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I've got an Oral-B/Braun electric toothbrush and was wondering how hard it would be to make charger that would be powered via USB (I guess 1A should be enough).

The needed amount of windings on the coil could easily be calculated when measuring the voltage on a coil with a known amount of windings when put onto the charging station while the toothbrush is charging (I got 0.035V with 1 winding which would mean that around 143 windings would be required to charge on 5V). Another requirement would definitely be a DC-AC converter with the right frequency (could be obtained in the same way as the voltage/windings).

So what else should be considered?

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closed as too broad by brhans, Voltage Spike, uint128_t, Dmitry Grigoryev, Andrew Jan 30 '17 at 14:39

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ compare the V/f input of the default power source with what you are applying to get the same reactive power of stored energy or inductance and conduction losses. Start by measuring the DC resistance which must be much smaller than the inductance. \$\endgroup\$ – Sunnyskyguy EE75 Jan 26 '17 at 18:12
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for giggles I measured my Braun Type 3757 120V 60Hz on my RLC meter and it gave me the following readings on the AC plug.

@1kHz L= 95.37 H, Rs=92kΩ Rp=O.L. Cs = 267pF Rs=92kΩ

The Rs changed from 72 kΩ and ~100H before I powered it above, so remanence may have affected the readings.

But at 120Hz it gave ambiguous large readings.

So I knew it didn't operate at 60Hz.

Then I scoped the magnetic field with a probe at 27us distorted cycle or 37kHz.

It is rated at 0.9W, so you will need to step up the voltage and use around 200mA RMS to match performance. Flyback operation will not work with existing coil here due to the high value of H, but you might be able to make an auto transformer out of it with additional windings as primary and modify or bypass the internal switcher.

Hope this helps you.

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Some wireless charging systems have communication channel which only enables wireless charging output when there is a compliant device nearby. Not sure if toothbrushes have such system, and if they do, what will happen if toothbrush starts receiving power without communications from charger. Also, it is not very likely in your case, because you were able to detect the power even without toothbrush present.

My advice is to check it yourself and see. Measure the frequency of the coil, then get an arduino and motor driver breakout, and you should be able to prototype the charger pretty quickly.

One word of caution: do not solely rely on the calculations when you design your charger coil. Instead, just measure induced voltage on a coil using original and your home-made charger, and make sure they are the same. Then load the coil so it output voltage halves, and make sure that both chargers produce the same voltage in loaded coil, too -- some cheap designs may actually rely on power supply inefficiency to properly limit charging current. Only then you should try it with an actual toothbrush.

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