help with joule thief

Discussion in 'General Electronics Chat' started by lokeycmos, Jul 31, 2011.

  1. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    so i built this circuit and a hand wound transformer. doesnt work. im getting the exact same voltage out as im putting in. i wound the coil from directions from make magazine. he said 7 to 10 turns. what it looks like is the transistor is not ocsilating. it appears that the battery in is going straight through the secondary winding and going to the load back to ground as if the transistor is completely off. i attached a couple pics for you. TY

    http://www.evilmadscientist.com/article.php/joulethief
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Can you take another picture and this time uncover the transistor? A clear close-up would also help.

    hgmjr
     
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
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    The photo you took is not sharp enough but it looks like the orange wire (that came from the toroid via the green wire) is going to the left side of the transistor (if I would hold it in front of me). On the link you sent the left transistor terminal is indicated as E = emitter.

    Question: Is your transistor inverted?
     
  4. iONic

    AAC Fanatic!

    Nov 16, 2007
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    If your transistor is the 2n3904 it's pinout is EBC, looking at it facing the flat side. It looks like you have the collector connected to ground.

    praondevou, you responded .263ns faster than me! The darn server must be located in Canada and gave you preferential treatment.
     
    Last edited: Aug 1, 2011
  5. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    i did get it working. i can light up an led from .5 volts. im kinda confused about my measurements. with .5v in i measure .4 across the led even tho it is fully lit. i know the transistor is oscillating. my frequency meter shows 130khz from base to ground. can someone explain my confilicting measurements? TY
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    You are probably using a DVM which is measuring a value that is closer to the average DC voltage?

    If you want to try something neat, disconnect the cathode of the LED and connect it to the positive power rail. The LED will still light.

    hgmjr
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    The Joule Thief spends most of its' time storing energy in the transformer, and then dumps it in a short blast through the LED. You measure such a low voltage because the voltage is only present for a very short period of time. It happens so rapidly (130kHz) that it looks to you like it's on continuously.
     
  8. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    You could add a peak detector to allow measuring with DVM. Use a diode & filter cap across LED, cathode to + of cap.
     
  9. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
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    could you give me a rough idea of what size cap to use?
     
    Last edited: Aug 1, 2011
  10. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
    432
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    i used a 1n4148 diode and i tried a 10uf, a 1uf and a .1uf cap. not getting the results i expected. im very interested in figuring out how to acuratly measure the output. if nothing else this is a great learning experience.
     
  11. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    i am completely dumbfounded. analog meter gives me the same readings in and out. the circuit works because .5v lights the led,but it would be very nice to know the output so i can recharge batteries with several almost dead batteries.
     
  12. iONic

    AAC Fanatic!

    Nov 16, 2007
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    The circuit works good with alkaline batteries that no longer power other devices and therefore draining the battery very low is not a concern. If you are using rechargeable 1.2V batteries there is no easy way to indicate that the battery needs recharging other than to measure the battery voltage itself. The voltage drop across the LED is not a good indication of the battery voltage.
     
  13. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    what i meant was using almost dead alkalines to charge nimh batteries. thats where it would be nice to know the output
     
  14. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    The circuit may not work to charge a battery. Once a charge builds on the battery needing recharge, it would have an EMF which opposes the charging and charging would stop. Somebody correct me if I've got it wrong.
     
  15. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Not sure. Sgt!
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Well, what the Joule Thief lacks in efficiency, it makes up for in simplicity.

    Meaning that it's easy to make, but only about 60% efficient - but if the battery is run down too much to run your other items already, you're really not losing much.

    While a Joule Thief will keep an LED lit with a nearly exhausted battery, the current that the LED is getting is very small; it would not be worth trying to charge NiMH or NiCD batteries with it.
     
  17. iONic

    AAC Fanatic!

    Nov 16, 2007
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    My thoughts exactly, you would probably need several run down batteries and several days!!! Maybe it could be used as a trickle charger for NiMH??
     
  18. lokeycmos

    Thread Starter Active Member

    Apr 3, 2009
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    im almost too embarrased to post this. i feel dumb. using the cap and diode to measure peak voltage DOES work. i was putting my DVM across the cap AND the diode. if i put meter across just the cap i get 3 volts. it is a good learning experience! thank you for all your help!
     
  19. acmefixer

    New Member

    Aug 4, 2011
    17
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    In the pic it looks like the blue wire (top end) isn't plugged into anything. And the resistor is plugged into the same row as the orange wire which is then plugged into the coil's green wire. This doesn't look right to me.

    When I get into one of these dilemmas where I can't figure out what's wrong, I pull out all the parts, put them in a plastic bag and start out new with fresh parts, so that if there's a bad part (or two, or three), it won't ruin my day.
     
  20. acmefixer

    New Member

    Aug 4, 2011
    17
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    If you want more efficient JT, try my Supercharged Joule Thief. It is much more efficient than a conventional JT, typically 70 to 85 percent. It's easy to build one, if you don't believe me. You'll get the same bright LED with half the current.
     
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