Joule Thief 101

[sm0ky2]

Smoky2:

A coil at it's SRF is either dead and blocks AC if you model it as a parallel resonant tank or it's dead and offers no resistance to AC if you model it as a series LC tank.  In either case the inductance is nowhere to be found.  Above the SRF it just looks like a capacitor.

Why should a coil at its SRF enhance the performance of a Joule Thief when it is effectively dead and not functioning properly? 


If you really wanted to be sure you could inject a signal into the coil and look for the SRF.

In broad general terms, the "buzz" about a coil operating at its SRF on the free energy forums is a bunch of BS.  You are effectively turning the coil into a piece of wire or an open circuit.  There is nothing exciting about that.  There is no "secret sauce" related to hacking a JT and turning it into an oscillator and running it at the SRF of the main coil.  There is a very decent chance that the oscillation would in fact die at the SRF because the main coil of the JT becomes inert at the SRF frequency.

MileHigh

The first two points go hand in hand. We have had scopes all over this thing, across the transistor, across the diode, across the coil, across the base resistor, even observed the ends of the battery at times.
When all other factors of the system remain the same, coil, core, voltage, transistor, led.
and all you adjust is the resistance across the base - you are changing the injected feedback signal, and how it applies to the next iteration of the cycle. This signal is not present at points other than a resonant node. It is inversely canceled out by its counterpart in the circuit. no reflection, no collapse of the parasitic capacitance at the peak.

To the latter issue, I agree, most of what people talk about in these regards is total b.s.
what I am doing here is presenting the facts along with the electrical theory, and experiments to get rid of all the mysticism and "magic" ideas floating around.
When this is all out in the open, where people can understand and experiment with this, then concepts like "secret sauce" need no longer apply.
You can't look at a Resonant circuit, in terms of how a component at SRF would perform in a non-resonant circuit.
You have to look at the whole picture.

The coil will always retain a positive value for inductance, because of the 3rd parasitic relationship of the circuit at SRF.
The resistance of the circuit has a parasitic inductance, and the ferrite will still magnetically charge.
That is the whole point of this exercise

- the "indoctrination" is that instinctive feeling you get when you head down this road, and makes you want to stay away from the SRF. It is perfectly normal after years of accredited education. We all go through it......

[Magluvin]

Smoky2:

The information in Magluvin's book is pertinent and says it all.  You stay away from the SRF of the coils in a switching power supply because at the SRF the coils crap out and don't function as coils anymore.  You even stay away from having a harmonic of your excitation frequency line up with the SRF of the coils.  The excitation is a pulse train with sharp edges so naturally the signal is very high in harmonics.

A coil at it's SRF is either dead and blocks AC if you model it as a parallel resonant tank or it's dead and offers no resistance to AC if you model it as a series LC tank.  In either case the inductance is nowhere to be found.  Above the SRF it just looks like a capacitor.



MileHigh

I posted that as an example of it is said that we shouldnt partake in srf functions when designing a power supply. I dont believe it is saying that the inductor is dead.

What Im seeing it says is that the rise in current if the inductor when the transistor turns on(in a typical regulator design) is too quick for the driver design. What  that tells me is that the on time is too long in this case.  So what Im looking at is how to design the circuit to accommodate that higher freq of operation.

As with the cores SRF, what Im thinking is that if we design the coils SRF to match a lower multiple of the cores SRF, there may be some advantages as Smoky says. Will have to be tested.

Havnt built a JT yet. But from what Smoky says about the variable resistor on the base, it must be adjusting the on time and getting the SRF mode to ring.

I find it to be interesting stuff.


Back when the Russian guys were playing with tv flyback transformers, I believe they were talking about the cores SRF function in what they were doing. What did they call it, NMR? But, there were claims of dangerous radiations from the cores running at core SRF freq.  I would like to avoid that if possible.

Mags

[MileHigh]

Well at least differing opinions were aired and that's always a good thing.  Perhaps there are some people building Joule Thieves that will do their own investigations and also get inspiration from the comments in this thread.

Let's assume that your design goal is a fairly bright LED with no flickering (to the human eye) with minimum power draw from the battery.  There can be other design goals, the one I am suggesting would seem to be the most logical one for me.

It brings up the issue of apparent brightness.  I was told once that it is a zero sum game.  A very bright flashing LED with a short duty cycle will have the same apparent brightness as a medium bright LED with a longer duty cycle such that the power draw from the battery is approximately the same in both cases.  Is that really true?  I don't know.

Nonetheless, it suggests there is a "Goldilox" core size and an associated Goldilox Joule Thief transformer configuration to give you a "sweet train of pulses" to keep the LED lit with minimum battery draw.

I doubt anybody would go that far but if we end up fighting WW IV with rocks then who knows?

MileHigh

[Magluvin]

So. Since this is a JT thread, might as well start there and test these things. A JT is the simplest circuit to play with.

Any suggestions as to what works best making one of these? Has anyone wound their transformers bifi? I figure more capacitance, lower freq SRF.

I have lots and lots of parts.  100mhz scope(200 if I do the hack for the hantek 5000 series.)   Near dead batteries of all sorts.

Mags

[Magluvin]

Went to beginning of the thread and getting stuff together.

Mags