Joule Thief 101

[MileHigh]

Just a few comments on the switches.  Assume that they are all semiconductor-based switches and here is where I am pretty much out of my element because I have not done this type of discrete electronics stuff in more than 20 years.  I am sure a lot of new stuff has come out since then.

S3 is probably the easiest, perhaps just a few FETs in parallel or MOSFETS.  Naturally you are trying to get the ON resistance to be as low as possible.

S2 is there because there would be too much power lost in the diode alone.  You need to normally bypass that diode to prevent the power loss.

More or less the same thing applies to S1 you need a semiconductor switche with the lowest resistance possible.  The whole premise for the design is that you have a big coil with current flowing through it where you occasionally "top off the current" with a battery and occasionally draw some energy from the coil when you light the LED.  So resistance is the enemy and it probably is not a practical circuit because of this fact.

MileHigh

[Lidmotor]

 I have been looking into ways to get energy to power low energy devices for years.  The video I posted showing the loop antenna tuner was done to see if perhaps ambient RF could enhance a Joule Thief.  The tiny solar panel supplied the input energy.  The crude experiment was interesting but inconclusive.  I did find this today about a company in the UK that has a patent on a device called the 'Freevolt' that does harvest enough room RF to run tiny loads. 
http://www.engadget.com/2015/09/30/freevolt-free-energy/

MH:
  Your 'outside the box' JT is interesting but I really don't understand it.  It did remind me of one design that was shown years ago where the oscillator had a second button cell power source that was dedicated strictly for transistor switching.  The drain on the button cell was tiny and the cell could run a long time.  I never built one but it looked interesting.  Maybe Bill remembers it and can link to it.  To me that was a good example of 'outside the box thinking' that most experimenters don't do.  I know a guy who built a JT using dollar store green steel twist tie wire. How weird is that?  I think he called it 'Penny'.

---Rusty

[Pirate88179]

Bill:

Two questions for you about Joule Thieves.

1. I am under the impression that for all these years you were believing that when a Joule Thief was up and running and driving a LED that it was in resonance.  Is this correct?  I thought I saw a comment from you about that.

If that's the case then I am assuming that now you have a much better understanding about how one works.  Is this true?

2.  I am also pretty sure that when you started stringing 10 or 20 LEDs together in series, that you have stated many times that each individual LED was the same apparent brightness as when you had just a single LED in the circuit.  It was only when you got to something like 40-plus LEDs in series that you started to notice that the individual LEDs were starting to get dimmer.

Is the above statement true?

MileHigh

1.  Not exactly correct.  At first, I thought that when you "tuned" (using a vr to the base) and the led got a lot brighter, that it was a type of resonance.  I was told this back then and, it seemed to fit because you could adjust the resistance and get the led to be very bright...move the vr a tiny bit up, or down and the led dimmed.  Later, we just called this the sweet spot.  What I am saying is that we found that you could go past the sweet spot and have to adjust back to it.  Also, about this time I learned how to properly measure the amp draw (mA draw, ha ha) in these circuits and found that you could tune the base vr to achieve a very low mA draw.  You could watch the led while doing this and tune so you got a decent brightness with as low an amp draw as was possible given that circuit's parameters.  Thus, you could tune for max brightness, or tune for max running time with still usable light.  So, I don't think we still called it resonance, but it is possible that we did after that time.

2.  Close.  It was those led Christmas lights (man, what a bargain for $3/ string of 100 leds)  I found that I could light 100 of them with the Fuji AA battery JT and they were very bright.  Using the same circuit, I added another 100 and could not see any difference in the light output.  (This is where a light meter would have been very useful back then) Stefan told me that those lights must have been in series as that is the most efficient way to hook them up like that with many leds.  300 leds made it noticeably dimmer and 400 leds brought it down to about 1/2 brightness of what the 100/200 leds were.

I know this is going back a few years (7) and my memory is not 100%.  Before I did the 100 leds strings I think the most leds I lit from a JT had been 4 or 5 and that was using a "standard" JT circuit.  Once I learned how to mod the Fuji, I was off to the races, ha ha.  That was also the time I learned how to zap myself many times even if I was being careful.  Some of those really hurt, ha ha.

Bill

[tinman]

Here is a totally radical Joule Thief "concept car" that I designed "paper napkin style" in my head today.  The chances of it outperforming a regular Joule Thief are almost zero but that's not the point.  The point is to think outside of the box and try something new.  If I had a bench and I was genuinely interested in this stuff I would build it myself.

There are three switches, and they are all programmed by a 555 timers or perhaps a super low power microcontrller.  The premise is that the power for the switching would be provided by an external fresh battery and you assume that that battery would last long enough to drain up to 100 batteries that actually power the Joule Thief.  Thus it is perfectly valid to have an external battery to power the timing system.

The basic principle of operation is that current is always flowing through the coil.  The current is occasionally given a boost (like boosting the International Space station in its orbit) by connecting the battery with S1.  The LED is occasionally lit by opening the bypass switch S3.  S2 is there to complete the current loop, and the diode is there to give the current a bypass when S2 switches.

It's radical, but it does give you 100% full control over how much current is flowing in the loop and thus the brightness of the LED when it is switched ON.  It also gives you full control over the ON/OFF duty cycle of the LED.

Note that the issue of the sloping voltage/current waveform for a regular Joule Thief is eliminated.  The assumption is that the inductance is quite large and the current through the LED when it is ON is nearly flat and unchanging.

Think outside of the bloody box.

MileHigh

You are so funny some times MH.
Throughout this thread,you have been stuck on one circuit that you deem to be !the! JT circuit.
Now your posting different circuit's,and telling everyone to !think outside the bloody box! lol

Fact is MH,most of us here have been doing that all along,while !you! where stuck in the box.


Brad.

[tinman]

I have been looking into ways to get energy to power low energy devices for years.  The video I posted showing the loop antenna tuner was done to see if perhaps ambient RF could enhance a Joule Thief.  The tiny solar panel supplied the input energy.  The crude experiment was interesting but inconclusive.  I did find this today about a company in the UK that has a patent on a device called the 'Freevolt' that does harvest enough room RF to run tiny loads. 
http://www.engadget.com/2015/09/30/freevolt-free-energy/

MH:
  Your 'outside the box' JT is interesting but I really don't understand it.  It did remind me of one design that was shown years ago where the oscillator had a second button cell power source that was dedicated strictly for transistor switching.  The drain on the button cell was tiny and the cell could run a long time.  I never built one but it looked interesting.  Maybe Bill remembers it and can link to it.  To me that was a good example of 'outside the box thinking' that most experimenters don't do.  I know a guy who built a JT using dollar store green steel twist tie wire. How weird is that?  I think he called it 'Penny'.

---Rusty