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Small Induction Coil Charger

Discussion in 'Electronic Design' started by Quack, Aug 9, 2005.

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  1. Quack

    Quack Guest

    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 :)

  2. Robert Baer

    Robert Baer Guest

    "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
    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
    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.
  3. Joerg

    Joerg Guest

    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
  4. Chris Jones

    Chris Jones Guest

    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.

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