Joule Thief 101

[MileHigh]

I seriously doubt that a normal Joule Thief can be made to keep the LED on all the time but it can be tried on the bench or in simulation.

The key thing about the Joule Thief transistor is that it snaps ON or it snaps OFF due to positive feedback.

What makes the Joule Thief transistor snap ON is the end of the discharge cycle of the LED.  The potential at the coil-LED terminal drops when the LED discharge is ending.  That drop in potential on the output (right) side of the transformer makes the input (left) side of the of the transformer raise the potential of the base resistor to snap the transistor ON again.

So, it would appear that a Joule Thief can not keep the LED lit all the time because to start a new energizing cycle where you snap the transistor ON, the LED must complete it's discharge cycle and go off such that the energy in the coil is completely depleted first.

You can see how this "winging it" electronics talk can be so fruitless.  No schematics and no timing diagrams and no explanation of the normal operation of the circuit under discussion is the typical backdrop for having a meaningless conversation about electronics while pretending it actually means something.  I have seen discussions of up to 50 postings back and forth that all meant nothing.

On a thread about two or three years ago Poynt got involved in a Joule Thief discussion and some beautiful comprehensive Joule Thief timing diagrams were posted.  I don't recall if they were simulations or scope shots but I think they were supplied by Poynt.  I tried a Google search for Joule Thief timing diagrams but could not find any.

[Magluvin]

I dont want to bring it up further than this, but I wish to reiterate my slight objection to using the circuit I had shown earlier with the led across the transistor.

I went through a search of jt circuits to get a quick view of what seemed most common and some that are variants.

The most popular circuit is the one with the led across the transistor.

The circuit below, which I have labeled as 'wrong'(even though it still works I suppose) seems to take away efficiency by draining the battery when switching on and also draining in series with the discharge into the led. Now below that pic is one that I have labeled 'right' in which I moved the led across the coil instead where it doesnt have the battery draining when the led lights. 

What Im thinking is that the popular circuit 'wrong'(probably the first ever that started it all?) is probably the worst circuit of them to use when we talk efficiency. I would bet that it drains the battery faster than the 'right' circuit. Just being that this is JT 101, that should be discussed a little, maybe.

The 3rd circuit shows a battery being charged as an output and is where I would expect it to be in the circuit. If the battery and the diode were across the transistor and in the 'wrong' circuit, the source battery would be drained as much as the load battery is being charged!    Thats not good at all, let alone the source battery has to pump the inductor. Drain and more drain. I want to go the most efficient route here. Never built one but I know how it works. I noticed this issue(to me it is an issue ) very quickly just looking at the current paths.

I read once before that a jt is really no more efficient than running the led direct and the only advantage was that it ran on virtually dead batteries. Well if they were testing for efficiency with the 'wrong' circuit, then maybe so.

Had anyone discussed this before possibly?

Mags

[Magluvin]

Looking at the 3rd pic in my post above, it seems the batteries are not correct polarity for the circuit. Picked it from a big list on search. But the output deal is what I wanted to show an example of.

Mags

[sm0ky2]

If I remember correctly (this was a good 7-8 years ago or something...)

The JT circuit sprung from several replication attempts of BruceTPU's self-oscillating circuit.
Which flashed an LED (slowly) for a confirmed 30+days off a capacitor and a resonant tank, which had everyone excited.
Noone was able to replicate what bruce was doing, most of them stated arguments that sound a lot like MH's discussion here.

However, when a battery was added, the circuit performed nicely. Eventually it was found that the batteries don't need to be fully charged. All this was done without the basic operating principals understood.

does the circuit "work" as it is built by a majority of the unknowledged replicators?
I suppose that depends on what you are trying to "do" with it....

If your goal was to light an LED,. then yes I supposed a JT works, no matter how you build it.
There are voltage step-up circuits that will do the same thing much better.

I guess I have the experience of watching the whole thing evolve, from an outside perspective.
I saw the obvious fact that no one was listening to Bruce.
Maybe because they were trained by the industry, perhaps because they could not comprehend the principals.
But at the end of the day, this thing spread around like candy, kids were building them everywhere.

hey LOOK!! this thing can light an LED with a dead battery!!! <<-- this is not even what makes the JT special.....
it is basically a side effect from one particular configuration, that caught on as a fad.

the name "joule thief" was coined some time after the device had been in circulation. The (2 possibly 3) people involved in propagating its' name took the information from the threads and put it in a logical, replicatable form that everyone could easily build. With no knowledge of electronics, signal processing, electrical engineering, physics, magnetics, or any other field that applies to the operation of this circuit. Anyone can copy the design and light an LED with it.

If that is as far as you want to take this technology.. umm,. the door is that way.

To claim that this is not an Armstrong Oscillator, is rather an absurd statement.
Even in its' most simplified form, it still remains such.
The fact that people ignorantly destroy the resonance of the tank, is quite frankly irrelevant.

There was a lot of questions posted recently, I will try to address them here. sorry if I missed one.

@ Mags - on the secondary load,
to put is simply, yes the load affects circuit resonance. Diodes can be used, if resistance can be kept within nominal values. Also, another inductor of greater impedance can prevent destructive feedback.
https://www.youtube.com/watch?v=h9RgjAgSQOg

It is almost a lose-lose situation to try to force the load to be resonant with the coil.
because the primary circuit has a resistance and impedance that differs from that of the ferrite with coil
There is a "mirroring" technique, but it can't really apply to the JT, at least not in the way we use it.

rather the transformer in its' entirety is made to be resonant, and the load is separated by a rectifier circuit,
or appropriate impedance, to prevent destructive feedback from destroying the resonant waveform.

In the above circuit, the number of turns on the secondary coil of the JT, was increased from that of the primary coil, until a resonant node was found.
The second inductor is much larger, with a much greater number of turns on the coil.
The impedance keeps the voltage from fully being achieved in the larger inductor.
Timing of the primary oscillator truncates the amplitude of the waveform, and it presents itself as a lower voltage , higher current signal.
--Note here that the secondary larger ring, is NOT self-resonant with the JT portion of the circuit.
   it oscillates with the resonant freq of the JT, but the ferrite and coil in the second inductor are much different.
The second inductor cannot be made to be self resonant with the coil that is around it.

[When the same large ring is used directly in a JT, the voltages spike to around 90V DC, and almost no current.
some LEDs can pass it through (arc?) , others get damaged.]

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@ MileHigh

What is the effect of discharging a magnetic inductor (current source) through a coil, when the inductor was magnetically charged, with the lowest possible reluctance?

Does the inductor (current source) then discharge with the most energy possible, because losses are minimized?

Why would you intentionally try NOT to do that?

As for operating JT circuits with voltage sources other than a battery -

You seriously need to do some research. I can name no less than a dozen people on this forum that have posted videos of a JT running from an earth battery. I myself have done this.

https://www.youtube.com/watch?v=xrrFsiMXrvA

There are also many other videos out there using all kinds of voltage sources.

the desk was a woodentop, metal frame desk, upon which sat a lamp, a computer and monitor.
The desk measureably sat at about 43V DC, we assumed because of the electronics sitting on it.
and Yes it powered a JT, because we tried it.

With a standard Joule Thief circuit the LED does switch off so the persistence of human vision does come into play.  I do appreciate how you stated that the persistence of human vision is a complex process and not necessarily a one-size-fits-all proposition.

MileHigh

This is a yes, and a no...  more recent JTs use faster reacting LED's. And will in fact flicker rapidly.
Many of the originals used a certain type of Red LED, found in college electronics kits.

These LED's, when powered on, then switched off, take some time to turn off. The light dims gradually.
In most scenarios, using these LEDs in a JT circuit, cut-off time of the LED is longer than half the frequency.
Thus, by the time the LED fully dims, it has already received another pulse and lit back up again.
There is no perceivable "off" condition, by the human eye, or by luminescent monitoring equipment.
What IS perceived, is a dimming of the light. But not a complete off-state.


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[sm0ky2]

What makes the Joule Thief transistor snap ON is the end of the discharge cycle of the LED.  The potential at the coil-LED terminal drops when the LED discharge is ending.  That drop in potential on the output (right) side of the transformer makes the input (left) side of the of the transformer raise the potential of the base resistor to snap the transistor ON again.

So, it would appear that a Joule Thief can not keep the LED lit all the time because to start a new energizing cycle where you snap the transistor ON, the LED must complete it's discharge cycle and go off such that the energy in the coil is completely depleted first.

I'm not sure what you are trying to say here. Makes no sense to me...

a JT can oscillate without the LED present in the circuit.
the LED is only there so you can "see" when the circuit is oscillating.
There are other ways to see this, without using an LED wasting away your energy....

Amplitudes of both voltage AND current increase when the LED is removed.

the "on" - "off" state of the transistor is a function of the inductor/battery circuit, NOT the LED.
you can change the location of the LED, or remove it completely.

The signal at the base from the inductor is what turns the transistor on.
It boosts the voltage from the "dead" battery to above the cut-on voltage of the transistor.
That's what makes the transistor turn on (and the LED light up).
Inductance.
It is a factor of the number of turns on the primary winding.
This is why it is usually such a low number (8-15 turns)

More turns = higher voltage. at some point, you exceed the operating voltage of the transistor.