Joule Thief 101

[Pirate88179]

Mags, you can also check out the original JT topic where there are hundreds of schematics and design variants that we tested.  Many, many folks posted a lot of neat devices over there.  It is really fun to play with the variables and try different circuits.  It really all depends on what you want to do.  Do you want a lot of light?  Or, do you want some light that lasts a very, very long time?  Or, do you want to try to get as much of both conditions as possible?

This is what we played with back then and, we all learned a lot about electronics from working with these circuits.  Check TK's video collection as he has some great circuits he made on his youtube channel.  I have a lot of JT circuit videos but, TK actually explains what is happening and why in his vids.  Mine are more like..."Holy crap, this lights up!  Wow!!"

Bill

[MileHigh]

Smoky2:

This is the minimalist version of the circuit, that uses no consideration to system losses.

I am not sure what you mean by the "system losses" because we are back to the issue of the informational scope of the clip that I posted.  The clip aims to simply explain how a Joule Thief works and no more than that.

"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.

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.

stating that it works and that we don't need to know any more than that to turn the lights on, is all fine in dandy.

That's a straw man argument, I never said that.

But compare an old style filament bulb to a newer LED bulb of the same luminescence.
and compare their energy consumption

Now, do the same with the Joule thief.

My response to this may surprise you and a lot of people, but not if they were paying attention to the beginning of this discussion.

Nobody is going to argue about an incandescent light bulb compared to an LED light bulb.

You are implying that a Joule Thief gives you even better efficiency than an LED light bulb.  For purposes of a fair discussion let's put aside the "Thief" part of the Joule Thief that can extract energy from nearly dead batteries.  In other words, let's just look at lumens per watt of supplied power.

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.

So what if we just compare a flashing LED using a very efficient timing and switching circuit and a Joule Thief?

The answer is that a flashing LED light will beat a Joule Thief light hands down.  Now the Joule Thief doesn't seem so glamorous, does it?

I am going to take a guess here:  I am willing to bet you that LED lights don't flash to save power.  There are several reasons for this.  The first reason is that you are saving so much power anyway compared to incandescent bulbs that it is not an issue.  The second reason is that you simplify the design and save costs and have a more reliable light.  The third reason is just pure speculation:  If you are under lights for a long time each day, you are better off if they don't flash because even though you can't perceive the flashing, you might be able to perceive it subconsciously and some people might be prone to getting headaches just like some people don't like flashing fluorescent lights.

MileHigh

[tinman]

Thanks Brad. Thats pretty cool. So if it was a long core without the separation there isnt any current through the core? Seems odd.  But Ill take your word for it.

I wonder if you interleaved the laminations(insulated) some near the center to increase surface area if there might be helpful to generate more charge.

In this circuit below from a pdf on this thread, to me there seems to be a problem. Not that the circuit doesnt work. But the led is across the transistor in the same direction.  I understand that the batt doesnt have enough voltage to conduct the led. But when the transistor turns off, the inductors collapse current flows through the led AND the battery, thus further discharging the battery more besides the transisitor switching. So wouldnt it be better to put the led across the transformer winding where the batt isnt being drained during the led discharge also?

Mags

Well personally,i would put the LED across the emitter and core-providing the core is conductive. This way you reduce the power consumption of the device,due to the removal  of the charge build up in the core.

[sm0ky2]

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.

[sm0ky2]

I am trying to bring everyone up to speed on this, because once we all get it, we can go to the next level.

If people are still getting hung up on the very basics, we must communicate the information more effectively.

Simply put, once the JT is operating at a resonant frequency, we can remove the LED/load completely,
and couple to it, using the inductor as a transformer. By winding a secondary onto the ferrite.

With an appropriate capacitance, this secondary coil can be set resonate with the frequency of the rest of the circuit, and used to power a load.