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Small Induction Coil Charger

Hi Everyone,

I have a small device which i am currently designing, and i want to use induction charging. As the environment may get wet, the device must also be sturdy and droppable, so no clip/screw parts to break.

The device draws around 15mA. It is a circular device, about 3cm diameter and about 6cm tall.

no its not a dildo! (6cm and 15ma? :) ).

(Measurements are of the current prototype but can of course be adjusted as required for the final version)

There is _spare_ room in the base of this device, below the battery, imagine a 5mm thick coin of 30mm dia, then you have an idea of the spare room available. (not much!)

Considering i can use my choice of battery type etc how should i go about designing the charging system ?

What _needs_ to be in the 'device' as a bare minimum to charge the chosen battery type ?

Any good examples around ? Most of the google searching turns up broad 'how stuff works' descriptions or 'free energy nonsense'. I have not seen a clear and simple schematic yet.

I found one google post (lost the link already, was very old anyway) about someone using a simple coil and zenner limiter on the 'device side', and an xtal to drive the coil on the 'charging base' side. Is it really that simple ? No bridge/rectifier in the device ?

Smallest possible for this.

Also please consider 'the device' only needs to draw around 15ma from the battery, so unlike an electric toothbrush this probably can get away with something very small and simple. Yes, i opened up my toothbruth first, and its guts are far too large to fit in my device :)

Currently i am running 'the device' on 2x3v lithium cells of 150mah each (in series). I must keep the device between 4v and 7v, unregulated. I will change the batterys for whatever battery type which will allow the simplest 'device side' charger ?

Thanks for any tips, suggestions or links to decent info :)

Alex.

Reply to
Quack
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"Induction charging".. Consider:

1) How big a pick-up coil (turns, diameter, core type) 2) Frequency(ies) (ie: waveform) used to excite the coil. 3) shielding by enclosure around coil, etc. Treat the coil as a secondary to a transformer (that is what it is, when doing its function), and either use a FWCT rectifier, or a FWB rectifier. The first takes more wire but is more efficent for output.

Also, try re-enginering the unit for a lower drain.

There seems to be a number of different Lithium-based rechargeable batteries. I recommend them, as they are excellent for standby (low or zero current) service, but still drive a load when needed. They appear to allow long-term over-voltage (to a certain degree) when charging. Like any other rechargeable, the charging current should not exceed published values; C/20 is a good "universal" safe maximum. So one might use a low power micro that is on standby, and some event does a "wake-up" for useage. DigiKey carries the Panasonic line, and datasheets are available on the web.

Reply to
Robert Baer

Hello Alex,

Not advice, just some ideas:

A series resonant concept with two core halves doesn't seem to cut it here, considering the available space.

Try to find a rechargeable battery that occupies half or a little more. Then place a coil inside the cavity with it, followed by a Schottky rectifier (or silicon if cost is an issue) and the works.

Run a similar coil on the outside and feed it an ISM frequency such as

13.56MHz. Adhere to FCC limits here, especially with respect to stability of that oscillator (needs to be crystal controlled) and harmonics. You may have to make the circuit(s) resonant.

Regards, Joerg

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Reply to
Joerg

Hi Joerg,

Thanks for that. Not knowing anything about transformers etc, this seems to be on the right page.

what else would i need in 'the device', you mention "and the works" ? mind sketching a simple diagram? :)

Also, any clear/simple examples of the base charger available ?

Alex.

Reply to
Quack

Could you buy one of those Braun toothbrushes and use that either for learning or as a pre-approved charger, and build your own device to work with it?

Unfortunately I think the more toxic NiCd is better for this application because it can be trickle charged for long periods without causing damage.

Chris

Reply to
Chris Jones

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