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Thread: Reconfiguring joule thief for different input/output

  1. #1
    Flashaholic* Fallingwater's Avatar
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    Default Reconfiguring joule thief for different input/output

    I have a small 2xAAA, 3-5mm-LED light (this one) that has no driving circuitry whatsoever - it just hooks the LEDs straight to the two cells. This, of course, gives abysmal output.
    I'm looking into building a small circuit to drive the three LEDs at about 30 or 40 mA each from fresh cells. Now, I know how to build a joule thief, but the stock design is only good for 1.2/1.5V input and output for one LED.
    How do I modify it so that it works for my application?
    Thanks.
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  2. #2
    Flashaholic nzbazza's Avatar
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    Default Re: Reconfiguring joule thief for different input/output

    Is that a torch in your pocket or are you just pleased to see me?

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    *Flashaholic* Mr Happy's Avatar
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    Default Re: Reconfiguring joule thief for different input/output

    The circuit is very malleable. I'd suggest trying it with 3 V input and put the three LEDs in series on the output. Experiment a bit and see how it goes. You might have to try different numbers of turns and different cores for the windings to see what works best.

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    Default Re: Reconfiguring joule thief for different input/output

    I'd love to see what works for you as I have the same dilemma. I've never tried assembling a joule thief before but there's always a first.

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    Default Re: Reconfiguring joule thief for different input/output

    You might try this circuit which is more efficient than the plain Joule thief. I used it to modify a bike light, see this thread.

    Although the above page is in Czech, the figure captions are in English and you should have no trouble building the circuit. You could try to decrease the value of R1 to set the output current to your liking (I used 8 ohms to get 18 mA which makes my two Nichia GS in series very bright).

    An English-speaking Czech colleague kindly translated the page for me (but I don't have the result here at hand). The most interesting information is that you can make the circuit even more efficient by replacing the BD433 transistor by a more bulky BD243C.

  6. #6
    Flashaholic* Fallingwater's Avatar
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    Default Re: Reconfiguring joule thief for different input/output

    Quote Originally Posted by Mr Happy View Post
    The circuit is very malleable. I'd suggest trying it with 3 V input and put the three LEDs in series on the output. Experiment a bit and see how it goes. You might have to try different numbers of turns and different cores for the windings to see what works best.
    Will do, thanks

    Cemoi: I'm not sure whether your circuit can fit in the small space available inside the light. Pretty sure a simple joule thief can though...
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  7. #7
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    Default Re: Reconfiguring joule thief for different input/output

    Quote Originally Posted by Fallingwater View Post
    I'm not sure whether your circuit can fit in the small space available inside the light.
    I managed to make it fit on a 0.6"x1.4" (1.5cm x 3.6 cm) PCB but it was tricky, and this is maybe still too big to fit in your light.

  8. #8
    Flashaholic* Fallingwater's Avatar
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    Default Re: Reconfiguring joule thief for different input/output

    Quote Originally Posted by Mr Happy View Post
    You might have to try different numbers of turns and different cores for the windings to see what works best.
    Could you specify a bit? Do I just wrap more/less wire (and what effects does it have)? Do I have to wrap a different number of windings for the two wires?
    Eat, drink and be merry, for tomorrow you may die.

  9. #9
    *Flashaholic* Mr Happy's Avatar
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    Default Re: Reconfiguring joule thief for different input/output

    No, I can't really tell you much more, I don't have enough knowledge of the subject. For more information you would have to read up on inductor design and winding calculations.

    What I do know is that there is a bit of an art to tuning such circuits. You can't always tell how they are going to work until you try them out, and the number of turns on a coil is something you may have to experiment with to get the best results. For a typical ferrite core or bead, I would guess that a number of turns between 20 and 60 would be a reasonable range to experiment with. To reduce the number of variables, I would keep the number of turns equal on both windings.

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    Flashaholic* FRANKVZ's Avatar
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    Default Re: Reconfiguring joule thief for different input/output

    Quote Originally Posted by Cemoi View Post
    You might try this circuit which is more efficient than the plain Joule thief. I used it to modify a bike light, see this thread.

    Although the above page is in Czech, the figure captions are in English and you should have no trouble building the circuit. You could try to decrease the value of R1 to set the output current to your liking (I used 8 ohms to get 18 mA which makes my two Nichia GS in series very bright).

    An English-speaking Czech colleague kindly translated the page for me (but I don't have the result here at hand). The most interesting information is that you can make the circuit even more efficient by replacing the BD433 transistor by a more bulky BD243C.
    Translated: http://translate.google.com/translat...hl=en&ie=UTF-8

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