To the likes of Floor and sm0key,
                MarkE , TK, MH  etc. are trying to educate, read and learn before making
   comments, everything they say can be verified. If any of them err and you point
   out they'll retract or amend as necessary, we should all be on the same side!
               John.

Indeed,but dont always asume that the guru's know everything,and if you feel that you are correct,then stick to your gun's-just as has happened here in this thread.

Quote MarkE post 47-Guess what?  I disagree with your claim that L1 is not coupled to L2.

Quote TinMan post 49-Have you heard of the Miller effect capacitance ?

Quote MarkE post 54-Yes, and if you understood it in relation to this circuit you would realize that Miller capacitance suppresses switching:

Quote MarkE post 81(after MarkE took the time to look a little closer)-This tells us that Tinman was right that Miller capacitance is the energy source.

So as you say Minnie,they'll retract or amend as necessary,but if you feel you are right,then stick to your gun's. I will say though,i am yet to see a resonant JT in action,but that dosnt mean one dosnt exist. Maybe a combination of the right core material,turns of the right size wire,and maybe a small cap across the driven coil might hold a chance of true resonance-but lot's of experimenting would be needed.

   Tinman,
            Thanks for your response. I know TK fairly well by now and he's always
 helped me, using his own time and money. You can't ask for more than that!
  Thought for the day, nobody really knows gravity. There's still a fair way to go!!
            John.

TK
Remove the 220 ohm base resistor,and you have my original circuit.

Not quite. My ferrite-cored coils are much lower in DC resistance than the air-core coils you used, and the BC33-25 transistor is very different from the 2n3055. But nevertheless we seem to have similar functions.


Here's a scopeshot that I just took, from the transistor Collector referred to the Emitter. It is essentially identical to MarkE's one-LED scopeshot from his breadboard build.

"This tells us that Tinman was right that Miller capacitance is the energy source. " I am sure that Mark meant that : capacitors are a energy storage medium, not an energy source. Is that how you understood that, Tinman?

Scopeshot with Base signal included. Essentially identical to the shot that MarkE showed.

(The Tek 2213a delayed timebase function was used to display the single pulse nicely centered on the screen.)

@ MarkE,TK,and MH.
Insted of saying rubbish,how would we go about building a resonant JT,or makeing my circuit opperate as it dose,but with a much higher power output?.Maybe a small cap in series with a diode across the collector/base?.

I'm not entirely sure that a resonant condition is what is wanted in order to get more efficiency or higher output from a JT-type circuit. After all, a resonant tank stores energy, and as the tank components aren't perfect, they will inevitably be dissipating some of that stored energy in unwanted ways, like by Joule heating of the components and RF radiation, removing it from being available for "output".

It might be true that a resonant condition could enable a circuit to extract some energy from the "ambiance", like a tuned receiver of the electrosmog harvester or crystal radio type does, allowing some of this "outside" energy to be put to use by the circuit.

The old JT threads on this forum have a lot of information as to making JTs as efficient as possible by carefully tuning coils and circuitry. The closest thing to a "resonant JT" that I can think of is the Slayer Exciter-type solid-state Tesla coil kind of thing.

Bill:

The answer to your question is that in a JT circuit you are not supposed to change the value of the base resistor.  In any generic transistor switching circuit, the base resistor value is normally chosen to provide the minimum amount of current to fully switch on the transistor.  It's a standard design exercise when designing a switching circuit.  How much current does the transistor need to switch?  What is the current gain of my transistor?  How much voltage is available behind the base resistor?  What is the voltage drop across the base-emitter diode?  Add a 10% margin of safety and then you can determine the value of the base resistor.

When you varied the value of the base resistor you varied the way the JT circuit responded.  The way to answer why the LED got brighter would be to look at the waveforms with your scope, construct a timing diagram, and then analyze the timing diagram.

Without analyzing the timing timing diagram and making proper measurements you are just observing.  Think of a small transistor radio.  As long as you stay away from the tuning section, chances are that varying the value of any other capacitor, resistor, or inductor in the circuit will make the sound from the speaker get louder or softer.  Do you know why?  Assume the answer is no.  So does that mean that there are 15 "volume controls" in a transistor radio?  Obviously the answer is no.  By changing the value of a random component you were skewing the circuit with an observable effect:  The volume got softer or louder.  However, the right component value to change is the setting of the volume control pot.

There are no "rules" saying that you can't change the value of the base resistor.  However, there are real design principles:  In a switching circuit you choose the value of the base resistor to ensure that your transistor is fully saturated when it switches on and there is minimum amount of power expended to do that.  So in theory there is truly a "right" value of base resistor for a given Joule Thief configuration.

If you want to change the brightness in a Joule Thief chances are varying the value of many of the individual components will do that.  But the point is to make an intelligent design choice as opposed to the transistor radio example where you just change values willy-nilly and observe the effects without actually knowing why the effects are happening.

Going back to your example, you were not "tuning" the circuit, but it more like you were "skewing" the circuit.  It's very possible that when the LED got brighter than normal the overall efficiency of the JT circuit went down.  Certainly you were not finding any "resonance" because a JT circuit does not resonate.

You also mentioned that when the battery voltage got lower you could play with the base resistor value to bring the LED brightness back up.  That's all fine but the true JT circuit is not normally changed as the battery voltage lowers.

All of the answers to the question of why the LED gets brighter or dimmer come from using your scope and making good measurements and analyzing the timing diagram.  In a generic sense the LED gets brighter because the (inductor + battery) is dumping more average power into the LED.  So you start by looking at your timing diagram and observing the charge/discharge timing for the main inductor.

MileHigh

I tried some small capacitors between B and C and also other places already and they don't work. The oscillation frequency is surprisingly high, over 500 kHz. (But dropping slowly as batteries discharge.)

Thanks for cheking that.  I hoped that when the supply voltage appears the capacitor would put an initial current 'kick' into the base emitter to switch the transistor on. Probably the 1 mH coil in the collector is already high enough to oppose hence reduce this current kick.

Gyula