Joule Thief 101

[MileHigh]

Brad:

I have given my response in my last post.

Go back to posting #514 and answer the questions.  Be a man and go for it.

Below is a scope shot of a JT type circuit in resonance.
I used a center tapped transformer,and have two LED's on each half of the center tap.
The circuit frequency was raised and lowered until maximum amplitude was had across the LED's,and minimum current draw on the P/in. When out of this resonant frequency range,the LED's would dim,and the I/in would increase. From this simple test,we can easily see why resonance is important in a JT circuit.
The second scope shot is across the 100ohm P/in CVR on the ground side of the circuit.

Brad

It's nothing more than a pulse circuit running at an operating frequency and you found a sweet spot.  Presumably you changed the operating frequency by varying the base resistance.  It's not operating like a regular Joule Thief.  No schematic, no surprise there.  No resonance.

MileHigh

[Pirate88179]

MH:

I think I just figured out where I got the term resonance when tuning my early JT circuits.  It was from a video made by our own host, Stefan, showing an early Dr. Stiffler replication.  Stefan demonstrates how you can tune the circuit to hit resonance and shows the scope shot as well as how bright the leds were when this happened.  (I also learned how to make my first Avromenko Plug after I watched this)

https://www.youtube.com/watch?v=qRw_sCzhFnk&index=436&list=FL0bTBCRogMzrYTQT3pbhxwg

Now, I realize that Dr. Stiffler's circuit is different from a JT but I "assumed" that when I was tuning my base resistor that I was hitting resonance when the leds got their brightest.  It was easy to go past the "sweet spot" as we later called it, and you would have to turn the pot the other way to hit the spot again.

Anyway, I finally remembered why and where I got the idea about resonance in our JT circuits and this is where it started with me.

I'll bet I watched Stefan's video over 10 times.   It is a nice blast from the past as they say.

Bill

[MileHigh]

Bill:

I watched the clip.  Wow, nine years ago in the old days of YouTube when the video quality was so inferior sometimes.  Let's assume it's an oscillator that goes into some kind of LC tank circuit.  If fact Stefan says "LC tank" in the clip.  Thee is no schematic but I am going to assume that the pot he is adjusting is purpose designed to change the frequency of the oscillator.  That's in contrast to changing the base resistance of a Joule Thief with a pot which just fries the normal operation of the Joule Thief.

So it certainly looks like the oscillator is fed into some kind of resonant LC tank circuit - a real resonator - real resonance.  That's why the amplitude of the sine wave goes so high.  It also looks almost certain that it is not related to a regular Joule Thief at all.

MileHigh

[Magluvin]

Indeed, for just about any gas engine there is a power band.  Don't racing car drivers shift gears and try to stay in the power band to maximize their acceleration as they increase in speed if that's what the want to do?  It's maximum horsepower out vs. engine RPM.

However, in the language that we use on the forums the power band is akin to a sweet spot that is quite wide as opposed to being a narrow sweet spot.  The point being that it is a sweet spot and not "resonance" in the engineering mechanical or electrical sense.

MileHigh

But what is it that happens that there ends up being a 'sweet spot'? Why not as good above or below that spot?

Mags

[Pirate88179]

Bill:

I watched the clip.  Wow, nine years ago in the old days of YouTube when the video quality was so inferior sometimes.  Let's assume it's an oscillator that goes into some kind of LC tank circuit.  If fact Stefan says "LC tank" in the clip.  Thee is no schematic but I am going to assume that the pot he is adjusting is purpose designed to change the frequency of the oscillator.  That's in contrast to changing the base resistance of a Joule Thief with a pot which just fries the normal operation of the Joule Thief.

So it certainly looks like the oscillator is fed into some kind of resonant LC tank circuit - a real resonator - real resonance.  That's why the amplitude of the sine wave goes so high.  It also looks almost certain that it is not related to a regular Joule Thief at all.

MileHigh

Yes, exactly.  But, this is where I got it in my head about resonance and the JT circuits back then.  I knew I got it somewhere, just could not remember until now.

Bill