Joule theif : try out !

[MarkE]

omdano that transistor will have a hard time starting at low voltage.  For low voltage designs you want a transistor that conducts at zero voltage, like a JFET, or a depletion mode MOSFET.

[omdano]

omdano that transistor will have a hard time starting at low voltage.  For low voltage designs you want a transistor that conducts at zero voltage, like a JFET, or a depletion mode MOSFET.

In short , it wont work to light the led  ?

[TinselKoala]

omdano, why don't you start, like many of us did, by making a common Basic Joule Thief as a testbed. Use a common NPN transistor; almost any will work in the Basic circuit and will run on a depleted AA or AAA battery, down to 650 mV or so, lighting one or two or three LEDs brightly.

Then you can start experimenting on your own, by substituting in different transistors, different values for the base resistor, different winding ratios on the inductor (doesn't need to be a toroid) and circuit geometry variations. This way you will learn what does what in the circuit and you will be able to tell whether you are actually getting any improvements, since you will have a basis for your own comparisons.

Please start with something you know will work, and make your variations from that point. In that way, you may discover something on your own and actually _add_ to the JT base of hard-won knowledge.

The extremely low voltage JTs will also, unfortunately, only give out a small amount of light. That they work at all is an electronic "miracle". To get a brilliant, steady light from a 0.025 V JT.... will be an even greater miracle.

For your amusement: JT with 4 blue LEDs in series, running on a depleted AG4 (LR626) button cell, recharging with heat from the soldering iron:

http://www.youtube.com/watch?v=K7msKzlNzKw

[MarkE]

In short , it wont work to light the led  ?

No it won't.  All JT and JT like circuits can be coarsely described as having two states:  transistor (mostly) conducting, and transistor (mostly) not conducting.  In order to get energy into the transformer so that the transformer will immediately boost by the turns ratio, or so that the transformer can fly-back, we have to initially turn the transistor on.  All that we have available at that instant is the supply voltage.  If the transistor will not turn on then no energy goes into the transformer and we can never boost the supply voltage.

[TinselKoala]

No it won't.  All JT and JT like circuits can be coarsely described as having two states:  transistor (mostly) conducting, and transistor (mostly) not conducting.  In order to get energy into the transformer so that the transformer will immediately boost by the turns ratio, or so that the transformer can fly-back, we have to initially turn the transistor on.  All that we have available at that instant is the supply voltage.  If the transistor will not turn on then no energy goes into the transformer and we can never boost the supply voltage.

It may also be important to note that a JT circuit can oscillate, but without sufficient amplitude to turn the LED on. It will still be radiating RF, though. The DALM JT shown above, with BC337-25 BJT,   has been observed to oscillate off and on for many days, as the battery spontaneously regenerates (starting oscillations) and depletes (ending them), over and over, but without enough "boost" actually to light its LEDs.