charge batteries via inductive coupling

Hello I need help figuring out the coil structure for a toy I want to build. I have a rubber ball with a battery inside. I want to recharge the battery without a galvanic connection. I'm looking to get about

300 ma charge current. The ball will sit in a cup shaped recharger. And the coil in the charger will be powered by a wall wart with a nominal 12 vac output and around 500ma or less. What I need is help in figuring out the shape of the coils and their positions and the number of turns and so on. I am fairly good with most electronics but never could grok this inductive stuff. The math is above me. If I could just get a bit of advice on some basic aspects like for instance:
  1. Would the coil shape for the charger cup be better as a say 1.5" donut shape mounted in the bottom of the cup just below the surface with the other coil in the ball just above and stacked like two donuts, be better than a 3" coil in the rim of the cup with the ball's coil like a smaller donut in the hole of the larger charger coil's donut?

  1. How do I determine the number of turns I need to limit the current drawn from the supply to it's rated power output. assuming 60hz at around 12 vac?

3b. should I be looking at increasing the frequency driving the coil?

  1. I would prefer an air core but it would be nice to know if that was a mistake.

  2. given the coil on the primary side is x turns what should the coil on the secondary side be. I need 5 volts or more at 300 ma or so. To run the ciruitry and have the over head to charge the battery.

  1. How can I predict the effect of seperation on the power exchanged by the two coils? I would like at least an inch between them is this feasible? And if not what number should I use?

Reply to
TonyMatthews
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The coupling between the coils is proportional to the dipole moments of the coils and inverse proportional to the 3rd power of the distance between the coils. The dipole moment of a coil is roughly proportional to the volume of the coil x number of turns x core permeability. Ideally, the coupling is independent of the frequency; however it is easier to get better matching and lower losses at the reasonably high frequency (~tens of KHz).

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

Just buy an electric toothbrush from OralB, take it apart and use the elements it has. They charge in their stand without contacts by way of inductive coupling. There is no way that they operate at 300mA charge current though. I think you need to key that figure down a bit realistically speaking. Or maybe not... :-]

Reply to
ChairmanOfTheBored

Transformer that use a core needs to have the windings around said core in order to pass the energy from one winding to another.

An air core configuration is best because it creates the core coupling through the air, regardless of the distance between the windings (up to a point, of course). Winding orientation is also an important factor. The turns have to be parallel to each other to maximize coupling efficiency, and minimize any losses.

You'll have to mark the ball so that you know which side goes into the cup receptacle of the charging stand (or put some Braille on it for sight impaired applications).

Reply to
ChairmanOfTheBored

It only gets to 3rd power when the distance gets big. Look up the equation for the field from a coil vs distance and use this to figure how much field is cutting the receiving coil. Ideally, the ball will fit into the charger such that its coil is concentric with the driving coil.

x the current.

This last is important because as you make the coil bigger to get more moment, you raise the inductance thus making the drive to get a given current harder to do.

The efficiency depends largely on the number of pounds of copper you are willing to use.

.

Use Schottky recifiers. Since you are charging a battery, you know the load side voltage.

If the coupling is loose: Ideally, you want the load side resonant and to have its impedance matched by the charging circuit. Making the primary side resonant can make it much easier to drive.

Reply to
MooseFET

Read the Vladimir reply and the extending of it. After I pressed "send" I though of another comment I should have made.

[....]

It doesn't rise to the level of mistake but, adding a core will reduce the amount of copper you need to use. These days iron is much cheaper than copper so it would be worth adding some.

Remember that the iron must not go between the two coils. If the ball fits down inside a cup that makes the windings concentric, you want the core to be inside the ball. This would be troublesome from a structural point of view. If you have the cup coil's below the ball's coil, adding a core below the cups coil could improve the coupling.

ASCII ART:

OOO OOO on the secondary side be. I need 5 volts or more at 300 ma or so. To

Reply to
MooseFET

How about charging the ball _inside_ a solenoid, where the magnetic field is very strong. Put a marking on the ball to enable the user to orient it correctly, so that the coil inside the ball aligns coaxially with the outer solenoid. Don't use a toroid.

Reply to
gearhead

I see a problem with the geometry here. For maximum coupling, the base coil and the coil in the portable unit should align along a common axis. With a spherical device, how can you ensure that it will be placed in the charging base with its coil aligned properly?

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Reply to
Paul Hovnanian P.E.

Position the coil around the circumference, and along with the iron core, weight the ball so it falls or rolls when dropped into the cradle. Have the primary coil in the cradle positioned alongside where the ball secondary coil would be. This would enable a completely spherical ball, but I foresee positioning problems depending on the type of ball (fluffy??).

An alternative would be to make it a "girl" ball and create a cavity in which the charging coil could be inserted. Something along the lines of the Oral-B electric toothbrush and cradle (and other brands). 300mA might be a bit much if you want to keep the charging receptacle small though. You would be forced to use a larger size ball if you need a large receptacle.

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Reply to
John Tserkezis

It seems, since the OP says there's a battery "inside", you just adapt the air-filler port to accommodate a power jack.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
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Reply to
Jim Thompson

Some adults may prefer a toy that quits working a few days after Xmas. If not, three coils in the ball solves that problem.

Reply to
MooseFET

That assumes that there is an air filler port. I have a ball that makes all manner of noises when it is bounced and flashes LEDs. There is a port for the sound but if that was deleted, the ball would be near enough solid.

Reply to
MooseFET

Three coils on 3 axis solves this but not perfectly. A coil at a 45 degree angle has less coupling.

Reply to
MooseFET

(snip)

But, being a ball, a random orientation is likely - it IS a toy and the owner/operator may well not follow any written instructions.

Reply to
rebel

If you had read the thread, you would have noted where other mentioned such a need. A simple mark on the ball will suffice.

Reply to
ChairmanOfTheBored

Instead of a cradle, make sure the place the ball sits has the semisphere for the ball to sit in and a back, like a chair back, that runs up the back of the ball. Place three coils in the charger. Two in the base, and one up the chair back. Have the circuit detect which one is driving the coupled charging circuit the best, and turn the other two off. Or, if they are all in series, just leave them all on. One orientation will charge perfectly. Most others will charge well, and one or two orientations will charge miserably, but will still charge.

Reply to
ChairmanOfTheBored

MooseFET snipped-for-privacy@rahul.net posted to sci.electronics.design:

Orienting the ball is part of the problem. Any workable solution includes orientation.

Reply to
JosephKK

[..Ball being charged..]

Yes include:

3 coils in the ball solves this problem.

The shapes of the coils will need to be distorted a bit a the point where they intersect. I don't think this will reduce the coupling area enough to matter.

Reply to
MooseFET

Andy comments: Or... you can include multiple coils in the charger and phase shift them so you have a rotating charging field.... much like the rotating field in an induction motor...... That way, the charger is slightly more complicated, and the ball is simpler...

A 3 axis rotating M-field isn't hard...... just think about it for a couple minutes and you will understand....

Andy in Eureka, Texas

Reply to
AndyS

All of this assumes that the intended use of the ball can tolerate the asymmetry in balance or shape.

Here's another idea: Put a small motor in the charging base that will rotate the ball through contact with a friction wheel. Mount the wheel a bit off center so that the ball wobbles and eventually it will reach a position where a suitable coupling threshold is reached, at which time the motor shuts off.

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Paul Hovnanian	paul@hovnanian.com
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Reply to
Paul Hovnanian P.E.

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