Scott,
Thanks for the nice answer. 
Barry Ma
-------------
On Tue, 26 September 2000, "Scott Lacey" wrote:
 
> Barry,
> 
> These use magnetic coupling to transfer the charging energy. In essence, the
> transformer secondary is inside the toothbrush handle, along with the
> rectifiers and rechargeable batteries. The older models used line-frequency
> sine waves and tended to get warm (but not hot). Many of the newer models
> use high-frequency (tens of kilohertz or higher) square waves and do not get
> warm while in the stand.
> 
> As for safety, the lack of exposed contacts is a great advantage. Battery
> chargers can deliver substantial current - enough to cause burns to a
> curious child probing them with a pair of metal tweezers.
> 
> Scott Lacey

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