Induction chargers

Last updated: February 8, 2009.

If you've got an electric toothbrush with a plastic bottom, you might have marvelled at how it charges up apparently by magic standing on what seems to be a plastic stand! How do two bits of plastic charge up the battery inside your toothbrush when there is no direct metallic contact between them? How on Earth can electricity flow between two plastic insulators?

What's happening here is a neat trick called induction charging and we're about to take a closer look at how it works. If you don't already know the basics of how ordinary electric transformers work, you might like to take a quick look at our article on transformers before you continue. If you'd like to know about other kinds of chargers, you might prefer to look at our article on battery chargers.

Photos: An electric toothbrush standing on its induction charger. The battery in the brush charges up even though there is no direct electrical contact between the plastic brush and the plastic charger unit in the base.

How ordinary chargers work

Most of the small electronic appliances we use in our homes work on relatively low voltages—typically 5-10 percent as much voltage as hefty electric appliances like vacuum cleaners and clothes washers. That means we generally need to use transformers to "step down" the domestic voltage so it will safely power electronic gadgets without blowing them up. All those chargers you have (little boxes attached to wires that plug into things like your MP3 player and cellphone) actually have electricity transformers hiding inside.

It's easy to understand how these simple chargers work: electricity flows into the charger from the electricity outlet on your wall. Inside the charger, a transformer "steps down" the electricity to a much lower voltage. The low-voltage current then flows from the charger into the battery in your appliance. The important thing to note is that all three parts of the transformer (the primary coil, the secondary coil, and the iron core linking them together) are contained inside the charger:

Photo: Ordinary electric charging: all the components of the transformer are contained inside the charger.

How induction chargers work

So far so good—but what happens with something like an electric toothbrush, which has no power lead to plug into the wall? When you stand the toothbrush on its charger, how does the electricity flow from its plastic-coated base into the battery inside the brush when plastic is an insulator (that is, doesn't allow electricity to flow through it)?

It's not magic we have here—it's just another kind of transformer in a cunning disguise. An electric toothbrush and its charger use a transformer just like a cellphone or an MP3 player, but it's cleverly split into two pieces, with half the transformer in the bottom of the toothbrush and the rest of it in the charger base it stands on:

Artwork: Induction charging: half the transformer is in the toothbrush; half is in the stand.

The primary coil is in the charger base and it has an iron peg on top of it covered in plastic. The secondary coil is in the base of the toothbrush, which you stand on the iron peg. What's the peg for? It's not just to stop the toothbrush wobbling about: it's the core that links the primary and secondary coils together electromagnetically. When the toothbrush is standing on the peg, you've got a complete transformer that works by electromagnetic induction: energy flows from the coil in the base to the coil in the toothbrush via the iron peg that links them. The two ends of the coil in the toothbrush are simply hooked up to the rechargeable battery inside.

Photo (left): The bottom of an electric toothbrush has a hole in it so it sits securely on the "peg" on the base—actually the core of the transformer. The copper coil (right) is just above the plastic base, running around the hole.

A charger that works like this is called an induction charger. Safety is the main reason for using an induction charger in the bathroom: you don't need a power cable or exposed leads coming out of the base of your toothbrush, which typically gets wet. Electric shavers often use induction chargers for the same reason.

Photo (right): The copper charging coil from inside an electric toothbrush. The coil works as the secondary coil of a transformer. It's fitted at the very base of the brush and sits around the little iron peg on the charger stand when the brush is sitting on its base. The two leads from the coil connect to the rechargeable battery inside the toothbrush.

How do Power Mats work?

They're induction chargers too. The mat works as one part of a transformer (essentially like the stand of an electric toothbrush), while the bit you clip on the bottom of your appliances works as the other part. There's no magic involved: it's just electromagnetic induction!