"Convert" Electrical Shaver To Charge Via USB [solved]

[denisab85]

Danke schoen, Darkwing!

My results are just perfect: the charger has been found in a bag of all-this-odd-electric-stuff today in the morning.
As my girlfriend has said, one needs to say goodbye to a lost thing in order to find it ))))

[Darkwing]

Ha! Marvelous.

[futile]

@Darkwing

I'm looking to do the same as you for my Philips Norelco Oneblade. It uses a 4.3v 70ma power adapter, so I think I should beable to follow your steps with the Resister to get the same results. I gather the resister will reduce the voltage, and I'm assuming the shaver will only draw the current it needs to charge.

I did have a few questions for you before I attempt this:

• Whats the point of the LED in your setup?
• Does it matter what power is 'available' over the USB adapter (i.e.. some usb adapters can provide 2.1A others 1A or 500ma)?

cheers,
Futile

[mariush]

You could use a fixed 4.3v linear regulator like this one, for example:  http://www.digikey.com/product-detail/en/texas-instruments/TPS73643DBVR/296-17498-1-ND/698477

Connect input and enable pins to 5v, ground to ground and output to output and done. This linear regulator doesn't even require additional capacitors at input and output, adjust resistors are built inside the chip, truly a one chip solution.

If you don't mind soldering some ceramic resistors and capacitors, a MIC5378 is 25 cents : http://www.digikey.com/product-detail/en/microchip-technology/MIC5378YC5-TR/1611-MIC5378YC5-CT-ND/5700662

[Darkwing]

@futile:

1. The LED is just for showing the "status" of the system. It let's me know whether current is flowing or not. It's in parallel to the resistor, that's important.

2. The maximum current supply of a USB port is not important, don't worry about it. USB always is (more or less) 5V and this is what counts. Current follows voltage and the current limiting resitor drops the voltage to let the desired amount of 70mA flow (Ohm's Law).

Hope that helps!

[futile]

@Darkwing
Awesome thanks for answering my questions. I was just a little concerned that different source usb adapters might change the 'ma' provided to the shaver and cause a problem. It sounds like it shouldn't be an issue though. I'm hoping to try and put it all together tomorrow or saturday. Can't wait to not have to travel with that bulky shaving plug!

I'll let you know how it goes, and post some images if all goes well.

@Mariush
Thanks for the alternative options, I'll definitely give it a try if the resister doesn't work out. I already have those to hand so will give em a shot first.

[futile]

So I don't have any background in electronics, but have played with USB and a multimeter before, so forgive me if I'm a little out of my depth.

After testing the official adapter itself. I seem to actually get around 6.12v at 85ma. Which is not the 4.3v 70ma that is advertised on the adapter casing. I think thats similar to what you got on yours though. I guess the shaver might be ok with a range of voltages and currents. I'm not sure how to test it while its plugged into my shaver.

Also, the back of my shaver states 4.3v 0.5w, the same that yours did.

I hooked up the 27ohm resistor (same as yours) inline on the cable just to test it out, and I appear to get 5.18v at 180ma. I tried 2 different USB wall adapters to power the cable, with the same results. So I guess thats all good, however I'm not sure why I'm seeing 180ma instead of 94ma, any ideas?



Am I to understand that the shaver should in affect only consume 2.2-2.3v when plugged in, and therefore the current would actually be less?

I was a bit fearful of plugging it into the shaver with the 180ma showing up, so any advise/assurance would be much appreciated.

[futile]

I did a bit more reading on ohms law and decided to try out a 68ohm resistor in place of the 27ohm one. Now I get 5.18v at 76ma, which seems maybe more what I wanted, but I probably need to better understand why you went with the 27ohm and the relevance of the 2.2-2.3v draw when shaver is plugged in.

updated:
After further reading on the subject (http://www.wikihow.com/Calculate-Voltage-Across-a-Resistor) I get how voltage drop applies over multiple resistors but couldn't see how it could happen with one resistor. I know it does because of those simple LED circuits where you need to get the LED working with a 9v battery.

I came across this posting (http://www.instructables.com/answers/Reducing-voltage-in-a-circut-for-the-layman/) that seemed to suggest you need to calculate the voltage drop and divide it by the needed current to get the ohms resistance. This online calc seems to use that calculation too (http://ledcalc.com).

voltage drop / needed current = ohms resistance

Using that calculation though, it would seem to suggest I would need a 12.57ohm (15ohm) resistance to provide 4.3v at 70ma from my 5.18v usb source.

But using the 27ohm resistor would result in 32ma:
0.88v / 27ohm = 0.032amps

So there a re a few bits I guess I'm confused on...

• Am I on the right track with the calculations, and do I potentially need a different resistor to emulate my shaver adapter?
• Why does my multimeter still measure the voltage at 5.18v (no drop), but instead the current is limited?

I think I'm probably just missing the fundamentals in electronics, but looking about didn't find a clear answer as to 'how a single resistor can provide a lower voltage', given that it seems to instead limit 'current'.

Any answers or suggested readings would be greatly appreciated.

[ajai.husin]

The 27 Ohms produce a very accurate current at 5V. The original power supply delivered 92 mA, we now have 94 mA – I'd say that's spot on. Ohm's law works quite well sometimes. 


[/quote]

Weeks later … *imaginejeopardymelody* 

Let me report quite a success on this!
I finally managed to make that cable and – surprise – it really does work just with a single resistor. Here it is:



I added some kind of status LED in parallel, whose current limiting resitor is measured in a way, that it is slightly over the verge of being 'on' at 5V. Below 4.8V its off and a bit higher as 5V it begins to shine very very brightly. It's like a little visual control for the USB power being 'ok' or 'not so ok'.



The connector to the Phillips I filed from an oldschool Molex harddrive plug. ^^



Here it is in action.



The 27 Ohms produce a very accurate current at 5V. The original power supply delivered 92 mA, we now have 94 mA – I'd say that's spot on. Ohm's law works quite well sometimes. 



Thanks to y'all for your suggestions on this little hardware-hacking project! 

Why my calculation is different outcome. Did i lost something here >> 5V/27ohm=0.185185A or 185mA and not 92mA?

[iainwhite]

Why my calculation is different outcome. Did i lost something here >> 5V/27ohm=0.185185A or 185mA and not 92mA?

Your calculation is only considering the cable, not the load circuit in the shaver.

Earlier in the thread, the OP said that he had calculated that the shaver needed about 2.2-2.3v to charge at 80-90mA - let's take values of 2.3v and 90mA - which equates to a resistance of 25.5 ohms.   The cable and the load form a voltage divider with resistances of 27 and 25.5 ohms.

Probably not the best explanation as I have no formal EE training, but you can hopefully see what I am getting at?