inductive charger core geometry

Greetings,

I am interested in the mechanism of the electric tootbrush charger. For those who do not know the electric toothbrush sits in a cradle while charging and there is no direct electrical connection.

What kind of a core geometry would be good for that type of an arrangement? I thought maybe a small rod or button on each side of the interface, but then supposed it might be better to have a U shaped core on each side to give the flux an easier path to return.

Please help if you can. All of my experience is with normal transformers.

regards, Yzordderrex

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Yzordderrex
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It might help somewhat. But I've torn a few apart and they just use a cylindrical core with a winding on either side of the gap.

It helps that they run at about 100 kHz.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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Electric Toothbrush with Inductively Coupled Charger

Simple circuit

Shows air core coils 1-1/2" and 13/16"

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Most that I came across ran around 60-80kHz. And yeah, usually just a big old chunk of ferrite with wire around it. Got to keep them profit margins up there ...

Really good contactless chargers operate the drive side in series resonant fashion.

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Regards, Joerg

http://www.analogconsultants.com/
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Joerg

Ok, on the Iron. I'll play around with that some - I was thinking a pair of planar core halfs. As far as the series resonant design goes, I've just about abandoned that idea. My SR simulations aren't cooperating with me. I think I may end up with too much variation in the gap which will detune the circuit. Have you any insight on the really good ones. A manufacturer name or any patent info would help as Ilike the series resonant approach. I'm looking at 500kHz to 1MHZ to run this thing. I'm thinking of a half bridge which will commutate very nicely with a lagging current.

regards, Bob

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Yzordderrex

I've made one of these things in the past. You need to spread out the resonance peak with a bit of resistance in the coil ie detune it somewhat. Having that resonance makes a big difference in the size of the charging gap. Typically I was getting good charge transfer at about 2x coil diameter.

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Dirk

http://www.transcendence.me.uk/ - Transcendence UK
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Dirk Bruere at NeoPax

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Ok, thanks Dirk. I'll take another look at it sometime in 1Q 09 and report back to the group.

Merry Christmas

Regards, Yzordderrex

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Yzordderrex

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Not if you want to run afoul of AM radio broadcasters' bands.

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