electric toothbrush contactless charger magnetic core geometry. (ETCCMCG)

AFAIR around 3/4" long and 1/3" diameter. You can open the charger and look. Unplug it from the wall outlet first :-)

They usually run around 60kHz. One has to mind regulatory hurdles for other frequencies. 1MHz would be a big no-no, results in egg in the face at the EMC lab.

The gap increases leakage inductance which can be (partially) circumvented by a series resonant design.

The base has a ferrite pin that couples the coil in the base to the coil in the handset. Can't remember the frequency, but it wasn't high. What was high was the voltage - +/- 100 or 200Volts AC. That surprised me, but it is probably obvious to those who know about these things....

Else they'd need a transformer. That might result in the cost to rise five cents which in turn could lead to hissy fits in the board room.

The whole thing is already a transformer...... why not use a lower secondary voltage? just less turns, thicker wire..... there is probably a good reason why it's such a high voltage. But I can't think what. Even the follow on electronics would be tricky at that high voltage. Seemed to be some sort of buck converter. It was totally potted in rubber gunge and I destroyed much of what was there but I did see the very narrow pwm pulses.

The tooth brush side is usually low voltage. At least it was in mine because it had to charge up two NiCd batteries. Plus that saves turns, meaning copper, meaning $$$ :-)

I opened up some of them too and what I saw make me guess they're doing some kind of trickle charging. So by current limiting. Which should not be too difficult as the "tranformer" coils are very loosely coupled. Don't put the thing on a metal surface while charging as most of the energy will get lost by warming up the metal.

petrus bitbyter

I saved the charger and pickup coil from my old Sonicare. The charger base (I haven't opened it. I want to save it for future use.) probably has a ferrite core that is positioned vertically alongside the toothbrush axis on one side. The rest of the base is open, so there is no core underneath where the handle sits.

The pickup coil in the handle is a 0.5 inch dia by 0.5 inch long ferrite slug with about 50 turns of #28 magnet wire around it. It generates about 25-30 V p-p (depending on how the pickup is positioned within the base.

The base unit runs at 80 kHz with an amplitude modulation of 120 Hz, which indicates that it is running off a full wave rectifier from the line input with no filter cap.