LED Joule thief type circuit help

Discussion in 'The Projects Forum' started by electronicsnewbie, Dec 7, 2009.

  1. electronicsnewbie

    Thread Starter New Member

    Dec 7, 2009
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    Hi,
    first post here :) I was wondering if somebody could help me with this little circuit. It a joule thief type circuit that powers a 3V led from a 1.5 battery. I have built the circuit and it works almost as it should but it's not quite as bright as it should be,the LED is brightish, but not brilliantly bright like a white LED should be. I would like some suggestions for component values to change, to make the LED brighter.

    Here is the circuit I used (I did not design it, found it somewhere on the Internet )
    [​IMG]
    Thanks for any help.
     
  2. whatsthatsmell

    Active Member

    Oct 9, 2009
    102
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    What are the specs of the led you are using? (forward voltage, current draw and mcd)

    Have you used this type led before in another circuit and was it brighter?

    Have you measured the voltage in the circuit across the led?
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    Since the signal is pulsing then an oscilloscope must be used.
    Maybe the choke has a resistance that is too high which limits the peak current.
     
  4. electronicsnewbie

    Thread Starter New Member

    Dec 7, 2009
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    Thanks for your reply. At the moment I don't have any "constants", because I'm using an LED I got out of a torch(took two AA's) and I don't (currently) have a multimeter though that soon will be rectified. But I am 100% certain that the LED should be brighter than it is. Also I tried a differen't orange led and that isn't quite bright enough.

    Cheers,
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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  6. BMorse

    Senior Member

    Sep 26, 2009
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  7. electronicsnewbie

    Thread Starter New Member

    Dec 7, 2009
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    Yes I would like that circuit if you have it please!
     
  8. BMorse

    Senior Member

    Sep 26, 2009
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    It is similar to the one you posted, but this one is more efficient.... and can be built to be really small if the "dead bug" style is used (see pics)...
     
    Last edited: Dec 7, 2009
  9. hgmjr

    Moderator

    Jan 28, 2005
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    What is the part number for your inductor? I strongly suspect that it has a significant DC resistance. Measure its DC resistance with your new voltmeter once you have it. First measure the resistance of your meter leads then measure the resistance of your inductor. The difference between the two readings is the DC resistance of the inductor.

    For this circuit to work properly you need to use an inductor with a DC resistance of less than 1 ohm.

    hgmjr
     
  10. hgmjr

    Moderator

    Jan 28, 2005
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    Here is a link to an inductor that would fill the requirements of low DC resistance.

    This inductor or an equivalent one should make a big improvement in the efficiency of your LED driver.

    All of the alternate inductor-based circuits that have been suggested will require a low DC resistance inductor for them to operate at their peak efficency.

    hgmjr
     
  11. hgmjr

    Moderator

    Jan 28, 2005
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    Of course if you are really industrious, you may want to try winding your own inductor. All you need is some coil wire or magnet wire of 30 gauge or so and a toroid with an outer diameter of a half inch or thereabouts. The inductance is not overly critical in the circuit you have chosen. 30 to 50 turns on a suitable toroid should do the trick. Most Radio Shack stores has some toroids and magnet wire that I have found to work fairly well.

    hgmjr
     
  12. Wendy

    Moderator

    Mar 24, 2008
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    The last experiment I finished using a CMOS 555 shows how to make an inductor. It is also realistic in that I did a really crappy job!

    CMOS 555 Long Duration LED Flyback Flasher

    A question for BMorse, do you mind in I turn that schematic into a AAC experiment? As a suggestion I'd look into some plastic battery holders for your breadboard, I'm buying a small selection myself, they have the single battery versions.

    It is definitely simple enough for newbies.
     
  13. hgmjr

    Moderator

    Jan 28, 2005
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    Bill,

    I agree with you that the joule thief is an excellent and elegant circuit for beginners. It is a very forgiving circuit with regard to the component values used. It also provides some very valuable experience in designing with inductors. If you understand how a Joule Thief circuit works you will have acquired valuable insight into the fundamental operation of a switched mode power supply.

    hgmjr
     
  14. electronicsnewbie

    Thread Starter New Member

    Dec 7, 2009
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  15. hgmjr

    Moderator

    Jan 28, 2005
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    The inductor you have is most likely one that has a fairly high DC resistance although I can't see any actual details on the specifications in the link.

    hgmjr
     
    Last edited: Dec 8, 2009
  16. BMorse

    Senior Member

    Sep 26, 2009
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    No, I do not mind at all..... go ahead if it will help others.....:rolleyes:
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    My solar garden lights use an inductor like that. Their LEDs are fairly bright and have a very wide angle for the light beams.
     
  18. DickCappels

    Moderator

    Aug 21, 2008
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  19. tkng211

    Well-Known Member

    Jan 4, 2008
    65
    2
    Suggest you replace the resistor R2 with a 1K Ohm trimmer connected in series with a 220 Ohm resistor (or experiment with different values of resistor, 560, 680, 820, 1K, 1.2K, 1.5K) to see how it affects the performance of the LED. After you get a more satisfactory result, you can further adjust the value of R1 to get better result if possible.
    Make sure that the transistors are connected correctly and the LED has not been damaged by the reverse voltage from the inductor. Maybe you can also add
    a diode 1N4148 connected in parallel with the LED but in reverse direction to protect the LED from breaking down by the reverse induced voltage .
     
  20. BMorse

    Senior Member

    Sep 26, 2009
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    The components selected for this circuit I posted are optimum for its efficiency any deviation from the circuit could result in poor performance/and or low LED luminosity.
     
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