How do electric toothbrush chargers work?

25 November 2007

Question

I’ve recently bought an electric toothbrush and the charger base has a plastic protrusion. The bottom of the toothbrush has a hole in it and you sit one on top of the other and it charges. But there’s no metal contacts. It’s plastic-to-plastic. How can that work?

Answer

chrisIt does sound very strange, doesn't it?

They don't want to use actual contacts because in a wet environment like a bathroom the contacts will get dirty. They'll rust and they won't last very long. What they do is something else which is kinda cunning. Inside that little protuberance there's a coil of wire which connects to the mains. Basically you've got a current running backwards and forwards through that wire, and that produces a magnetic field. It's an electromagnet.

Inside the toothbrush is another coil going outside the protuberance, again, under the plastic going round and round then connecting to the toothbrush itself. If you have a changing magnetic field going near a conductor, the field will induce a current in that conductor. It's actually how a transformer works. The current inside the first coil makes a changing magnetic field and that makes a current flow inside the second coil without having to touch it. That charges the toothbrush itself.

This takes advantage of the fact that although electricity will not travel through the plastic, magnetism will...

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