Overclocking my toothbrush

Question

I use these toothbrushes: http://www.amazon.com/Oral-B-Advanced-Toothbrush-Vibrating-Bristles/dp/B00336EUV8

They tend to run out of battery quite quickly. I'm happy to keep buying more because regular toothbrushes just don't get the job done in comparison.

However I've stumbled upon a neat little trick.

I've been taking them apart to insert 10440 size Lithium batteries in place of AAA cells.

As you can imagine this gives a boost in power which is quite amazing. I'm reasonably certain that it's not too much power, I'll ask my dentist about it next time I see her, but I am pretty much hooked on the extra power. More gum stimulation is healthy right?

Anyway, the real issue is that after a short amount of time (a few days of normal usage) the motor eventually gets more and more sluggish. It would start out going quite fast, then gradually slow down. Pausing for a while, then restarting sees it regain some power.

I can of course do some more controlled tests to better determine what's going on but the behavior seems like a temperature dependent effect to me. When I perform this modification to a "new" (i.e. its alkaline cell it came with has been exhausted and I put my lithium cell in it) motor it runs nice and powerful for a few sessions of brushing before this pattern of rapid loss in performance starts to set in. I check the voltage of the battery and it is above 3.8V (not depleted).

Whether it's the battery or a degrading motor that is limiting the vibrating power, I think I should be able treat it by limiting the current in the circuit. As it is, there is no circuitry for regulating current: the battery is directly connected to the tiny DC motor.

What modifications could I make?

I have very little space to work with so it seems like I'll only be able to insert one or two small components (diode, resistor), and the circuit must remain small.

My hope is that a well adjusted modification can keep the current supplied by a lithium cell to within a range that does not degrade the motor. This way I can make the circuit once within the assembly and I can transfer that into new toothbrushes (because these need to be replaced as they are worn out).

I'll be back to post measurements of current, battery internal resistance, and maybe temperature when I get around to doing this.

Answer 1

The motor runs faster b/c you're applying a higher voltage to it; the motor has to turn faster to generate the back emf to balance the battery voltage. Now, if you limit current, and the motor runs more slowly, it will again be generating less than the battery voltage, so the remainder of the voltage will be the drop across the current limiting device or circuit. The voltage drop across the current limiter, times the current through it, represents power that is wasted.

I assume your idea is that you still want the motor to run faster than the original device, but slower than the direct lithium-to-motor configuration. But you see, the concept of limiting the current here is kind of counter productive: You added a higher voltage battery, and inserted an extra element, and then burn off power to bring the motor speed down from what it would be w/o the current limiter. The more direct solution would be to choose a battery that's not so much higher than what the motor was intended to see. I.e., instead of jumping from 1.5V to 3V, maybe try to hit something like 1.8 or 2V. Of course, batteries don't come in arbitrary voltages like that, but the point is, don't overkill it with 3V, then try to kludge it back to where a lower battery voltage would get you to directly.

Probably the best solution would be to modify the motor itself. If you rotate the brush assembly relative to the magnets, the motor's speed/torque characteristics can be changed. It requires mechanical finesse, but you might be able to mod the motor to work well at 3V. Or you might be able to outright replace the motor.

Answer 2

Have you tried using a normal recharegable AAA battey?

They're only 1.2v, but they have a lower internal resistance, so it might make up the difference. It's worth a try.

Answer 3

You are more than doubling the source voltage to the motor. TBH I doubt the motor can handle it, the brushes are probably dying. If you do add a resistor, the speed of the vibration will be reduced (which I presume you don't want). If you are open to the idea of a lower speed, add a resistor in series and vary this resistor's value till the speed is what you want. Now check if the motor lasts longer. I can't help in any other way because I have no clue where the fault it.

Answer 4

I'm guessing, but the significantly higher voltage is making the motor run too hot. It sounds like the bearings are ceasing up when hot. They might be getting significantly hot even if the overall unit is just unusually warm. The heat can hurt the lubricant, and differential thermal exapansion of different materials can make them bind.

Overdriving a motor by this amount is not a good idea. Remember that this is a high volume consumer product, so I'm sure significant engineering effort went into reducing cost so that everything is designed to work just within its limits.

Answer 5

At its core it is either a a 1.5V or 3V motor. This looks like lower end product (compared to the $150 toothbrushes) that is meant to be discarded after it is worn out. My guess is this probably a more common 3V DC motor that was being under driven. You should be fine.

If I am wrong and it does burn out your toothbrush, well you probably weren't far away from replacing it from bristle condition anyhow.

If you really are curious tear this apart when you are ready to replace and see if you can identify the motor from markings and searching online. Also would be worth while at this point to try other voltages and observe temperature/health of the motor.

You'd be surprised how tolerant low voltage DC motors are. We had a 6V DC 24mm Dia off-balanced hobby motor from some device we replaced. I hooked up my bench power supply and surprisingly it was able to handle 24V 10 minutes before getting too warm to hold. 30V killed it when the enamel burned off the windings.

I don't doubt the toothbrush can take the extra power without significantly reducing the life of all the components but i am more concerned about the increased impact on your gums. I'll tell you what i tell my wife. "If you set it down and it can jump off the shelf by itself, you need to slow it down or you are going to hurt yourself." You would be better to just to ask your dentist as already suggested. Be sure to bring it in the "overclocked" toothbrush with a fresh battery so she can determine if this is too much vibration.

Stay safe!