Smoky2:
I am not sure what you mean by the "system losses"
in general I am referencing resistive and inductive/reactive losses in the USE of the JT.
compared to the PROPER USE of the Armstrong Oscillator.
or in the comparative example, the resistive and inductive/reactive losses in the use of a filament
compared to the use of an LED.
I never anywhere stated that a JT circuit was better or more efficient than a household LED.
that is topic for another discussion, wherein I use the parts already inside the LED lightbulb to form a JT circuit,
throw away the extra misc. components found inside, and power the LED with a mostly dead battery,
and compare that to an unaltered LED powered by the Mains.
"Minimalist version of the circuit" is another issue. When is a circuit a Joule Thief or not? I think that there is a simple answer to that one. If the circuit can power a LED with a battery whose output voltage is lower than the normal drive voltage for the LED, and the LED is driven using the technique of a discharging inductor acting as a current source, then you have a Joule Thief. If the circuit does not meet these two conditions then it is not a Joule Thief.
hmm, there are a lot of different devices referred to as a "joule thief". But at some basic level, we have to agree that there are certain aspects, features, and components of the circuit, that define is as the 'fad' known as a JT.
I am not sure if I would use the same criteria you offer above. Mine would be more like:
1) transformer (or suitable equivalent switching device)
2) inductor
3) low voltage power source
4) optional load
the LED is optional, and serves only as an indicator that the circuit is in operation.
The fact, or should I say phenomena, that people are amazed by, and use the LED as a source of light, is quite frankly irrelevant to what is or is not a joule thief.
The entire argument of it using the "last bit of current in a battery" is complete hogwash,
you can run these off nearly any voltage potential, from any source.
from the earth itself, broadcast radio signals, to the voltage built up in the metal frame of your computer desk....
The things TK and Bill did, without a standard "battery" are worth going back and looking at.
What is a Joule Thief?
a Joule Thief is: An Armstrong Oscillator
Most of the instructables, and do-it-yourself JT webpages use a very simplified (minimalist) version of the oscillator,
and do so with completely mismatched components.
No thought was given to most of their designs other than
the switching range of the transformer vs the inductor, and the cut-on voltage of the transistor and diode.
Furthermore, taking an equivalent circuit replacement works for digital electronics. We do it all the time.
But taking an analog circuit, and forcing it to be digital, you lose certain qualities of the signal.
go talk to an old guitar player about digital equipment vs their older counterparts, and hear what he has to say.
there is no JT "standard" for the transistor, the resistor, the ferrite, or the coil.
Some here have put forth a considerable effort to standardize the components, but this was an aftermarket thought.
Not the definition of the device.
What I am trying to do is teach others how it was designed to be used in the most efficient manner.
Obviously I can't comment on the various oscillator circuits that you have made reference to, but I suspect that many of them may not in fact be Joule Thief circuits as per the two criteria that I outline above.
again, I'm not sure I can agree with your observational criteria.
A lot of the efficiency is due to the fact that the LED in a Joule Thief is flashing and taking advantage of the persistence of human vision. To accomplish this the Joule Thief has the overhead associated with the lossy energizing of the main coil and the associated overhead for the timing circuit.
MileHigh
Most of what I have been talking about is not necessarily comparing the Joule Thief to another circuit.
But comparing the Joule Thief to itself, under different operating conditions.
What those operating conditions are, and how to use them to build the best possible JT circuit.
Efficiency of the JT vs other devices can only be done analyzing the duty cycle of the power across the transistor.
This is generally done outside the linear mode of the transistor, and at frequencies far from a resonant node.
Comparison in this manner shows that the Joule Thief is a rather inefficient circuit. We can and have done better.
a JT in resonance, sometimes cannot even be measured.
Equipment can get destroyed, and capacitors explode, stray voltage spikes in unexpected parts of the circuit.
This is because people don't pay attention to the impedance of their oscilloscope,
or that a diode can create a return current path, which is preferred by the current when resistance through the coil peaks.
DMMs are usually the first to go, people think since they run it through a diode that its no longer "AC"......
There is a whole range of mathematics and rules that must be adhered to when it comes to resonant circuits.
these have been around for 200 years, people mainly ignore that which we do not use.
I don't get too deep into these concepts here, because most of them apply to much larger resonant circuits, than a simple JT. - but they CAN and sometimes DO apply, when you are taking measurements of the JT circuit in resonance.
Also note, that a resonating inductor produces large amounts of interference to the surroundings.
If not properly shielded, this can disturb instruments and equipment nearby.
Our circuits are not designed to operate in this manner, it is a whole other branch of technology that never went anywhere. We went with the predictable, more consistent route.
It is now our time to experiment with this.
As it pertains to "flashing" LEDs::
persistence of human vision varies from person to person. One human can see very fast flashes, where another human cannot perceive them. There is an "average", based on a number of test samples, but generally any testing done to the JT is done using an assessment of the actual circuit, not some arbitrary visual aspect.
Also, there are frequencies your brain cannot process. points where the LED will appear to dim to you, but in fact it is producing a greater amount of "light" than a lower frequency you were able to see.
We know by the diode data sheet, how much "light" is produced with a given voltage/current put through the diode, and we also know the decay function or Cut-off time, that this "light" is dissipated over after the pulse cuts off.
What we "see" from the LED does not matter.
I think what you will find, is that in most set-ups, the LED itself never fully turns "off".
Therefore, the persistence of human vision doesn't even come into play.
The LED itself doesn't even matter. more accurate testing can be done using other components as a load.