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constant current power supply

Discussion in 'Electronic Design' started by [email protected], Aug 1, 2013.

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  1. Guest

    Perhaps this isn't the forum for this but I thought I'd try. I have three Norelco electric shavers. Two are rechargeable types with built in chargers,and two 1.2volt nicads. I've replaced the batteries in both of these portable units several times over the years and now it seems like neither one will charge it's batteries anymore.

    The third one uses the same 2.4 volt motor but it runs off 120VAC. That onehas a bad power supply as well. I have tried to repair these shavers but Ithink the problem on each one is in the hybrid circuit and/or other proprietor y parts and since these are quite old I'm pretty certain that no partswould be available.

    So what I would like to do is simply install two batteries in each shaver and build a 50ma. constant current charger to use with each. I have been charging my AA nicad's and NMIH's like this for years without ever having any problems. I can remember to take them off charge after 14 hours or so.

    Can anyone please suggest a circuit idea that might fit the bill, or perhaps point me in the right direction? Although I rather like it, my wife is getting tired of the beard. Thanks, Lenny
  2. Ecnerwal

    Ecnerwal Guest

    There are many possible ways. I favor the brute force and simplicity of
    using an LM317 adjustable voltage regulator in constant current mode.
    It's probably not cheap enough for Jeorg...

    Rather than try to ASCII sketch it here, I'll suggest you search for
    that - it in the data sheet, and it's available multiple other places on
    the web. You will need some sort of basic DC supply (roughly between 7
    and 40 volts - 12 is often easy to find) to feed it - some sort of
    wall-wart or computer supply that's disused is usually the
    cost-effective approach rather than building one from scratch.

    You could even get fancy and put a shut-off timer on it; or not.
  3. mike

    mike Guest

    Use a 6V wall wart and a series incandescent lamp to set the current.
    Gives indication of charging.
    Fails open circuit == safe.
  4. miso

    miso Guest

    Just get a new Norelco with a lithium battery. I'm using the "James Bond
    Spectra" edition and still get about 140 minutes per charge. Over the
    decade, blades probably add up to the cost of the razor.

    Back in the day I also replaced the nicads, but only once per device. I
    think you would get about 5 years on the nicads, but after a decade you
    might as well get improved technology.

    Norelco has great price fixing on those blades. Close to $30, while
    their top of the line razors are about $160 on sale, sometimes packages
    with a spare set of blades.
  5. Guest

    I found a circuit. Thanks everyone for the tips. I figure a 56 ohm resistoron the LM317 circuit should do it. Lenny
  6. Guest

  7. ehsjr

    ehsjr Guest

  8. Guest

    You're correct about the 25 ohm resistor Ed. I built my charger circuit using an LM317T. I set the current for .05A with a 22 ohm resistor, and it works quite well with a supply of 12V +/- 50 %. So charging the shaver's
    batteries directly from a an automobile cigarette lighter jack would work fine. And I can easily make that an option, however I would prefer to have the shaver work with the supplied coil cord as originally intended, and that is from 120VAC.

    I'm sure that I can figure a way to drop the AC line down to a safe operating level and then (hot), rectify and filter the DC to run the IC. And although Norelco was doing this, and as far as I can tell without power line isolation, I would not feel comfortable with that arrangment.

    A 12 volt transformer would be the most simple way to accomplish what I'm trying to do, but I don't think I've ever seen one small enough to fit inside the shaver. I have enough room for something that would measure about a one inch cube.

    So, this probably sounds like a stupid question, but I have to ask. Does a 120V 60HZ transformer this small even exist? This primary supply would not even have to be regulated. I'm not experienced enough to try to design a switcher for this application. I would just like to keep this real simple if possible. Of course if I had a bit more real estate to work with this wouldbe a non issue. Thanks for any further assistance. Lenny
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