Re: How does my cordless toothbrush recharge w/ no metal contacts?

Mike,

You are correct: ordinary plastic is an insulator and does not conduct 
electricity (the fancy term for this is "dielectric material").

However, electromagnetic forces and waves pass easily through plastic.  For 
example, a magnet will be attracted to steel even if a thin sheet of 
plastic separates the two. 

You probably know that electricity flowing through a coil of wire creates a 
"field" of magnetic force.  If the electrical current changes with time, as 
with ordinary household alternating current (AC), then the magnetic field 
will also change with time.  If another coil of wire is placed in this 
changing magnetic field, electricity will be induced to flow in the second 
coil, even though there is physical contact between the wires in the two 
separate coils.

This is the principle behind an electrical transformer.  I have just 
described an "air core" transformer; most transformers use steel to help 
channel the magnetic fields and provide an efficient coupling of electricy 
between the two coils (called "windings"), but this is a refinement, not a 
necessity.

Not having taken apart your toothbrush and charger, or having seen the 
engineering drawings, I can only speculate on how the manufacturer has 
arranged to make a rechargeable unit without metal contacts. 

My best guess (and I'd be willing to be a cup of coffee on this) is that 
the recharger base and the tootbrush handle each contain a winding forming 
half of a transformer.  When the handle is placed in the recharger base, 
the coils in each are brought into proximity and a reasonably efficient 
transformer is created, allowing AC current in flowing through the 
"primary" winding in the recharger base to induce current in the 
"secondary" winding in the handle.  The windings are probably wrapped 
around U-shaped pieces of steel that will form two halves of a ring 
separated by plastic when the handle is placed in the charger base.  This 
would make a pretty cheap but fairly efficient transformer AND there would 
be no metal contacts exposed on the base or handle.

Some simple circuitry in the tootbrush handle (for example, a rectifier, 
current-limiting resistor, and zener diode) would convert the AC current 
induced in the secondary winding to DC at the proper voltage to charge the 
tootbrush battery.

There are other alternatives, albeit a bit more exotic:

The first is to use capacitive coupling instead of magnetic coupling, 
wherein the plastic in the handle and base act as dielectrics in a pair of 
capacitors coupling the charger base to the handle.  Metal plates inside 
the base and handle act as plates in a capacitor.  I don't think this would 
work very well without some additional circuitry to convert the 60 Hz AC 
current to a much higher frequency.  

The second is to extend this idea further and covert the 60 Hz AC to radio 
frequency energy.  The base would become an RF transmitter, and the handle 
an RF receiver.  This will also take more circuitry than the simple 
transformer-based approach would be terribly inefficient.  

That's why I'm guessing the manufacturer followed the KISS principle ("Keep 
it Simple, Stupid") and went with the economical, rugged, and reliable 
transformer-based design approach

Steve Czarnecki