Get your new Braun /Oral B brush to fit the old-style charger.
Step 1: Braun Electric Toothbrush Charger Hack
Having used Braun Oral B electric toothbrushes for years, I was disappointed to discover that the design of the induction charger base has changed on the latest units. The old charger base had a slot on the back to take excess cable and a bracket for wall-mounting, but the new charger base is smaller and will only sit on a horizontal flat surface. This is much less convenient than the old base which I have mounted next to the shaver socket in our bathroom. A quick eMail to Procter & Gamble did not shed any light on any technical / electrical differences. Therefore, I decided to take some electrical measurements on old and new chargers. Since these are induction chargers, there is no access to measure voltages and currents directly, so I used a ten turn 'search coil' and a multi-meter to compare the ampere-turns output from the charger bases.
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Step 2: Measurements
The original 'search coil' I tried, the white coil shown above, was taken from a scrap RFID tag. When these showed that the outputs were roughly comparable, I repeated the test using a coil taken from a scrap Oral B toothbrush. (The brown coil shown above.) This coil has dozens of turns, so the readings were higher, but confirmed that the output from the old charger at 110V was about 50% higher than the new charger at 230V:-
Old charger model 4728:-
230V 590mV O/C 460mA S/C (White coil)
110V 410mV O/C 290mA S/C (White coil)
110V 12V O/C 75mA S/C (Brown coil)
New Charger model 3757:-
230V 280mV O/C 170mA S/C (White coil)
230V 8.7V O/C 45mA S/C (Brown coil)
Step 3: Grab a Needle File
Although the outputs of the old charger at 110V are about 50% higher than the new charger at 230V, I decided that they are close enough as should make no difference. The charge control circuit inside the toothbrush should keep the charging current within acceptable limits, since NiCd and NiMH cells are very forgiving in their charging requirements. Therefore, I modified the 'spigot' of the old charger base, onto which the toothbrush fits, by filing it to the same shape as the new charger. I was a little concerned that I might break through the plastic and expose the metal core, but this did not happen. The toothbrush now fits perfectly, charges perfectly, and is wall-mounted, as desired. A little polishing with fine wet-and-dry abrasive and metal polish should restore the finish of the spigot, when I get chance.....