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Name: Michael C.
Status: Student
Age: 20s
Location: N/A
Country: N/A
Date: September 2002

My 5 year old son has asked me this question several times (I do not know the answer). My rechargeable toothbrush (Braun) has a plastic casing. The recharger unit is also completely plastic coated (no electrodes or terminals visible). How does the electricity pass through the plastic casing to recharge the battery?

I believe the following to be the case, but I am not 100% certain.

That is an innovative way to transport electrical energy (at least to me). If I am thoroughly understanding your charger, the transfer of power is done by induction. If there, in fact, is no electrical conduction between the two pieces then it is definitely charged by inductively coupling two coils. This is the basis for how transformers work. You pass a time varying wave form, or AC voltage (residential house voltage) through a primary coil (usually wrapped around a iron core). This primary coil is in the base of the charging unit.

This voltage is responsible for a time varying magnetic field which emanates from the primary coil and "magically" through space and plastic. This time varying magnetic field is received by the secondary (or pickup) coil in the toothbrush. The secondary coil "feels" this magnetic field and generates a time varying voltage at the coil terminals proportional to the field strength around it. Next, (and this is just a guess) the time varying (AC) is converted, or rectified to a time constant voltage (DC voltage) and used to charge the batteries in the toothbrush.

Darin Wagner

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