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Inductive charger

Former Member

I'm going to design an inductive charger and I have a lot of open questions and don't really where to start. I know about the principle how it works. I need help with several things.

The idea is to get 5-7VDC out oin the secondary side, and something like 100mA would be nice.

 

First of all, what switching frequency is suitable concerning size and efficency of components, and also regarding type approval issues? I think electrical toothbrushes uses approx. 60 kHz so maybe that is a suitable frequency.

Secondly, does anyone know about a design example I can use for a start?

Does anyone have any suggestions for suitable inductors? Supplier, part number etc?

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  • 0
    This was found in an Interplak Model PB-12 electric toothbrush but similar designs are used in other appliances that need to be as tightly sealed as possible.

    A coil in the charging base (always plugged in and on) couples to a mating coil in the hand unit to form a step down transformer. The transistor, Q1, is used as an oscillator at about 60 kHz which results in much more efficient energy transfer via the air core coupling than if the system were run at 60 Hz. The amplitude of the oscillations varies with the full wave rectifier 120 Hz unfiltered DC power but the frequency is relatively constant.

         E1           CR2          R1                                E3
  AC o----+----+--|>|-----+---/\/\---+----+----------------+-------+  Coupling
          |   ~|  CR1     |+   1K    |    |                |        ) Coil
        +-+-+  +--|<|--+  |          |    / R2             |        ) 200T
    RU1 |MOV|     CR3  |  |      C1 _|_   \ 390K           |        ) #30
        +-+-+  +--|>|--|--+   .01uF ---   /          CR5   |     E4 ) 1-1/2"
       E2 |    |  CR4  |       250V  |    \ MPSA +---|<|---|----+--+   
  AC o----+----+--|<|--+             |    |   44 |         |    |
              ~        |-     R3     |    | Q1 |/ C    C3 _|_  _|_ C2
                       +-----/\/\----+----+----|     .1uF ---  --- .0033uF
      CR1-CR4: 1N4005  |     15K               |\ E  250V  |    |  250V
                       |                R4       |         |    |
                       +---------------/\/\------+---------+----+
                                        1K
    The battery charger is nothing more than a diode to rectifier the signal coupled from the charging base. Thus, the battery is on constant trickle charge as long as the hand unit is set in the base. The battery pack is a pair of AA NiCd cells, probably about 500 mA-h.

    For the toothbrush, a 4 position switch selects between Off, Low, Medium, and High (S1B) and another set of contacts (S1A) also is activated by the same slide mechanism. The motor is a medium size permanent magnet type with carbon brushes.

                                           S1B
                              S1A  +--o->o
                 D1           _|_  |       R1,15,2W
             +---|>|---+------o o--+   L o---/\/\---+
    Coupling |         |                   R2,10,2W |
       Coil  +        _|_ BT1          M o---/\/\---+
       120T (          _  2.4V                      |
        #30 (         ___ .5A-h        H o----------+
     13/16"  +         _                            |
             |         |        +-------+           |
             +---------+--------| Motor |-----------+
                                +-------+
  • 0 in reply to enrico.migchels

    Hello,

     

     

    I am interested in the " Electric Toothbruh Inductively Coupled Charger " schematics.

     

    Please let me know if someone has tested this circuit.

     

    Does it work well without a magnetic core coupling ( and only with a air core coupling ) ?

     

     

    Thanks in advance

     

    Best regards

Reply Children
  • 0 in reply to Former Member

    Hi Philippe!

     

    After starting this thread I made several tests with inductive charging. From my experience I will say that inductive charging without a magnetic core gives such a low efficency so it more or less useless. Inductive charging as we know it today is not very useful at all. You are dependent on placing the inductors exactly right compared to each other, and you need a magnetic core in between. It limits the use of the technology very much.

    Furthermore, it's very hard to find sutiable inductors for this. You have to wind them by yourself or you have to buy customized inductors in large volumes. 

     

    There is a team that has developed a solution with hexagonal inductors printed on PCB. This solution looks very iterseting to me, but this team has a patent for it.

     

    I wanted to make a nice inductor charging solution, but I have more or less given up on this.

    Sorry! :-(

  • 0 in reply to Former Member

    Roger

     

    Did you ever sort this out?

     

    I have a charger - 110v and have the circuit of it if it would help.

     

    It is in Eagle Cad format if you have Eagle on your PC or I could just send a .jpg of the screen.

     

    Let me know

     

    Cheers