EEVblog #284 - Braun Toothbrush Teardown

[stryker]

Thanks so much for doing this Dave.  I've wondered for a long time what was inside my toothbrush!

A question on the inductive charge coils tho.  At one time you had the CRO hooked up to both coils, and the waveforms were a mirror of each other.  How is it that the induction effects don't cancel each other out?

Cheers ! Geoff

[madworm]

Nice!

Here's some more data ;-) I've got the same charger, only my "pickup coil" was inversed.

[TbayBoy]

Exactly what I have been wondering about, inductive charging.. thanks (I'm still searching out research material)

[NiHaoMike]

I believe the nearly flat bottom is from a transistor turning on. I have taken apart a similar toothbrush and it basically used a self oscillating converter.

I also suspect the motor was also operating as a boost converter, since 1.5V is not enough to effectively switch a typical MOSFET. High quality pictures would help.

[IanB]

I haven't watched the video yet, but I HATE electric toothbrush designs (really hate). The reason? Somehow they don't seal them properly and moisture (condensation) gets inside and stops the electronics working. I mean, they know that electric toothbrushes are going to be subjected to immersion and steam, so why don't they make absolutely sure the enclosure is hermetically sealed?

Alright, I have to admit this invective is directed towards Philips toothbrushes, and Braun may be better. I just have to hope that Braun have got it right. But with the ridiculous prices of rechargeable brushes I can't afford to experiment. Gaaarrrhhh!

[Psi]

I haven't watched the video yet, but I HATE electric toothbrush designs (really hate). The reason? Somehow they don't seal them properly and moisture (condensation) gets inside and stops the electronics working. I mean, they know that electric toothbrushes are going to be subjected to immersion and steam, so why don't they make absolutely sure the enclosure is hermetically sealed?

Alright, I have to admit this invective is directed towards Philips toothbrushes, and Braun may be better. I just have to hope that Braun have got it right. But with the ridiculous prices of rechargeable brushes I can't afford to experiment. Gaaarrrhhh!

Yeah, when it comes to electric toothbrushes ya really have to buy quality.
As you say, the cheap ones just die from moisture getting inside.
I went through 2 of the cheap ones before i got a proper sealed one.

[IanB]

It's frustrating watching these videos! You want to shout at the screen sometimes.

Dave, apparently the instructions for taking it apart and disposing of the battery have two steps:

Step 1: twist the handle on the base (what you did)
Step 2: invert the handle and push the pointy end down on a hard surface (what you didn't read) -- it implies the insides and battery will then pop out the end

[EEVblog]

It's frustrating watching these videos! You want to shout at the screen sometimes.
Dave, apparently the instructions for taking it apart and disposing of the battery have two steps:
Step 1: twist the handle on the base (what you did)
Step 2: invert the handle and push the pointy end down on a hard surface (what you didn't read) -- it implies the insides and battery will then pop out the end

I did read it, it didn't work 

Dave.

[nitro2k01]

To me it's pretty clear what's going on with the double coils. It's a center-tap transformer. Using half-wave rectification is just generally ugly. Let's not go there. Using a full bridge rectifier requires four diodes, i.e. 2*2 diodes in the current path, with their associated voltage drops. A center tap transformer only requires 2*1 diode in the current path, so you shave off a few tens of a volt from the transferred voltage.

As for current limiting, this might very well be done on the primary side, which would also (hopefully) prevent damage to say your mobile phone if you held it up against the coil. This is probably also why this pulsing charge strategy is in place, to let the microcontroller's capacitor recharge every now and then. The issue would not just be low supply voltage, but preventing ripple on the microcontroller rail, which it is implied if the cap inside diode is charged higher than the peak ripple voltage outside the diode at all times.

[IanB]

I think Dave determined that it was a centre tapped coil with full wave rectification--he mentioned it in passing later on, but didn't really go into it.

Also I think the gaps in the charging pulses are most likely to give a steady voltage reading when the micro is checking the battery voltage.

With 100 mA of charge current on an AA NiMH cell there is no need for any particular charge current regulation. It's way below the current that would lead to any overcharging issues. Most likely the micro just stops charging when the cell voltage reaches a particular value like 1.45 V.

Lastly I noticed signs of moisture ingress when Dave took the bottom off. It didn't look like it was very well sealed.

[david77]

Yeah, there was a bit of crud around the bottom of the brush. But I guess the only way they could make it absolutely watertight is to seal the case completely in wich case you could not replace the battery. The electronics in Dave's brush still work so it doesn't seem to be a big problem.

I think it's a good thing they potted the charger thing. It's going to sit on your hand basin or quite close to it in the bathroom. It might get dropped into water wich could be very nasty. Not everybody's got RCD protected outlets!

[mikeselectricstuff]

I wonder if that chip included a charge pump to give more MOSFET drive voltage - there seemed to be a few caps above it, and 1V is pretty low to turn a MOSFET  hard on.
 

[IanB]

Actually the battery is not replaceable in those toothbrushes. The battery removal is for recycling and disposal according to EU regulations. Once you twist the bottom off it is broken and does not go back together again.

[david77]

Are you sure, Ian? I've replaced a couple of batteries in similar toothbrushes.

[IanB]

Well the battery is not "officially" replaceable. You can see how the metal strips were welded to the battery terminals. That's not to say you can't do your own home brew battery replacement of course, and then glue the case back together.

[G7PSK]

I have taken a Braun tooth brush apart and put it together again more than once. You need to push the shaft in fairly hard you may even have to stab it down onto the bench top. The whole inside will slide out,what happens is a build up of residue from toothpaste which creeps inside this is often the cause of failure it gets into the switch and other contacts. 

[Ferroto]

The reason the chips cannot be read with the camera unless the light is at a specific angle is because directly shining the light at the object at the same angle as the camera causes all shadows to disappear and the full intensity of the light to hit the object. This makes it difficult for the camera to contrast different shades.

You might want to try reflecting the light off a surface such as a wall or mirror and indirectly light the subject this will allow the subject you are filming to have greater contrast.

http://www.photography.com/articles/taking-photos/bounce-lighting/

[NiHaoMike]

Yeah, there was a bit of crud around the bottom of the brush. But I guess the only way they could make it absolutely watertight is to seal the case completely in wich case you could not replace the battery. The electronics in Dave's brush still work so it doesn't seem to be a big problem.

I think it's a good thing they potted the charger thing. It's going to sit on your hand basin or quite close to it in the bathroom. It might get dropped into water wich could be very nasty. Not everybody's got RCD protected outlets!

Sonicares use a resonant linear motor so they can be fully sealed.

I wonder if it could be possible to use a supercap along with the resonant linear motor and capacitive touch sensing so the entire toothbrush (except the replaceable tips) could be potted with no moving parts and therefore last almost forever.

[IanB]

Sonicares use a resonant linear motor so they can be fully sealed.

That is the theory. Unfortunately they don't seem to seal them properly even though the design suggests they should be capable of full sealing. I have had two Sonicares in succession fail due to moisture ingress. I learned the risk and I am very careful to keep the third Sonicare in a dry environment.

[MikeK]

That is the theory. Unfortunately they don't seem to seal them properly even though the design suggests they should be capable of full sealing. I have had two Sonicares in succession fail due to moisture ingress. I learned the risk and I am very careful to keep the third Sonicare in a dry environment.

You're not supposed to use them in the shower.

[madworm]

I have had two Sonicares in succession fail due to moisture ingress.

Me too ;-( Now I've got a cheaper Braun one. No more sonic and playing melodies.


chip rot

The bottom part was terribly affected by moisture. It refused to charge at some point. Took it apart and salvaged the mosfet chip and two buttons. Planned obsolescence. There's a documentary with that name as well. It covers the issue of light bulbs burning out way too fast and many other things.

[mikeselectricstuff]

I wonder if it could be possible to use a supercap along with the resonant linear motor and capacitive touch sensing so the entire toothbrush (except the replaceable tips) could be potted with no moving parts and therefore last almost forever.

Capsensing would be tricky in a wet environment.

[SeanB]

Still trying to figure out what is wrong with the old wooden stick with fluff on it, or the plastic equivalent. Cheap, waterproof and disposable without any ROHS stuff.

But then again if it was potted, and used a reed switch with a small ferrite magnet to operate it it might last longer, but then they would sell less.

[MartinX]

Very interesting to see the waveform from the coil, I wonder if it changes if you put back the ferrite beads, they have to do something, I bet it has to do with EMC emissions.

[JoannaK]

Very interesting to see the waveform from the coil, I wonder if it changes if you put back the ferrite beads, they have to do something, I bet it has to do with EMC emissions.

I think they might be used for circuit tuning... I did my Master's degree (IT, embedded systems) at wireless power/data, and I noticed how important proper tuning of send/receive elements (coils+caps) were for power trasfer.