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  • EEVblog #284 – Braun Toothbrush Teardown

    Posted on May 30th, 2012 EEVblog 40 comments


    Teardown Tuesday.
    What’s inside a Braun electric rechargeable toothbrush?
    And some basic measurements on wireless power transfer and charging.
    Chips Used:
    EM6682
    TSM7401

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    38 responses to “EEVblog #284 – Braun Toothbrush Teardown” RSS icon

    • Perfect! I’ve almost got the same model. Saves me the trouble of ripping it apart before it has died ;-)

    • Ross McKenzie

      Dave,

      Did you investigate the effects of returning the two ferrite beads that you removed at the start of the “teardown”?

      Cheers,

      Ross

    • Excellent teardown! I once opened an older model Braun toothbrush and also noticed those ‘ferrite’ beads. I think those are for absorbing hydrogen from the battery (for safety reasons!). I’ve noticed my Pelican torches have similar looking beads and those are designed for use in explosive atmospheres.

    • The “ferrite beads” are actually platinum catalyst pellets for the hydrogen gas from the NiMH cell.

      Also, the IP67 rating stamp on the bottom of the (mains powered) charger should well explain why it’s sealed off as well it is: you generally don’t want your consumers to be electrocuted. :>

      • Someone else on Youtube mentioned the pellets too. They look and feel and are placed an awful like like ferrite!
        Missed the IP67 rating stamp. Yeah, makes sense given it’s proximity to basins full of water.

    • Dave, didn’t I see *two* LEDs on the board? Your schematic only shows one.

    • The markings on SOT23s might be checked against this excellent database:
      http://www.marsport.org.uk/smd/mainframe.htm

      6Cs = BC817-40 NPN
      A4 = BAV70 diode

    • I think that double coil is there to increase current handling like in a normal CT transformer.

    • Okay, your next project must be a homemade x-ray machine to look inside of the charger stand ;-)

      Many Greetings from Hamburg/Germany

      Homeland of BRAUN equipment :D

      • I opened up a potted toothbrush charging station a few years ago for a work project. From what I can remember, there were no ICs at all in it. It was transformerless with an RC-network to limit the voltage. The oscillator was built with discrete transistors and diodes, perhaps with some feedback from the coil.

    • The EM6682 is probably derived from a specialized microcontroller for watches !!
      This is the reason of the swatch group having buyed that.
      In fact, the swatch group has a local hand on every aspect and every subcontractor for manufacturing watches, so a chip company makes sense :)

    • Hi Dave,

      I’m very sure that Braun just used these antiparralel Coils to get their current rectifies without having two times the voltage drop of the diodes like you have in bridge rectifiers. That might be the best way for efficiency and conserving the battery without a big capacitor.

      Best Regards from Germany,
      Hakon

      • I think you are on the money here. I would also be surprised if the EMI is not also reduced due to a balanced current waveform in antiphase – particularly due to the ringing artifacts which seem like they extend to several hundred kHz.

    • look here :
      http://www.emmicroelectronic.com/webfiles/product/asic/sens_act.pdf

      even they use a very low voltage mosfet, they seem to put a charge pump for feeding the gate :) makes sense when the battery is low !

    • It’s quite simple to get all the stuff out of the tube:
      You have to pull the ring at the top around the plastic shaft where you put the brush on.
      On my Braun, it is a metal ring. Once you removed it, you can pull out the whole insides from the tube.

    • How to disassembly a Braun electric toothbrush:
      http://www.blinkenbyte.org/braun-battery-replacement/index.html

      Tried in mine, because original battery has gone completely off.

    • Out of fun I measured my Braun 3757 after this episode. It’s a newer model and it uses 31khz. http://dump.501gu.de/braun_3757_scope.png

    • Great digging. Next time I need a few million custom chips I know where to go.

      I wonder how long some gold contacts would last to couple the base to the toothbrush? I am thinking that they would probably last the expected life of the brush. It would sure simplify things tremendously.

    • I disassembled my borken toothbrush (almost same model) after your teardown. Circuit layout was different, but still using TSM7401 and an EM micro. The coil had only two leads. Mine stopped working because it wasn’t sealed properly, so moist got in there and everything got rusty, the motor shaft wasn’t even able to turn anymore.

    • I did a teardown on a oral-b toothbrush. Wasn’t too exciting since it was a $2 non-electric one, lol.

      Ok, it was electric and I’m glad I didn’t make a video of it, because getting it open wasn’t pretty. It involved a hammer and diagonal cutters. I’m pretty sure it was glued shut, or maybe I’m just an idiot.

    • Can someone explain thet micro to us a bit. I assume this is not a one time programmable device, it looks like its more of a you buy it preprogrammed from the factory kind of deal. Am I correct, that hobbyist can’t play with these?

      • That’s correct, you supply the factory with the program and they make the chips with that program in ROM. If you want new software, you’ll have to order new batch of chips with new ROM.

        Based on datasheet page shown that chip isn’t even available in flash version, so for development they most likely have some kind of in-circuit emulator.

        Mask ROM isn’t really that uncommon; for example Microchip also can provide you with ROM-masked PICs, but if I remember correctly, you’ll need to meet some quality criteria, buy a lot of chips and whatnot before they even consider selling them to you.

      • Correct.
        I think they have a special dev version of the chip is reprogrammable some way, or allows you to do the development. But this brand and style is likely very much not hobbyist friendly. I doubt a hobbyist could even get them.

    • Definitely keep up the tear Down Tuesdays. I like the variety of products and their associated technologies you are discussing.

      Big THUMBS UP!!!!!!!!!!

      B

    • WTF?

      Double coil to increase current handling? antiparralel coils? conserving the battery without a big capacitor? balanced current waveform in antiphase?

      Are you kidding me?

      None of you recognise a simple full-wave rectifier circuit?

      Really?

      The two parts of the appliance form a simple transformer with a centre tapped secondary winding (that’s the ‘two’ coils thingy).

      The rectifier diodes are connected across the winding in opposite polarities with the cathodes connected together to form the ‘output’ connection.

      The centre tap is the 0V (common or return) line.

      During each half of the A/C cycle, one of the diodes will be forward biased and conducting, allowing current to flow from the ‘output’ to 0V. The other will be reverse biased and non-conducting.

      This sort of circuit is very common in these sort of low voltage ‘wireless charging’ appliances due to the voltage drop across the two conducting diodes in a bridge circuit (the only thing you did get right). For silicon that drop can be between 1.2V ~ 1.4V.

      • I wasn’t going to tell them :)

        BTW, Cost is also a factor. The manufacturing costs for this configuration are less than half that of a single winding with a bridge rectifier chip or four diodes.

    • Time to open my Braun – it has seized mechanically after 12-15years of daily service, the battery was still decent…

    • Presumably the charge pulsing is to enable the microcontroller to measure the battery voltage without any charging ripple in order to determine when the battery is fully charged. It would need a better ADC than the one in the EM6682 to determine state ofcharge from battery voltage.

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    2 Trackbacks / Pingbacks

    • [...] here: EEVblog #284 – Braun Toothbrush Teardown | EEVblog – The … This entry was posted in Uncategorized and tagged braun, older-model, once-opened by LordOhai. [...]

    • [...] The entire problem can be expressed in a single word — coupling. You need to look at how the signal generator is coupled to the first coil, then how the two coils are coupled to each other, and finally how the load is coupled to the second coil. Looking at the first of these, the primary coil must be designed so it has sufficient inductance at the operating frequency to impose a suitable load to the frequency source. This essentially is about the output capabilities of the source, the frequency and the inductance. For the chosen frequency the inductance must be high enough so as not to appear as a short circuit, but also not an open circuit. Then, the 2 coils must be designed so that the magnetic field from one induces a voltage in the other. The shape, size, core (air?) and distance will all be a factor. Finally we need to look at the load and ensure the power output from the secondary is suitable for it (e.g. the load appears as neither a short nor an open circuit). Then, we need to go back to the beginning because loading the second coil will change the behaviour of the first one. It must still represent an appropriate load to the signal generator. There are no hard and fast rules about exactly what frequency you need or exactly how many turns each coil must be, or how much power you can get out. But you need to find the right balance of all of these. There are some good examples you might wish to investigate. The one which comes to mind is the rechargeable electric toothbrush. This delivers enough power to a totally sealed toothbrush to charge the batteries inside. I have fairly recently seen a video of someone taking one of these apart to investigate it — here. [...]

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