it from falling out
of the holder. If it wasn't flat and the toothbrush would reside in some
kind of cradle a lot of nasty gunk could accumulate in there caused by
the fact that it's often wet when you return it. I could imagine mold,
staph and so on.

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

 

[Fred Bloggs]

I need to transfer some energy accross a platic barrier.

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

 

[Joerg]

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

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).

 

[Kevin G. Rhoads]

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

painting?

"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 ;-)

 

[Klaus Kragelund]

"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.

That sounds like good work behavour - I understand better now

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.

Yes, sometimes it is better to abandon a non-optimal solution

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

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

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 ;-)

Great, and thanks for the lengthly explanation

Regards

Klaus

 

[CWatters]

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).

My someone is paranoid

 

[Joerg]

My someone is paranoid

Nope. But married ))

 

[Rich Grise, Plainclothes Hippie]

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).

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

Cheers!
Rich

 

[Rich Grise, Plainclothes Hippie]

[HUGE dissertation snipped]

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

Cheers!
Rich

 

[Joerg]

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

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 :-(

 

[Klaus Kragelund]

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 ....

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

Regards

Klaus

 

[Joerg]

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

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.

 

[Kevin G. Rhoads]

The capacitive coupling method would only come into play if only a

signal and not power were to be transferred

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.

 

[Joel Kolstad]

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

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.

 

[Joel Kolstad]

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.

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.