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I'm not going to change my habits based on the answer, but I did wonder from a theoretical standpoint. I get idea of wireless charging. I still have flashbacks to my power generation & electric motor courses in undergrad (3 phase systems, lossy magnetic field circuits, etc.). So basic intuition, I have a vague sense of. What I don't know is whether today's wireless chargers for rotary toothbrushes are made with enough monitoring and control logic to be aware of when a load (i.e., a toothbrush) is docked and drawing power, so that it dials down the EM field when there is no toothbrush. As a related 2nd question, does the toothbrush switch loads when full charged so that the charger can tell, and dial down the juice?

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  • \$\begingroup\$ Transformers (which a pair of charging coils is) mostly already DTRT when it comes to no load. \$\endgroup\$ – Ignacio Vazquez-Abrams Dec 22 '15 at 21:39
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A robust modern product would probably have circuitry to detect the toothbrush load back in the base and turn itself off. Unfortunately the competitive market and cost pressures to keep product prices down means that the products are often designed as simple as possible. So likely no extra special circuitry.

Some tooth brushes may have a magnet that activates a reed switch in the base to activate the charge circuit. Other base changers are just going to be a many turns primary winding that exhibits a good amount of inductance. This winding is across the power line when the base is plugged in and will act as a transformer primary when the toothbrush handle is placed in the holder. A coil in the toothbrush handle acts as the transformer secondary and induced current flow occurs and charges the battery. The secondary load characteristics are reflected back to the transformer primary and cause more current to flow from the mains AC connection.

With a carefully designed transformer setup like this the primary current could be very low when the secondary in the handle is not in position. Is it low enough to justify the simplest design where it is 100% connected to the power line? In today's world probably not!! In the past? Highly likely.

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  • \$\begingroup\$ I believe that your 1st paragraph speaks most directly to the question. And your remaining 2 paragraphs are a good refresher for me about transformer theory. Thank you! \$\endgroup\$ – user36800 Dec 23 '15 at 22:57
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I can't give the detailed working information about how a toothbrush holder delivers power because I don't know that but I can tell you that theoretically no power has to be consumed if there is nothing sapping power from the magnetic field.

Think about a coil of wire attached to a sinusoidal voltage source. If the coil is made from low resistance wire and is reasonably "reactive" to the frequency of the sinewave, the current drawn will be near enough 90 degrees to the voltage waveform i.e. it will have a power factor of virtually zero and very little power will be used up.

As the toothbrush (or whatever) comes up close its coil can convert some of that magnetic field into electrical energy using the principle of induction. Therefore real power can be withdrawn from the mag field and the phase angle that was 90 degrees becomes somewhat less. The power factor is no longer zero and this signifies power is taken from the voltage source and delivered to the toothbrush. It does look like the toothbrush uses AC power frequency like this: -

enter image description here

Operating at a much higher frequency is quite possibly a sensible thing to do and this is exactly what passive RFID tags utilize. A lot operate at 13 MHz and you can make quite small coils and, to get extra distance they are resonant tuned with capacitors. All done without incurring hardly any off-load power: -

enter image description here

I've built bigger ones to supply energy to telemetry electronics mounted on a spinning rotor in a turbine.

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  • \$\begingroup\$ Thanks for the background info, Andy. I get that most of the current is out of phase with the voltage if there is no coupling. The higher the frequency, the more power loss even in that scenario (I would expect, but am not totally sure) because it would resemble radio waves being transmitted. \$\endgroup\$ – user36800 Dec 23 '15 at 22:55
  • \$\begingroup\$ It's not radio waves, it's magnetic fields. You get a radio wave with a proper EM antenna but not with a coil of wires of a few cm diameter at a few MHz. Mag fields die down as distance cubed so much much gets very far! However you are right in one sense if ferrites are used to "focus" the field because it's hard to find a ferrite material that has low losses at (say) 10MHz but it still won't be a major issue. \$\endgroup\$ – Andy aka Dec 23 '15 at 23:02
  • \$\begingroup\$ Ah, OK. I've only studied EM at the theoretical level, in the context of microwaves & optics (decades ago!). So your comment gives me a sense of the order of magnitude at which that way of viewing the fields is not useful. Thanks. \$\endgroup\$ – user36800 Dec 24 '15 at 18:58
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EEVblog did a teardown of one of these a while back, mildly interesting and might help to answer some of your questions: http://www.eevblog.com/2012/05/30/eevblog-284-braun-toothbrush-teardown/

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  • \$\begingroup\$ Interesting...sad how much I've forgotten about analog electronics since moving into communications algorithms. \$\endgroup\$ – user36800 Dec 23 '15 at 22:56

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