I am not going to write about how to make a Joule Thief but rather I simply want to add a little more information for everyone. As I come across more, I will add it to this page.
A list of reference web articles/pages will be at the bottom of this page. After I got my joule theif working I did more research and some of my own experiments. The references below are some of the better articles I've come across which better explain how to build a joule thief as well as what you can expect duriung its operation.
To read the wiki article, Click Here
* The first thing I want to show is a small table of the inductance values of the toroid bifilar inductor vs. the frequency of the pulses that I measured.
NOTE: Some of the inductances will show 0/## this is because my inductance meter could not measure an inductance of only one winding. In these cases I also measured the inductance of the two windings.
All of the ferrite toroids were considerably different. It is rather interesting that I have an extremely small inductor at 616 μH, the OD of which is ~6mm and it is ~ 3mm tall, and the 223 μH inductor has an OD of 18mm and is 12mm tall. These are considerably different physically and very different inductances and yet the frequencies are very close.
The red listing is, IMHO, where we want to be, if possible, even a lower frequency would be better still. I'd like to see if I can get it to ~ 500Hz! The reason for this is that the fewer the number of pulses per second, the longer the battery should last. Assuming all pulses are the same height/voltage and width/duration.
The human eye cannot tell that something is pulsing when the pulsing is fast enough. Here in the USA we use AC current of 60 Hz and in Europe they use 50 Hz. So, if there was some advantage to it, and it could be done easily, getting the frequency down to 100Hz, would also be acceptable.
You're probably thinking that we'd notice a difference in how bright the LED appeared. All I can say is that I didn't notice any difference between the LED going at 5KHz with the same LED going at 1MHz so I doubt we would notice any difference between 5KHz and 500 or even 100 Hz.
I ran two tests - one using the 5KHz inductor and another using the 1MHz inductor - and there was a substantial difference in how long the 1.1V batteries lasted. I initially thought this was because the 1MHz inductor has 200 times more pulses per second. However, after thinking about it, I decided that it is only the percentage of time that the LED is on during one cycle that determines how much total energy is used and so determines how long the battery will last. Thinking about it a bit more, I thought almost the exact opposite, that it might be due to the length of time the inductor is "charging" that determines the life of the battery. The shorter the LED is off, the longer the battery will last. Or, put another way, the less current used, the more efficient the circuit is. I ran a few more tests on Feb. 22, 2014, and discovered that this looks correct. The longer the LED is off, the lower the frequency and the lower the amount of current is used by the circuit. All of this should mean that this is more efficient and should cause the battery to last longer. This does use more windings and so more wire. I believe that the items that determine the on vs total time are: the core, the wire size, the resistor value, and the LED. I'll have to run a few tests to see how each of these affects the ratio of LED on time to the time of one cycle. We get some indication that different inductances with the same core and the same wire do not change the on-time/cycle-time ratio in the above grid testing the Inductor and LED.
The failure mode for the 1MHz inductor was very interesting: the LED would light for about 10 seconds then turn off for about the same length of time while the battery recovered some of its voltage. It would rise to a voltage of about 0.62V and this would turn on the LED again until it hit a low of about 0.51V, at which point the LED would turn off. This cycling continued throughout the day, another eight hours, when I finally replaced the 1MHz inductor with the 5KHz inductor and ran the battery down to about 0.37V.
NOTE 1: I have noticed similar frequency changes depending on the inductor value. However, when R3UK recommends that the more windings the better, I'm not sure he's correct. I'm not really sure what he means by "better" since when I've added too many windings, the LED doesn't light at all. Look at the tests done and listed here we see that the more windings of the same kind of wire increases the inductance which decreases the frequency but the on-time/cycle-time ratio does not change and so the battery life remains the same. So, what is "better" about this arrangement? On the other hand, more wire was used but this did not change either the performance of the LED, making it brighter, or increase the life of the battery, so there was no improvement there, nothing better about that. On the other hand, using more wire takes more time to wind the inductor. That's not an improvement, and then there is less wire left over to build another unit. So far, I find nothing better about using more windings yet.
Date: 02/22/2014 - Today I ran a few more tests and this time I was also measuring the current the circuits used. I used the three LEDs: Blue, amber,and white. It was very clear that the more windings, the lower the frequency, and the less current the circuits used. This, then, is a very good reason to use more wire to lower the frequency, within reason.
NOTE 2: When I used so much wire that the inductances were around 3.5 mH, the LED did not light up at all, so that is definitely not better. Too high an inductance as well as too low an inductance will cause the LED to not light up. So far I know it works from ~< 1μH to > 800μH. In the transistor test grid above, we see that 800 μH still worked fine so the LED fails to light up somewhere between 820 μH and 3.5 mH. I don't think it is worthwhile to do more tests to determine the inductance value that stops the LED from lighting up. However, since it now turns out that the more windings the less current is used, perhaps more tests to see how low a freqency is possible with a reasonable number of windings would be worthwhile.
Other tests will need to be done to determine how to change the on-time/cycle-time ratio to light the LED for the shortest time possible while having the longest cycle time and still have an acceptable brightness over the life of the battery.
NOTE 3: LEDs that can be used: Many other articles mention that blue or white LEDs should be used. I have used Amber LEDs to make a few Christmas "candles" for windows which have difficult or no access to the AC. These work very well with dead batteries of around 1 Volt. Any LED that is listed as a 3V LED or a 20mA LED should work with the Joule Thief, so you don't have to limit yourself to blue or white LEDs.
So, what is it we want? This all depends on what we want to do. If we just want to have a little night light or flashlight, then making the unit as efficient as possible may not be all that important. After all, we're using "dead" batteries. If, however, we want to put up a light that lasts as long as possible, then using the best transistor, ferrite core, number of windings, and LED all must be considered.
Amber LEDs were used in our Christmas window-lights to better simulate candles. The ferrite cores were salvaged from a dead CFL bulb plus one from something else. No doubt they are not the most efficient. But, they do get the job done.
If we want the most efficient joule thief, assuming this means using the least current, then we want:
Also see the LED Tester built from this basic circuit.
Low Voltage Battery Traces: Batt~0.57V, 0.2V/Div & Batt~0.41V, 50mV/Div @ 10μs, Amber LED
I also made a replacement Piano Light "bulb" out of two LED modules with three LEDs in each of them, the base of a dead LED light bulb, and the circuit from inside a 12V, 500mA switching power supply.
Here are a few of the better articles I've come across explaining the Joule Thief:
Title: Joule thief
Synopsis: A good explanation, schematic, and O-scope trace which shows what my scope showed.
Title: MAKE A JOULE THIEF
Synopsis: Clearly shows how to connect the windings of the ferrite cord inductors.
Title: Making A Simple Joule Thief (made easy), by ASCAS
Synopsis: The one I used to make mine, no explinations but it works.
Title: "Joule Thief" Circuits, crude to modern... by Dave Kruschke
Synopsis: Here Dave shows how this circuit works using a mechanical switch!
Title: Bifilar coil
Synopsis: How to wind it & the different connections
Title: The Joule Thief!
Synopsis: R3UK has some very good explinations, experiments, and results. Ideas on better transistors to use than the 2N3904
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