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While working on these tests I noticed that the voltages were lower than I expected. I located my oscilloscope calibrator and confirmed that the Tecktronix 453 scope was reading all voltages lower by a factor of 10. I have adjusted all measured voltages in the grids below to their correct values as of 09/03/2014. Yes, you guessed it, I was using a 10X probe. I'm glad I didn't pick up the 100X probe!


To read the wiki article, Click Here


The above image shows the inductor wound with separate wires, but winding them with the two wires together is more efficient, faster, and easier to do: "if you wire in parallel so they interleave you can up the efficiency, sometimes by as much as 40% - mickeypop." Note: The comment passed on by mickypop was received on the Instructable, "Itty Bitty Joule Thief by Victor805" - see the link below. The next image shows how to do this for a simple joule thief where both wires are wound with the same number of turns.



The above image is used with permission of Victor805 where it was used in his instructables: Itty Bitty Joule Thief by Victor805 and permission of Steve where the original was taken and modified by Victor. Steve's article on the joule thief is at: Joule thief - getting power from dead batteries


* 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.Inductance

(μH)Frequency

(KHz)0/<12,000 (=2 MHz)0/421,000 (=1 MHz)6917.512317.22234.96165.0The two inductors with 0/## were air core different diameters of the same length of cat 5 pair wire about 42" long. I simply wanted to see if such a bifilar inductor would work.


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.


Transistor Tests @ 1.28V, 495-3849-ND @ 800 μH, 44 LoopsTransistor

Freq

(KHz)On

(μs)1 Hz

(μs)On Time

( % )Peak

Volts2N22222.6120384313.02N39041.2+320820394.42N44011.2-330840395.6


Inductor and LED Tests @ 1.28V & 2N3904From

Core Order #

O.D.

(mm)Wire

Guage( # of )

LoopsmA

μH

Freq

KHzOn

(μs)1 Hz

(μs)On Time

( % )Peak

VoltsLED

ColorDigi-Key495-3851-ND12.5241138.75274.290240373.6BlueDigi-Key495-3851-ND12.5302523.15701.4280690303.6BlueDigi-Key495-3851-ND12.530255.457022.818.843.243.53.1WhiteDigi-Key495-3849-ND10321156.51318.3120235542.4AmberDigi-Key495-3849-ND10324439.28002.2280460542.4AmberDigi-Key495-3849-ND10321156.61387.8440127.5315.6WhiteDigi-Key495-3849-ND10301254.21588.1642.5122.5345.4WhiteDigi-Key495-3849-ND10322241.64973.70100270375.6WhiteDigi-Key495-3849-ND10324430.68001.78220560395.6White

NOTE: The test for the 3851 core with a white LED was done on 11/27/2014 and it looks like I used a different white LED & 3904 transistor.


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, R3UK recommends that the more windings the better, I'm not really sure what he means by better. If you 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. The current used goes down which will increase the battery life and could definitely be considered better. On the other hand, if we're trying to charge another battery, it will take longer to charge which could be considered worse. So better, could be different for different uses of this circuit depending on what we're trying to accomplish.


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 current, 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 the more windings the less current is used, perhaps more tests to see how low a current 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:

  1. A core that needs the fewest loops and produces the lowest frequency.*

  2. The LED that causes the circuit to use the least current. This seems to be the white LEDs.

  3. The best transistor, which so far is the 2N4401.

  4. The most windings for the lowest frequency, within reason.

  5. The best size wire to reduce the current used. It seems that larger diameter wire is better.*

* I'll have to run more tests using the same wire and number of loops on several different cores to give you those results. So far, the larger core, Digi-Key 495-3851-ND with 25 loops, has a lower frequency and uses less current than the 495-3849-ND core using 44 loops.




Core vs Current Tests @ 1.28V, 2N4401, White LEDFrom

Core Order #

O.D.

(mm)Wire

Guage( # of )

LoopsmA

μH

Freq

KHzOn

(μs)1 Hz

(μs)On Time

( % )Peak

VoltsLED

ColorDigi-Key495-3848-ND10.0301155.92079.5250105484.8WhiteDigi-Key495-3849-ND10.0301154.21588.1642.5122.5345.4*WhiteDigi-Key495-3849-ND10.0301153.61498.3340.0120.0335.0WhiteDigi-Key495-3874-ND6.3301140.319011.432.587.5374.8WhiteDigi-Key495-3847-ND10.0301136.94665.2675190394.8WhiteDigi-Key495-3851-ND12.5301135.86244.88120205594.8White

*

This was the original reading as seen in the "Inductor and LED Tests" grid above. All other measurements were made on the same day with the same Volt/current meter, power supply, Inductance meter, and Oscilloscope.



The non-Simple Jouel Thief


The non-simple Joule Thief uses different numbers of windings/loops for the collector and base of the transistor. I'll do a few experiments to find the best winding ratio in order to find the lowest current used for one of the ferrite cores I have. In the image below I show that I will start with 20 loops for both sections and go down from there until I find the lowest current used. The second test I'll start with about half as many loops, 12, and I'll start with the base using 8 loops.


Helpful hint: It does matter which two wires you connect together which end up going to the positive battery terminal. If your core doesn't work select the other pair of wires. Or wind your wires as can be seen in the Wikipedia photo at the top of this page.




Non-Simple Jouel Thief, 1.20V, 2N4401, White LED, 495-3847-ND, 30 gauge Wire

#

LoopsShort

μHLong

μHTotal