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.

[amyk]

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.

The moisture comes in from the top, where the shaft meets the housing it doesn't seal well. My ideal design would be a brushless motor with a rotor in the wet part and the stator inside the housing (which would not need a hole into the interior), like those used in aquarium pumps. The downside is that it requires more power and control electronics than a regular brushed DC motor. Sealing the bottom and buttons isn't difficult even with replaceable batteries, since they make waterproof torches in the same form factor with an o-ring and threaded cap.

[madworm]

Perfectly sealed toothbrush units... that's never going to happen.

Less opportunity to rip off the consumer, just like with inkjet printers that "suddenly" require maintenance because some nefarious droplet counter has reached its maximum value.

The sponge used for head cleaning might be full and overflow.

The "service" costs 5x more what a new printer costs and the fact that this "error" is software resettable is kept a secret and/or the tools are not available to the end consumer.

[saturation]

I've been using Sonicare since late 1990s, before they were bought by Phillips.  I've never had a problem with them except the batteries dying.  Its very annoying because since the discharge rate is fairly slow, the batteries can not be deep cycled or reconditioned to get them back up to speed.  So, by 2 years, you get only half the rated AH, and the battery slowly deteriorates from there. 

4 years ago, Sonicare came out with the only AA powered version; sans inductive coils and batteries the thing is cheap and lightweight.



Because the motor is identical to the sealed brushes the brush power is identical, and you don't have the charging issues.  Instead, most folks don't know how to seal o ring joints properly, leading to leaks.  Its not entirely user friendly that way, because an o ring seal must be impeccably clean for it to be watertight, else the o ring will leak where the dirt is.  That said, my brushes have served me well for over 4 years without leaking, I have 2, so that comes to nearly 8-product years of use and I submerge these brushes in 4" of water during cleaning, which is every use.

Never drop an electronic toothbrush.  They are not built with shock/vibration resilience and the extra mass of the batteries when it falls will add to the injury.  A tiny droplet size leak is an other wise water tight casing means very slow to nil drying out, and inevitable death. 

[vk6zgo]

I know this is an old thread,but I just had occasion to look at my electric toothbrush.
My daughter bought it,don't know where,it worked for about 3 minutes & stopped,never to go again.

It is a Braun,even though it also says "Oral B" on the handle.
The charger is about half the size of Dave's one

Same waveform from the charger, frequency looks to be about 25.6 kHz,but the last time the Tek 7613 was calibrated,Ronnie Reagan was in the White House!

Tried the unscrewing technique,but it just slipped around on the "peg" protruding from the charger.
Looks like it is fully sealed,so no fix,& only a fully destructive teardown possible -----bummer!

[amyk]

I know this is an old thread,but I just had occasion to look at my electric toothbrush.
My daughter bought it,don't know where,it worked for about 3 minutes & stopped,never to go again.

It is a Braun,even though it also says "Oral B" on the handle.
The charger is about half the size of Dave's one

Same waveform from the charger, frequency looks to be about 25.6 kHz,but the last time the Tek 7613 was calibrated,Ronnie Reagan was in the White House!

Tried the unscrewing technique,but it just slipped around on the "peg" protruding from the charger.
Looks like it is fully sealed,so no fix,& only a fully destructive teardown possible -----bummer!

It might just be a usual buildup of gunk around the base. They're built to be waterproof so the seal is tight even without the gunk.

[vk6zgo]

No gunk,it's virtually  brand new!

[Fezder]

just saw this video, nice! especially i liked ''scope probe-hack''

[madires]

Tried the unscrewing technique,but it just slipped around on the "peg" protruding from the charger.
Looks like it is fully sealed,so no fix,& only a fully destructive teardown possible -----bummer!

Seems to be the same model like I have (Professional 2000). Just after about 50 charge/discharge cycles the battery lasted only 4 days. At the beginning it was about 7-8 days. While searching for a way to open it (unscrewing method doesn't work) I stumbled across a recycling hint in the Japanese manual. The bottom part is fixed by a pair of snap-in thingies. You can open it by creating a torque with a large screwdriver. Afterwards it won't close as precisely as before but the sealing gasket is good enough to prevent any ingress of moisture. And it will open quite easily the next time :-) I replaced the 4/5 AA NiMH and I'm curious if the new battery will perform better than the original one after 50 cycles. 50% after 50 cycles is a bad joke! Never had such a bad rechargeable battery.

[AndersG]

Sorry for arriving late to the party, but I took apart a Braun charger and it is indeed potted, but I managed to strip the coil and ferrite and measure dimensions:

It is wound with 0.3mm wire on a bobbin OD 17mm, ID 7mm, h 5mm. Inside the bobin, there is a ferrite dia 6mm h=18,5. Plain wire, not Litz.

I created a quick and dirty Class-e/push-pull/multivibrator and wound my own coil. Image shows my mesurement pickup coil over the transmitter coil. Works a treat. It charges one of our brushes in 45 minutes and I also tested with a discarded brush. Measured the charging current which was approx 70mA into a rather tired battery with the original charger and 100mA with mine.




Now, why would anyone do such a daft thing? Well, I need a charger that runs off 12V in my boat

I also need to be able to turn it off after charging. The resulting output is much cleaner than the original. First overtone at 60kHz is -36dB down and second if -26dB down.

[Gyro]

A bit of a blast from the past, but I don't think Dave ever managed to complete this teardown due to the charging base being potted. I realised that I had a Lidl charging base for a rechargeable PIR night light / torch in my pile which isn't potted. Although the circuit may not be identical, it shares the same centre spigot arrangement, so it is likely that the coupling coil / inductor is similar.

In the Lidl torch, the coupling receiver is an identically sized inductor, but with a smaller number of turns of thicker gauge wire, as would be expected. I tested it with an old Braun toothbrush, with its larger diameter air cored coil, and the charging performance seems to be the same, so it's likely that it is a similar circuit.

The charging base uses a single transistor oscillator with a capacitive mains dropper. It's likely that the supply voltage to the oscillator varies quite a bit depending on the magnetic loading, the reservoir capacitor is rated at 400V and the transistor has a V_CEO of 450V. It's a bit surprising that such a simple circuit can tolerate such a wide load range without running into problems. Operating frequency is 143kHz off load and 166kHz on load (Braun toothbrush). The waveform seems fairly clean with some flattening at the base of the sine wave.

Anyway, the schematic and teardown photos are attached. I included most of the component values - it didn't seem worth desoldering the MLCCs at I'm thinking of using it as part of a high isolation PSU converter (https://www.eevblog.com/forum/metrology/power-supply-for-voltage-references/msg3398466/#msg3398466). I did note that the mains caps (and specifically C8) are not X rated, but at least look to be reasonable quality and are down-stream of a low current fuse. C8 also lacks a discharge resistor, relying on to current drain of the oscillator to bleed off its residual charge.

Hopefully of interest to someone.


EDIT:  Just looking back again at Dave's video, I noticed that the charging waveform on his Braun base was at significantly lower frequency, around 22kHz and with significant ringing. Maybe the higher operating frequency of the Lidl base accounts for the lack of observed ringing.

[Marty1]

Also sorry for arriving late, but this breakdown was the best, that I could find, but leaves me with some questions, that you might easily know the answer to.

IHAQ: Does charging of the internal NiHM battery terminate when it is full or does it charge for a fixed amount of time?

My findings and the reason for asking:

I'm using my third Braun-Oral-B electronic toothbrush since 2017 now. They were all of the type: 4729 (Oral-B PRO 600) with charger type 3757. They tend to last for 2 years and 10 months. Then the internal NiMH battery life gets so bad, that it's annoying.

After about 300 charge cycles the NiMH battery (a 4/5 AA Sanyo ) has degraded for about 60%.

The 4729 toothbrush always charges for 18 hours indifferently from when its battery was new versus now that the battery is old and degraded. It's the time that the green charging light keeps blinking for and then stops.

The old battery now has less than half the run time (9 minutes) versus when it was new (27:30 min).

Because the now wearing battery can hold less charge, it should be charged lesser/shorter. Otherwise it will overcharge and and overcharging degrades the internal NiMH battery even faster.

Question 1:
Will charging be automatically terminated when the battery is filled, or will it bluntly charge for a fixed 18 hours. Thereby overcharging the now aging battery and wear it out at an ever faster rate?

Question 2:
Because I don't want to buy a new tooth brush every 2 years, I now want to replace and upgrade its battery.
When I would upgrade from the standard 1400 mAh battery to a 2450 mAh  AA sized NiMH battery, will the standard 18 hours charge time be sufficient to load the new battery completely? Will charge termination take place when the battery is full? Or can I better opt for a 1900 mAh one that can take twice the abuse due to overcharging, but has a shorter run time?

[madires]

From my experience the Oral-B toothbrushes seem to have a standard charge control. Going for more mAh should work fine. BTW, I've switched to a LiIon model and I'm happy with it so far.

[Marty1]

From my experience the Oral-B toothbrushes seem to have a standard charge control. Going for more mAh should work fine. BTW, I've switched to a LiIon model and I'm happy with it so far.

Thanks for your reply. What I'm trying to determine is if the Pro 600 (type: 4729), will keep receiving a charging feed, even when the (already for 50% deteriorated) battery is full?
I'm under the impression that it keeps receiving a charge feed, because the greenly blinking indicator light keeps blinking for 18 hours. It did that when it was new, and now that it has a 50% deteriorated battery, the charging light still keeps blinking for 18 hours when charging.

[madires]

IIRC, my old NiMH Oral-Bs charged for about 15h.

[Marty1]

IIRC, my old NiMH Oral-Bs charged for about 15h.

If there is a charge controller active, then it should terminate charging, when the battery is full.
This is not what I observed. When the battery was new it charged for 18 hours and now that the battery can hold only 50% of its capacity, it is still is being charged for 18 hours.
Then I conclude that there is no charging controller active. Otherwise the 18 hours of charging when the battery was new should be halfed to 9 hours now that the battery lost 50% of its capacity.
The NiMH battery will therefore be overcharged for the remaining 9 hours. Because of over charging the battery will be damaged and loose its remaining capacity in an ever increasing rate.
Can anybody confirm this?

A lifetime of 300 charge cycles for a Sanyo 1400 mAh NiMH battery seem kind of short to me. When overcharging on a regular basis occurs, then that would explain it.

[Marty1]

The information I'm looking for can't be found, so I devised the following experiment in order to gather new information.

The hypothesis, is that the toothbrush is being overcharged, which wears out the battery at en ever increasing rate.

I halved the charge time of the brush and measured how long it would run for.

Low and behold, the runtime was the same with half the charge time.

It charged for 9 hours ((instead of 18 hours) and pulled it off the base / charger while the green charging indicator was still blinking.

The run time that I got after that was identical to when it was charged until the automatic cut-off /  termination after 18 hours of charging (green blinking indicator light).