also called Joule Thief

This project illuminates a white LED and
explains how a transformer works . . .

Page 1

Conventional torches come in all shapes and sizes.
From a single AAA cell to 4, 5 and 6 "D" cells, as well as "lantern" and "fisherman's."
This project uses a white LED to produce  illumination equal to a small torch.

White LEDs have different "characteristic voltages." A 1,000mcd white LED used in this project had a characteristic voltage of 3.5v and a 3,000mcd white LED had a characteristic voltage of 3.2v. Both LEDs were driven at 20mA and the 3candala LED produced a brighter, whiter light while the 1candella LED had a yellowish ring around the edge of the illumination.

A LED torch is one of the simplest projects you can build and it's very interesting as it uses a super-bright white LED.
In the history of LED production, red LEDs were the first to be invented and their output was so dim you could barely see if they were illuminated. You needed a darkened room to see them at all.
Then came green, yellow and orange LEDs.
As time went by, the brightness improved and it came to a point where the output would shine into the surrounding air. These were called Super-bright LEDs.
Then came the blue LED. At first it was dull, but gradually the output increased to a dazzling glare.
With the combination of red, green and blue, manufacturers had the potential of producing a white LED.
This was the dream of all LED manufacturers.
Since the illumination produced by a LED comes from a crystal, it is not possible to produce white light from a single crystal or "chip." The only way was to combine red, green and blue. As soon as the output of blue came up to the quality of the other colours, a white LED was a marketable product.
The more-recent way to produce while light is to illuminate a blue LED and surround it with a yellow phosphor coating. The yellow and blue combine to make white light. This is called additive mixing of colours.  
White LEDs are now with us and their output makes them a viable alternative to the globe.
There is an enormous array of LED torches on the market, from $2.00 "give-aways" to $200 "rip-offs."
Although a LED torch is passable for illuminating an area, it certainly does not have the illuminating capability of a $10 lantern, using a 6v battery.
A LED torch is more of a "fun-thing" to see how far LEDs have come in the past few years and see what can be done with a single cell and an handful of components.
When we first decided to produce a LED torch project, we wanted to fit the circuit into a 2-cell torch but a white LED requires about 3.4v to operate, and two cells produce only 3v. So we had to think of a number of ways around the problem. That's why we have produced a number of circuits.
As you know, a LED will not operate on a voltage below its characteristic voltage. It simply will not operate AT ALL.
This characteristic voltage depends on the type of LED and is about 1.7v for a normal red LED, while a super-bright LED is about 3.1v - 4v.
The exact characteristic voltage varies with the colour, the intensity of the LED, the current flowing and the way it is manufactured. This feature cannot be altered after it is manufactured and the EXACT voltage must be delivered, otherwise the LED will be not work or if the voltage is higher, it will be destroyed. This is the cold, hard fact. The supply voltage must exactly match the characteristic voltage.
This sounds a difficult thing to do, but a simple solution is to add a resistor in series and the voltage across the LED will sit at the exact value required by the LED, while the extra voltage will appear across the resistor. According to Ohm's Law, a current will flow though the resistor and this will also flow though the LED. This applies when the circuit is supplied with a DC voltage.
All we have to do is create a voltage higher than 3.4v and we can drive one of the latest SUPER-HIGH-BRIGHT white LEDs with a single cell, using a step-up-voltage circuit.
This will produce a series of pulses to the LED and the brightness will be slightly higher than if a steady DC voltage is applied. These are the things we will be covering in this project. 

This project explains the operation of a "transformer" in flyback mode. A transformer is one of the most complex items in electronics. Even a simple hand-made "transformer" requires a lot of understanding to see how it works. This project will demystify some of the features.

The web is filled with circuits similar to "
CIRCUIT A" below.
Here are 3 circuits:

Although they work, the performance and efficiency can be increased an amazing 300% by simply adding a capacitor.
We will look at the Joule Thief circuit and show the improved design.


The first circuit in this discussion is the simplest design.
It consists of a transistor, resistor and transformer, with almost any type of LED. The circuit will drive a red LED, HIGH BRIGHT LED, or white LED.
The circuit produces high voltage pulses of about 40v p-p at a frequency of 200kHz.
Normally you cannot supply a LED with a voltage higher than its characteristic voltage, but if the pulses are very short, the LED will absorb the energy and convert it to light. This is the case with this circuit. The characteristic voltage of the LED we used was very nearly 4v and this means the voltage across it for a very short period of time was 4v. The details of the transformer are shown in the photo. The core was a 2.6mm diameter "slug" 6mm long and the wire was 0.95mm diam. In fact any core could be used and the diameter of the wire is not important. The number of turns are not important however if the secondary winding does not have enough turns, the circuit will not start-up.

The transformer is configured as a BLOCKING OSCILLATOR and the cycle starts by the transistor turning on via the 2k7 base resistor.
This causes current to flow in the 60-turn main winding. The other winding is called the feedback winding and is connected so that it produces a voltage to turn the transistor on MORE during this part of the cycle.
This winding should really be called a "feed-forward" winding as the signal it supplies to the transistor is a positive signal to increase the operation of the circuit. This is discussed in more detail in Circuit Tricks.
This voltage allows a higher current to flow in the transistor and it keeps turning on until it is saturated.
At this point the magnetic flux produced by the main winding is a maximum but it is not expanding flux and thus it ceases to produce a voltage in the feedback winding. This causes less current to flow into the base of the transistor and the transistor turns off slightly.
The flux produced by the main winding is now called collapsing flux and it produces a voltage in the feedback winding of opposite polarity. This causes the transistor to turn off and this action occurs until it is completely off.
The magnetic flux continues to collapse and cuts the turns of the main winding to produce a very high voltage of opposite polarity.
However this voltage is prevented from rising to a high value by the presence of the LED and thus the energy produced by the collapsing magnetic flux is converted to light by the LED. 
The circuit operates at approx 200kHz, depending on the value of the base resistor and physical dimensions of the transformer.
The circuit draws 85mA from the 1.5v cell and the brightness of the LED was equivalent to it being powered from a DC supply delivering 10 - 15mA.

Before we go any further, there are a number of interesting circuits on the web.
The following two circuits need explaining. The first circuit is identical to our "Circuit A" except the design engineer did not do his homework. He only added 8 turns to the 100uH inductor and found the circuit did not start-up. His solution was to add another transistor and tie the base to the collector. What a waste of a transistor!
The second circuit is a very inefficient design. The second transistor is being turned on via a 1k resistor on the collector of the first transistor and when this "turn-on" current is not required, it is being shunted to "deck."
Our circuit uses the "oomph" of the secondary winding to saturate the transistor and this produces the highest efficiency.

Here is a circuit from one of the major chip manufacturers:

Apart from the circuit being enormously complex and expensive, 62mA is too high for many white LEDs. The maximum current must be kept to 20 - 25mA.

The first "poor design" got me thinking. Maybe the signal at the transformer end of the 220R needs to be stabilised to improve the performance of the circuit. I tried a transistor and it did not work.
But I actually thought of placing a small capacitor at the join and taking the other end to the 0v rail. This will allow rail voltage to enter the feedback winding of the transformer but prevent the signal generated by the winding being lost through the 2k7 resistor.
The following circuit is the result:

The brightness of the LED did not alter but the current changed from 85mA to 28mA.
The circuit instantly became 300% more efficient.
I could not believe it.
When I put the CRO across the LED, I realised why. The frequency of the circuit changed from 200kHz to 500kHz. The LED was getting more than twice the number of pulses per second.
That's why you cannot trust anything or anyone. This improvement has never been presented in any circuit on the web. Obviously no-one has done any experimenting at all. 
If the brightness of the LED is equal to a DC voltage of 4v and a current of 10mA, the circuit we have produced is slightly more efficient than delivering a DC voltage to the LED, even though there are some losses in the transformer and transistor.
This proves the fact that LEDs driven with a pulse, are more efficient than being driven by a DC supply.

Here is a photo of Circuit B constructed by a reader. He used a toroid (a circular magnetic circuit - or ring) and this has lower losses because the magnetic flux does not emerge (come out of) the end of the core. The magnetic flux keeps circulating. However if the flux is not very high, it does not saturate the core and there are no losses and the slug performs almost the same as a toroid.


You can clearly see the number of turns on the toroid.
The circuit is not very efficient because it does
not have a capacitor to improve the efficiency.

- Amazing!

This bike flasher uses a single transistor to flash two white LEDs from a single cell. And it has no core for the transformer - just AIR!
All Joule Thief circuits you have seen, use a ferrite rod or toroid (doughnut) core and the turns are wound on the ferrite material. But this circuit proves the collapsing magnetic flux produces an increased voltage, even when the core is AIR. The fact is this: When a magnetic filed collapses quickly, it produces a higher voltage in the opposite direction and in this case the magnetic field surrounding the coil is sufficient to produce the energy we need.
Wind 30 turns on 10mm (1/2" dia) pen or screwdriver and then another 30 turns on top. Build the first circuit and connect the wires. You can use 1 or two LEDs. If the circuit does not work, swap the wires going to the base.
Now add the 10u electrolytic and 100k resistor (remove the 1k5). The circuit will now flash. You must use 2 LEDs for the flashing circuit.


The original 30 turns + 30 turns coil is shown on the right. The circuit took 20mA to illuminate two LEDs.
The secret to getting the maximum energy from the coil (to flash the LEDs) is the maximum amount of air in the centre of the coil. Air cannot transfer a high magnetic flux so we provide a large area (volume) of low flux to provide the energy. The larger (20mm) coil reduced the current from 20mA to 11mA for the same brightness. This could be improved further but the coil gets too big. The two 30-turn windings must be kept together because the flux from the main winding must cut the feedback winding to turn ON the transistor HARD.
When the transistor starts to turn on via the 100k, it creates magnetic flux in the main winding that cuts the feedback winding and a positive voltage comes out the end connected to the base and a negative voltage comes out the end connected to the 100k and 10u. This turns the transistor ON more and it continues to turn ON until fully turned ON. At this point the magnetic flux is not expanding and the voltage does not appear in the feedback winding.
During this time the 10u has charged and the voltage on the negative lead has dropped to a lower voltage than before. This effectively turns off the transistor and the current in the main winding ceases abruptly. The magnetic flux collapses and produces a voltage in the opposite direction that is higher than the supply and this is why the two LEDs illuminate. This also puts a voltage through the feedback winding that keeps the transistor OFF. When the magnetic flux has collapsed, the voltage on the negative lead of the 10u is so low that the transistor does not turn on. The 100k discharges the 10u and the voltage on the base rises to start the next cycle.     
You can see the 100k and 1k5 resistors and all the other parts in a "birds nest" to allow easy experimenting. 
This is the first circuit you should build to flash a white LED from a single cell.
It covers many features and shows how the efficiency of a LED increases when it is pulsed very briefly with a high current. 
The two coils form a TRANSFORMER and show how a collapsing magnetic filed produces a high voltage (we use 6v of this high voltage).
The 10u and 100k form a delay circuit to produce the flashing effect.
You can now go to all the other Joule Thief circuits and see how they "missed the boat" by not experimenting fully to simply their circuits. That's why a "birds nest" arrangement is essential to encourage experimenting. 
Note: Changing the turns to 40t for the main winding and 20t for the feedback (keeping the turns tightly wound together by winding wire around them) reduced the current to 8-9mA.

The circuit can be made small by using a ferrite slug 2.6mm diam x 7.6mm long.
The inductance of this transformer is quite critical and the voltage across the LEDs must be over 6v for the circuit to work. It will not work with one or two LEDs.


The circuits we have presented above use a single transistor and a transformer to provide feedback. This feedback is a form of REGENERATION to turn the transistor on HARDER and HARDER to produce the maximum efficiency.
An oscillator can be produced with two transistors and an inductor, but there are some design-features that need to be applied to produce an efficient circuit.
The first circuit is a POOR DESIGN.

Poor Design - see text

In the circuit above, the base current is constant and will be very small through a 10k resistor.
Base-current is effectively wasted current or "wasted energy" and should be kept to a minimum.
The circuit consumes 10mA and the LED will see less than 4mA.
By reducing the 10k base bias resistor to 470R the circuit-current increases to 25mA but the LED is still not at full brightness.
Secondly, the base-current is shorted to the 0v rail via the first transistor and is completely wasted during part of the cycle.
But the main problem with the circuit is the fact that the driver transistor is not driven into full conduction at any part of the cycle and the circuit has very little efficiency. 

To solve this problem, the two transistors are connects so the "turning-ON" is provided by a transistor and it effectively reduces in resistance to a small value to turn ON the driver transistor.
Theoretically a current-limiting resistor should be added in the base of the driver transistor (about 47R) but this made no difference to the current taken by the circuit.

2-Transistor Joule Thief Circuit

The circuit turns ON via the 220k resistor and the voltage on the collector of the NPN transistor drops to nearly 0v. This action causes current to flow through the inductor and at the same time the 1n capacitor is brought towards the 0v rail and this turns ON the first transistor slightly harder. This action continues until the driver transistor cannot be turned on any more.
The 1n charges a little more and the current through the base lead reduces slightly. This action turns OFF the first transistor slightly and the driver transistor is turned OFF a slight amount.
The voltage on the 1n rises and very soon both transistors are fully turned OFF.
The magnetic flux in the core of the 1mH inductor collapses and produces a voltage in the opposite direction.
This voltage is added to the 1.5v rail voltage and the final voltage is high enough to illuminate the white LED.  
This keeps both transistors OFF and when all the magnetic flux has been converted to energy to illuminate the LED, the voltage on the collector drops. This lowers the top plate of the capacitor and since the capacitor is slightly charged, the bottom plate drops to a voltage less than rail voltage. This action turns ON the first transistor to start the next cycle.  

Another 2-Transistor Circuit:

This circuit is designed for mass production as the choke is a standard 33uH.
It can be made by winding 50 turns of 0.1mm enamelled wire on a 1.6mm diam ferrite slug.

With this we turn to a surface-mount chip that has been designed to carry out the exact same task as circuit B. The chip is called PR4401. The following is the promotion advert for the chip:


I could not find any sales literature on the internet, but the manufacturer requires 9,000 pieces to be bought at a cost of 36 cents per piece. This comes to $3,240 if you want to incorporate it into your project.
I have described the pro's and con's of this chip in another article "Circuit Tricks" and you should read it and work out what they really mean.

Kit of components $3.00  plus $4.00 postage

An equivalent IC (chip) has come on the market for 10 cents and it is a better chip.
Here is the circuit for QX5252F:

Using 220uH, the circuit takes 13mA an illuminates 2 white LEDs very brightly.
Using 100uH the circuit takes 30mA and the LEDs are really the same brightness.
Using 33uH the circuit takes 80mA and the LEDs are just about the same brightness.
Obviously the 220uH creates the most efficient circuit.

Here is the prototype:

The kit comes with a PCB, all parts: QX5252 Chip, tactile switch, 1.5v button cell, tinned copper wire for cell and heatshrink for cell-cover, 100uH inductor, very fine wire and 1M to make your own 100uH, 2 machine pins and length of fine solder, but only 2 LEDs and not the change-over switch.  All for $7.00 posted.
Email Colin Mitchell for details on buying the kit.

The prototype has been built on Matrix Board and shows the change-over switch used to test different LEDs.  You will get the Printed Circuit Board in the kit that has been generated from the layout. This is the easiest and simplest way to make a PCB and avoid any mistakes.

The inductor has been fitted via machine pins and it can be removed and different inductors fitted to see the results. A machine pin is hollow and allows to poke the ends of a conductor into the pin and it will make contact.
The current taken by the circuit changes according to the inductance and this will enable you to compare inductors and even find the value of an unknown inductor.
You need to use one, two or three known inductors and make a table of the inductance and current taken by the circuit.
The current may or may not be linear but we measured inductors from 33uH 1,000uH and recorded currents from 80mA to 2.5mA.
This enabled us to measure unmarked coils.

One more feature of this project is to wind your own inductor and see if it is effective as the 100uH supplied in the kit. 
The kit comes with fine wire and a 1M resistor.
Wind 250 turns on the resistor very carefully and as you come to the end of the winding, you can criss-cross the wire over the other turns to keep them in place.
Leave at least 4cm of wire at the beginning and end.
Now heat the wire very close to the body of the resistor with a hot clean soldering iron that has been fully tinned. The wire will get tinned very close to where it comes from the winding. Now wind 5 turns around the wire coming from the resistor and solder it in place. Break off the wire.
Do the same with the other end.
Fit your home-made inductor and the LED should be as bright as the 100uH.
Measure the current across the switch. It should be about 32mA.
The voltage is being converted from 1.5v to 3.5v and each LED will get slightly less than 6mA by the time you take the efficiency of the circuit into account.

Here is the waveform:

 What is happening?
The first part of the cycle shows the inductor being pulled down to the 0v rail.
This means the 1.5v for the battery is directly across the inductor and current starts to flow in the winding.
This produces magnetic flux (called EXPANDING FLUX) that cuts all the turns and produces a voltage in the turns that is opposite to the incoming voltage.
This means the effective voltage entering the inductor is very small and thus a small current flows. However enough current flows and enough time is allowed so the inductor produces magnetic flux.
This is shown in diagrams A and B.
The circuit then immediately turns OFF and the magnetic flux collapses very quickly.
This is shown in diagram C. This voltage is actually in the opposite direction to the original voltage and this is one of the most important things to understand.
The voltage produced by the inductor will be very high (possibly 10v or more) (and is added to the voltage of the battery. But as soon as the voltage reaches 3.2v, the white LED starts to turn ON and produce illumination.
The magnetic flux keeps collapsing and supplying energy to the LED for about 2uS and when the voltage falls to less than 3v, the LED turns OFF.
The QX5252 IC turns ON again and pulls the inductor to the 0v rail to start the next cycle. The IC operates at about 130kHz and if the inductor has not lost all its magnetic flux, it will add to the flux on the next cycle. 
You can see the inductor only has to produce about 1.7v as the voltage is "produced" on top of the 1.5v from the battery. The circuit will work down to about 0.9v
Understanding and interpreting a waveform is very important because the LED is only turned ON for a very small portion of the time but it is turned ON very brightly.
Our eyes detect this brightness and hold this brightness while the LED is completely turned OFF during the rest of the cycle. This effect is called PERSISTENCE OF VISION.
This is why a very small current will produce high brightness and create a highly effective circuit.
That's how the circuit got its name: Joule Thief. It appears to get energy from nowhere. But we know a LED can operate in pulse-mode and product a very high brightness while consuming a small OVERALL current.
This project will teach you 3 things:
The number of turns to create 100uH inductor, testing different inductors, testing different LEDs and determining the value of inductance by measuring the current.


When you build circuit "B," you will realise the specifications given in the .pdf for the chip, could be improved. We have achieved a supply current of 18mA for an equivalent brightness of 10mA. The chip requires 25mA. So, all the technology in the world has not surpassed a hand-made circuit.
The advantage of our design is the ready availability of components and you can change them to suit your own application.
If you want to increase the brightness, the 2k7 can be reduced to 1k5.
If you want to drive 2 LEDs, they can be added in series:

Adding a 100u across the battery will increase the current by 4mA and the brightness will increase slightly.
When 2 LEDs are placed in series, the current drops from 28mA to 23mA and the brightness from each LED is slightly less. This circuit is operating at about the maximum capability of the transformer. The actual limiting factor is the size of the "core." It can only "hold" a certain amount of magnetic flux and return it to the windings during the collapsing part of the cycle. A larger core will allow three or more LEDs to be illuminated.
The "high efficiency" of this circuit is due to the "pulsing of the LED." When a LED is pulsed with a high current for a short period of time, the brightness is equivalent to a lower, steady, current. That's why a current of 23mA from the battery will illuminate 2 LEDs with an equivalent brightness of about 8mA of steady current. It is very difficult to compare the brightness of one LED against another and these results are the best you can make by visual inspection. We are not driving the LEDs to their maximum but the output is very impressive.

The secret of this circuit is the transformer.
We normally think of a transformer as a device with an input and output, with the voltage on the input and output being connected by a term called "turns ratio."
If the output has more turns than the input, the output voltage will be higher.  This is called a step-up transformer. If the output has less turns than the input, the output voltage will be lower.
This applies to "normal" transformers where the voltage is rising and falling at a regular rate, commonly called a "sinewave."
But the transformer in this circuit is different.
The voltage applied to it is not rising and falling smoothly, and thus it does not work in normal "transformer mode."
The voltage is being applied and then turned off. When the voltage is applied, the primary winding (the 60 turn winding) produces magnetic flux. When the voltage is turned off, the magnetic flux collapses and produces a VERY HIGH voltage (in the REVERSE DIRECTION), in all the windings.
Our transformer is really a coil in flyback mode with a feedback winding.
The feedback winding delivers a voltage to the transistor to turn it on HARDER. If the winding is connected around the wrong way, the circuit will not work.
The other important factor about the transformer is the core material. There are many different types of ferrite. Ferrite is a type of iron which is powdered very finely so that the magnetic lines that pass through the particles do not create eddy-currents. These eddy currents absorb the magnetic flux. The material we have used is F29 and this is suitable for high frequency applications.
The circuit also employs a term called RE-GENERATION. This is the effect where a circuit is turned on slightly by a component (the 2k7 base resistor in this example) and then the transistor turns itself on more and more until it is fully turned on. The feedback winding is configured so that the voltage it produces (actually the current it produces) is fed into the base to turn the transistor on.
Thus the feedback winding is very clever. It produces energy and is delivered in a particular direction - in other words it can be a positive or negative energy. In this case it produces positive energy, to turn the transistor on harder.
This is called POSITIVE FEEDBACK as it turns the transistor ON during the active part of the cycle.
Now we come to the MAIN, PRIMARY or FLYBACK winding.
This winding produces a high voltage during part of the cycle (the FLYBACK part of the cycle) and this is passed to the LED.
If the LED is removed, the transformer produces a high voltage with a low current, but when the LED is inserted, an amazing thing happens. The energy from the transformer is converted to a lower voltage with a higher current.
What actually happens is the LED absorbs the energy and turns it to light as soon as the voltage rises to 3.6v.
We could achieve the achieve the same low-voltage, high current requirement, with less turns, but the number of turns has actually been determined so the core does not saturate.
The voltage for the LED is produced when the transistor is switched off and the magnetic flux in the ferrite core collapses.
The speed of the collapse produces a very high voltage in the OPPOSITE DIRECTION and that's why a positive voltage emerges from the end connected to the LED. These two facts are important to remember.
The other important fact is called "transformer action." This is the action of magnetic flux.
When a voltage is applied to a winding of a transformer or a coil of wire, a current will flow and this will produce magnetic flux. If another winding is present, the magnetic flux will cut the turns of this extra coil and produce a voltage in it.
However, there is a very important point to remember. The magnetic flux can be: EXPANDING, STATIONARY or CONTRACTING.
When the magnetic flux is expanding, a voltage will appear in the second winding mentioned above.
When the magnetic flux is stationary, NO VOLTAGE will appear in the second winding.
When the magnetic flux is contracting a voltage will appear in the second winding with REVERSE POLARITY.
The size (the amplitude or "value") of the reverse voltage will depend on the speed of the collapsing magnetic flux. If the flux collapses quickly, the amplitude will be very high.
That's how the transistor turns itself on and on until it is fully turned on. At this point the current flowing through the circuit is a maximum but the flux is not expanding so the base of the transistor does not see the high "turn-on" energy and thus the transistor suddenly turns off.
The magnetic flux collapses and the transistor sees a reverse voltage on the base to keep it turned off until the flux is fully collapsed. The current through the 2k7 enters the base to start the cycle again.
From this you will be able to see how the transistor and transformer work.

Now we come to the problem of flashing a white LED, using a 1.5v supply.
The following circuit performs this task:

The oscillator charges the 100u via the 1N 4148 diode and when the voltage reaches about 10v, the BC 547 transistors "zeners" (breaks down) and conducts. Energy in the 100u is then dumped into the LED to make it illuminate. This causes the voltage across the 100u to drop and the transistor comes out of conduction. The oscillator then continues to charge the 100u to repeat the cycle.
The zener voltage of the transistor is not 10v as approx 4v is dropped across the LED. This conforms with an article on the web that said the emitter-collector junction is equal to a 6v2 zener.
The 330R charging resistor produces a fast flash and the 1k produces a slow flash.
The current for the circuit is approx 22mA and any type of LED can be fitted.
Measuring the current-consumption of a circuit is a very difficult thing to do.  When you insert a a meter into the positive line (or negative line) of a circuit, you introduce extra resistance and the operation of the circuit will alter. You may think the low resistance of an ammeter will not affect the performance, but quite often the "ammeter " is really a "milli-amp meter" and the "shunt resistance" on the 200mA scale can be 4 - 7 ohms. This is quite considerable when a circuit is operating on 1.5v and drawing 30mA. This can be a loss of 100mV to 200mV and the current taken by the circuit will alter considerably.
That's why the best approach is to place a 1 ohm resistor in line with the positive of the battery and measure the millivolt drop across the resistor. Each millivolt drop will correspond to 1mA flow and this will change the circuit conditions as little as possible. The following circuit shows how this is done:

A 100u electrolytic across the circuit will reduce the impedance of the supply and keep the circuit working as normal as possible.

As a point to note: The White LED Flasher circuit did not start-up on a flat AAA cell.
Solution: take two flat cells and connect them in series and see how long the LED will flash. You will be very surprised. The circuit will draw about 30mA and the LED will flash very quickly.
The circuit will continue to work on two very flat cells until the flash rate drops to one flash per second.  
This type of circuit puts a very heavy "strain" or "noise" on the power supply. In other words it puts a heavy demand on the battery for a short period of time.
This is not a problem if the only item connected to the battery is the flasher circuit. But if the battery is also driving a circuit such as an mp3 player or microcontroller, the high-frequency noise may upset the operation of the electronics.

The oscillator transistor needs to sink a very high current for a very short period of time (as mentioned above) and thus it must be a "high-current" type. A "high-current" type improves the efficiency of the circuit. If the transistor cannot sink the transformer to the 0v rail, it effectively becomes a "resistance" in the network. Suppose the supply is 1500mV (1.5v  , 1v5) and the transistor can sink to 500mV, 30% of the voltage is dropped across the transistor and thus the circuit is using only 66% of the incoming energy. If the transistor can only sink to 0.75v, the circuit is using 50% of the incoming energy.
Some transistors can sink to 0.3v and thus the circuit is more efficient.

Now we come to the stability of the circuit. The circuit is very unstable and very unreliable. Touching the components with a finger changes the frequency of the flash-rate and connecting  CRO to the collector of the oscillator transistor inhibits the flashing. The oscillator keeps working but the zener transistor fails to operate.
This circuit is totally unsuitable for a commercial design and it reminds me of some of the original transistor flasher circuits. They required precise values of resistance and did not work when the supply voltage dropped.
Fortunately someone came up with the flip-flop flasher and changed everything. It is totally reliable and operates under all sorts of conditions.
Now we come to the design of a higher output circuit, to satisfy those who want to use a larger cell and drive 2 or 3 LEDs to maximum brightness.

To drive more LEDs, a higher output is needed. We have already mentioned, the limiting factor with the circuits above is the transformer. To achieve a higher output, the size needs to be increased. This is quite easily done by getting a larger core. It is the core that determines the amount of flux that can be stored. When turns are wound on a core, the result is called an inductor and when a second winding is added, the result is called a transformer.
Most of the inductors and transformers we use in the circuits in this article have an open magnetic circuit. This means the flux escapes out one end of the core and in general the result is not very efficient. But it has proved to be satisfactory.
An improved core is called a "pot core" and consists of two halves as shown in the diagram below:


The magnetic lines go around the "magnetic circuit" as shown in the diagram above and pass through an air gap. The air gap is to compensate for the DC across the coil (transformer). If the air gap is closed up, the inductor will saturate before the circuit is fully conducting and this may make the inductor less effective. All this theory is very complex and you really have to try the component to see the effect.
Our circuits use a simple "in line" inductor as shown above or a "bobbin" as shown below in the third item. The photo below shows the "slug" transformer used in circuits A, B, and C and the "bobbin" transformer used in circuit D. The size of each transformer gives some idea of the relative output. The centre inductor is a 10mH choke. This is unwound to get the bobbin for the transformer.

The bobbin is re-wound with 35 turns of 0.5mm wire for the primary and 20 turns for the feedback winding. The two pins connect to the primary and the 20 turn-winding is wound on top, with flying leads. The gauge of the wire is chosen so that the windings completely fill the bobbin. The feedback winding can be a thinner gauge, without any detriment to the operation of the circuit. By the appearance, you could expect up to 5-10 times more output from the bobbin.
But with a higher output, you need to provide some form of energy-limiting circuit to prevent damaging the LED.
The following circuit provides current limiting so that the LED will produce maximum brightness for the voltage range 1.5v to 0.9v.

This gives a choice to suit a variety of torches. The smallest penlight torch will only have enough room to drive a single LED while the larger "C" and "D" cell torches will drive two or three LEDs.
There are some slight differences between each of the circuits and you need to read the article if you want to deviate from any of the layouts we have given.
For instance, the 2SC 3279 transistor is capable of sinking 2 amps and this makes it a better driver for circuit-2 but its collector-emitter voltage is only 10v and it may zener in circuit 3, where the voltage is very near this value.

Circuit-1 drives one LED from a single cell

Circuit-2 drives two LEDs from a single cell

Circuit-3 drives three LEDs from two cells

The circuit includes a feature called "current regulation." You can also call the feature "voltage regulation" as both have the same effect of controlling the brightness of the LED.
It can also be called a "constant brightness" arrangement.
It's a feedback arrangement consisting of a BC 547 connected to the base of the main transistor.
When the voltage across the "detector resistor" rises above 0.7v, the BC 547 turns ON and prevents the main transistor operating.
This allows the LED to produce a constant brightness over a wide supply voltage. The circuit will theoretically work to 0.8v.
Do not remove the current regulating transistor as the circuit will over-drive the LED when the supply is 1.5v. The excess current will instantly destroy the LEDs.

The actual operation of the circuit can be explained in a little more detail.
When the circuit is turned on, the oscillator transistor produces a high voltage from the inductor and this is rectified by a diode to charge a 100u electrolytic.
When the voltage rises to over the total characteristic voltage of the LED or LEDs, they turn on and current flows though the 39R "detector resistor."
The voltage across the 100u will continue to rise and since the characteristic voltage of the LEDs has been reached, any further voltage rise will appear across the resistor. As soon as this voltage reaches 0.7v, the feedback transistor begins to turn on. The feedback transistor acts like a variable resistor as shown in the diagram below and some of the current from the feedback winding is passed to the 0v rail, through the transistor. The oscillator transistor sees a reduced "turn-on" effect and the output of the stage is reduced.

In this way the brightness of the LEDs can be kept constant throughout the life of the battery.

The circuit is actually being "pulled back" when a fresh cell is connected, by the action of the feedback transistor. As the voltage from the cell reduces, the oscillator circuit will not be able to produce a high output and the action of the feedback section will not be needed. Eventually the voltage of the cell will be so low that the LED will start to dim. This is the end of the life of the cell.

Caution: Do not allow more than 25mA to flow though a white LED (unless it is being pulsed) as it will be instantly DESTROYED. Other LEDs (such as low-brightness red LEDs) are much more tolerant - but white LEDs are easily damaged.

A number of circuits similar to this project have been presented on the internet. One circuit had twice the number of components and used 4 transistors.
The art of designing a circuit is to make it as simple as possible, while providing all the needed features. It is pointless making a circuit complex, as it simply adds to the cost and makes fault-finding more difficult.
But a note near one of the circuits was really annoying. It said the circuit "had not been tested, only a simulation was run."  While these simulation programs work in a number of applications, they certainly cannot take into account the characteristics of an inductor. This is one item that no-one can predict. It's performance depends on so many variables.
If you think you can design a circuit such as this on a simulator, and it will work, you are kidding yourself.
Electronics is not that simple.
Transistors exhibit different characteristics according to the current flowing though them and a circuit such as ours requires the main transistor to pass a very high current for a short period of time.
Fortunately, Japanese transistors are capable of passing a high current while some Philips transistors will fail to pass the test. The gain of a transistor under these stressful conditions cannot be determined from a data-sheet.
Circuits should never be presented in an article unless they have been tried and tested.
A simulation program cannot possibly take into account the effectiveness of an inductor in any particular situation, even though the inductance is known.
There are hundreds of ways to produce a 10uH inductor, or any inductor for that matter.
It can be air-cored or ferrite cored. The windings can be thick or thin wire. The core can be made of several different materials. On top of this it will depend on the frequency of the circuit.
The output voltage of an inductor that has been specially designed for a particular circuit can be 100 times higher than an incorrectly designed item. That's why it takes a considerable amount of "trial-and-error" to produce an ideal inductor or transformer.
The output voltage has a lot to do with the "Q-factor" or quality factor and this is a value that is associated with the way the inductor or transformer has been designed. The "Q value" is basically  the ratio of the supply voltage compared to the output voltage.
No simulation program can "guess" the value of "Q" and since the operation of the circuit is entirely dependent on this value, it has to be constructed.
I would not even attempt to put this type of circuit on a simulator.

There are many ways to go about designing an inductor or transformer.
You can sit down and study the theory of inductance, the effectiveness of ferrite material at different frequencies, the use of different wire gauges and the associated inductance formulae.
If you think you will be able to produce an inductor for this circuit entirely from theory, (with the first prototype working perfectly), you are kidding yourself.
There are a number of parameters you cannot specify in the formulae.
Even if you did come up with an answer, no electronics-designer would be satisfied with the first result. He would need to see the prototype and add or remove turns to see the effect. He would use thicker or thinner wire and note the effect. He would carry out all sorts of experimentation, including monitoring the battery current while noting the current though the LEDs to work out the efficiency of the circuit.
It could take 50 or more prototypes to arrive at the best design.
So, where do you start when designing a transformer or inductor?
No-one really knows where to start.
It all comes from trial-and-error and guessing a starting-point.
The easiest way is to copy an existing design.
But if you don't have something to copy, you can begin with say 10 turns. Note the output voltage and current taken by the circuit.
Increase the winding to 20 turns. Again note all details. From the figures you can work out if you are going in the right direction.
Continue collecting data with both additional turns and reduced turns as, sometimes, an unusual feature suddenly arises.
Keep working until you are satisfied with the results.
Even if you have studied inductor theory, you will still have to carry out the practical side of things.
Nothing takes the place of actually "doing-it."

In our 3 circuits, there are many different combinations of windings that will work.
The reason is the circuit is non-critical.
You have to understand the operation of an inductor in an entirely different way to the theoretical model to see how it operates.
This is called a "loose" circuit and a wide range of primary windings will produce the same result.
For example, a primary winding of 35 turns will produce the same LED brightness as 55 turns and the current from the supply will be the same.

The output of the transformer (on no-load) will be more than 200v and thus the circuit must not be operated on no-load as the voltage may damage the transistor.
If the LEDs are removed, the circuit will charge the capacitor to more than 45v and this is above the operating voltage for a 100u/25v electrolytic.
If you remove the LEDs and turn the circuit on, then re-solder the LEDs, they will be damaged. This is because the electrolytic will have charged to 45v.
Thus it is very difficult to experiment with the circuit to see how the transformer charges the electrolytic.
You will have to follow our explanation:

The electrolytic is charged by pulses from the inductor.
In circuit-3 the voltage across the electrolytic is 10v and it is delivering current to the three LEDs at a constant rate of 17mA.

CRO waveform - output of inductor

In the CRO diagram above, the pulses (or spikes) occupy about 10% of the total time.
The area under the graph (under each spike) is shown in orange and this represents the energy supplied to the electrolytic.
The inductor is capable of producing a very high spike when in flyback and this voltage allows a burst of current to pass though the diode and charge the electrolytic.
When the inductor is operating under no-load, it is capable of producing a spike of more than 200v, but this voltage is not allowed to be produced when the load is connected. The voltage-spike is limited to the characteristic voltage across the LED or LEDs, plus the voltage drop across the diode and minus the battery voltage. The voltage will be about 9v.
If we are drawing 17mA for 100% of the time, we must deliver 10 times 17mA for 10% of the time to keep the electrolytic charged. Thus a current of about 17 x 10 = 170mA is needed to pass through the diode to charge the electrolytic.
The other feature of the diode is it prevents the voltage on the electrolytic being discharged to the 0v rail via the transistor when it is turned on.

The frequency at which the circuit operates is determined by the inductance of the inductor. The cycle start when the power is applied and the transistor turns on to allow current to flow though the main winding. This produces magnetic flux in the feedback winding to turn the transistor on harder. This continues until the transistor is turned on fully and maximum flux is produced.
But the flux is not expanding flux and thus it does not cut the turns of the feedback winding and the transistor does not get the full turn on current into the base.
The transistor is turned off and this causes the magnetic flux to collapse. This flux is in the opposite direction and it produces a reverse voltage in the feedback winding to keep the transistor fully turned off.
The main winding also produces a voltage in the opposite direction and it delivers a pulse of energy to the electrolytic via the high-speed diode.
As soon as the magnetic flux is spent, (converted to electrical energy) the cycle starts again.
The combination of these two operations creates the length of time for one cycle.
In our case the circuit operates at approx 90kHz.

There is a lot of hype and confusion about the light output of some super-bright LEDs.
Sometimes there is very little difference when you compare the output of 1cd, 3cd and 6cd (6,000mcd) LEDs, when supplied from wholesalers.
One of the reasons is the difficulty in identifying each LED. They have no markings and if they are not kept in their correct bag, they can get mixed up! There are literally dozens of different types.
Secondly, the difference in brightness is due to the angle at which the light-beam emerges from the LED. This is due to the lens inside the LED and/or the way the LED is potted, producing a divergent beam or a narrow beam.
Almost all LEDs have a different illumination intensity, color and spot-size, depending on the manufacturer, beam angle and quality of the chip producing each color (efficiency).
Some have a blue appearance in the centre of the spot white light while others have a noticeable green fringe.
This project is an ideal way to test 2 or 3 LEDs at the same time. Since they are in series, they pass the same current and the intensity control will allow you to vary the brightness and compare the outputs.
When experimenting, keep a record of the type of LED by paining it with red or while nail polish. Keep the same reference on the bag from which they came. This will prevent them getting mixed up.





Current Case


140 to
















Firstly you need to decide on the type of housing you want to use. This will determine the circuit you will use, the number of LEDs and the shape of the PC board.
It's best to get a kit of components as the core for the inductor is supplied with winding wire and these are normally difficult items to get.
If you want to use the project for experimentation, circuit-3 has an adjustable brightness control.
The only extra components you will need are red LEDs to take the place of the white LEDs, when you are setting up the circuit.

If the circuit does not work, you have two choices. You can buy another kit or carefully work though the assembly and see where you made the mistake.
Things like the orientation of the transistor, diode and LED need to be checked but the general reason for the project not working is the connection of the transformer. Simply reverse one of the windings.
It does not matter which way the windings are wound on the ferrite core. By simply reversing one of the windings, the transformer will work. Do no reverse BOTH windings as this will not solve the problem.

Before experimenting with any of the circuits, there are a number of things you must be aware of.
The inductor is capable of producing a very high voltage when no load is connected and this can cause damage to the oscillator transistor, the electrolytic and/or the LEDs.
We have already mentioned some of the ways the components can be damaged and the most critical component is a white LED. It will not tolerate excess current, even for a fraction of a  second. Ordinary red LEDs are very tolerant and this gives you a false sense of robustness.
The circuit is capable of charging the electrolytic to more than 45v and if a white LED is connected when the electrolytic is fully charged, it will illuminate very brightly and die.
The situation does not occur when the circuit is operated normally and this means experimenting with the circuit is risky if you don't know what you are doing.
One solution is to use 2 red LEDs to take the place of each white LED.
You can take all current, voltage and efficiency measurements with the red LEDs and when the circuit is operating as required, the LEDs can be replaced with a white LED.
Don't let the sensitive nature of a white LED deter you from experimenting - simply substitute them.
This project has been specially designed for experimenting. The main reason for using a hand-made inductor is to allow different arrangements to be tried.
One point you will have to remember:
The energy from an inductor in flyback mode depends on the amount of ferrite in the core. The core supplied in the kit can only supply enough energy to fully illuminate 2 white LEDs.
When 3 LEDs are used, the maximum current it can supply is about 15mA.

We have just about covered everything, but a few experimenters have provided some circuits that should be included.
All Joule Thief circuits need a feedback arrangement to keep the oscillator producing a waveform.
The simplest is to have an additional winding that provides this waveform (an increasing voltage) that turns the transistor ON more and more until the transformer is saturated and cannot supply energy via the winding. At this point the circuit IMMEDIATELY stops increasing and starts to collapse. The voltage (energy) in (from) the winding is reversed and turns the transistor OFF COMPLETELY to assist in the circuit collapsing.
In place of the feedback winding, a transistor and a few components can be used. It basically does the same thing and the result(s) are the same as it is easier to use a 2-wire inductor when building products for mass distribution. 2-leaded inductors are plentiful and cheap.  
We are now going to look at some of these circuits and see how to drive the more-powerful 1-watt LEDs.
All the above Joule Thief circuits consume about 20 to 50 mA to illuminate 5mm white LEDs.
The next circuits consume over 100mA and are classified as HIGH POWER circuits.
When you are designing HIGH POWER circuits you are more likely to damage a transistor from over-heating and you have to be prepared for failure.
The first fact to remember is this: The transistor in all Joule Thief circuits is turned ON for about 50% of the time and then the collapsing magnetic field produces energy to illuminate the LED for about 50% of the time.
This means the peak current is about twice (or more) the average current supplied to the LED (the current you will measure via a multimeter).
These are only approximate figures to give an idea of what is happening. A 1 watt LED has a characteristic voltage across it of about 3.3v  to 3.6v and this means the current will be 330mA.
If we are pulsing the LED, this current will increase to more than 500mA for very short pulses and this current must be handled by the transistor.
When a transistor has to supply a high current, the collector-emitter voltage increases and it may be 0.5v at low current but increase to 3v when the current is increased.
That's why the "hidden" operation of the circuit is completely different to what you are expecting. By "hidden" we mean the short pulses of high current. 
In fact you can't measure the current in the normal way because the resistance of the shunt in a multimeter will lower the current considerably and produce a false reading.
In addition, the high frequency of the circuit will produce a completely inaccurate reading.
All current readings must be taken across a 1 ohm resistor in the supply and using the millivolt scale to produce a reading of 1mA for each 1mV on the scale.
These circuits have been provided for your experimentation. The effectiveness of their performance will depend on the driver transistor and the inductor.
Some transistors work much better than others and some inductors produce much better outputs than others.

In the following circuit, Samuel Budiyanto budiyantosamuel90@gmail.com  used a 1N5819 Schottky diode, in place of the UF4004 and a 1.0mH coil from a disposable CFL, lamp (150 turns, 0.25mm on 5mm ferrite rod).
The current was 180mA from 13.7v and the output was 360mA @ 5v. 
This is about 80% efficient. The 1k was increased to 10k.


The next circuit has some interesting features - the two 1N4148 diodes and the two 1 watt LEDs in series.
Samuel Budiyanto budiyantosamuel90@gmail.com  suggested using  2SD882 in place of BD139 and decrease 47n to 10n.  Experiment with it yourself.


There are lots of JOULE THIEF circuits on the web and many photos and diagrams showing how the components are connected. 
The operation of the circuit is very complex and takes a lot of description to fully explain it.
Even though this project uses only a few components, you should build a few of the different design and see how they compare.

Here are a few circuits from the web:

Don't use more than one cell or the LED will illuminate when the circuit is not working  !!


All the circuits described above use and inductor with feedback winding and it is very easy to show the output of the feedback is moving in the opposite direction to the output of the main winding and we call this "out-of-phase" and it is 180 out of phase.
This is the signal (or pulse) that is needed to turn on the transistor harder and harder to create the first part of the cycle.
It is the magnetic flux from the main winding that passes through the core and produces the voltage in the feedback winding.
But in the following circuit the two coils are surface mount inductors and they are 10cm apart and can be connected either way to the circuit and the LED illuminates.
Thus there is no magnetic interaction between the two components.
But here's the secret. The inductors are about 100microHenry to 470microHenry and these are what we call the "high-speed" components in the circuit.
The 1n2 capacitor is what we call the "slow component."  Unless you take on this way of thinking, you will not be able to see how the circuit works.
The circuit starts by charging the 1n2. The resistance of the 470uH + 470uH is very small and the capacitor charges quickly.
As soon as it gets to 0.6v, the transistor start to turn ON. But the transistor has to turn on fairly hard to reduce the voltage at the join of the two inductors and thus the voltage on the capacitor has to rise higher to create a current through the 6k8 resistor. 


This circuit is impossible to describe via "logic" and so  it was necessary to put a dual trace CRO on different connections to see what was happening.
The first thing to point out is this:
The two inductors are not near each other and do not produce or have any magnetic influence between them.
Thus there is no feedback in the normal sense of a transformer or feedback winding.
Secondly, the circuit is very reliable at self-starting but the values of inductance and capacitance need to be in the range shown. The inductors used in the experiments ranged from 100uH to 330uH and the capacitor from 1n to 3n3.
 All the circuits described to date in the "Joule Thief" category worked on the principle of FLYBACK, in which the transistor is turned ON very hard and immediately turned off so that the magnetic flux in the inductor or transformer produces a very high voltage.
This circuit works differently.
The turning ON and OFF of the transistor is much smoother and the voltage across the capacitor is a perfect sinewave.
But the mystery is this: how is the transistor turned ON more and more to get to the saturated state.
All we can say is this: The circuit starts to turn ON by charging the 1n capacitor and current flows through the two inductors.
This puts energy into each inductor and when the transistor starts to turn ON, the voltage at the join of the two inductors decreases.
You can see this on the oscilloscope and when the transistor turns ON more, the voltage out of the end of the second inductor charges the 1n with a sinewave to 3v. This is sufficient to put current through the base resistor to fully turn ON the transistor. The charge on the capacitor gradually flows into the transistor and it decreases. When it drops to 0.6v, the transistor turns OFF and the energy in the first inductor produces a high voltage spike to illuminate the LED.
At the same time current flows through the second inductor to charge the capacitor to repeat the cycle. 
The first question you ask is: How can the second inductor produce an output voltage of 3v when one lead is being taken to 0v.
The answer: The current through the inductor at the beginning of the cycle is fairly high and this produces a lot of magnetic flux.  When the left lead is taken to 0v, the magnetic flux collapses and if it collapses fairly quickly, the inductor produces a high voltage in the opposite direction to the original voltage. That's what happens in this case. The voltage may be double but the output current will be half. Or alternatively, the current multiplied by the original time for the effective current to flow, will be equal to the output current and the duration it will flow. The output current will depend on the voltage produced when the flux collapses and the resistance of the circuit it is being delivered to.
The only simple way to describe this is to say the energy absorbed by the inductor in the first place will be equal to the energy it delivers (minus losses).

What we are talking about is the watt-second. In other words, volts x amps for a period of time.
Obviously, in this case it is milliamps and less than 1 volt for a few micro or milliseconds.
But it is the fundaments we need to get across.
The value of 1 volt x 1 amp for 1 second is the joule, and someone came up with the clever name JOULE THIEF, to describe these circuits.
There is no creation of energy from "no-where " in these circuits.
It's just that energy can be converted from a: "dribble over a long period of time" to a "short, sharp, big bang." - like pulling back the string of a bow or hammering with a sledge hammer.
An inductor can create a conversion.
You can pass a current through an inductor from a 1v supply for say 1 second and when the voltage is turned off, the inductor will produce an output voltage of say 2v for 1 second but the output current will be half. Or the output can be 4v for 25% of the original current. You can get all sorts of results but the combination of the voltage, current and time cannot be greater than the original combination.
This is simply called the CONSERVATION OF ENERGY.
ENERGY IN = ENERGY OUT (minus losses).
That's why this circuit appears to create MAGIC. But it simply converts one of the parameters to a higher value and one or more of the other parameters is reduced in value.

The next circuit works amazingly well.
Obviously designed by someone "fooling around" as it does not conform to convention.

Another miracle of electronics.
This circuit has never appeared in any text book, website or forum. It appears to be fake.  But when you construct it, the results are amazing. The circuit draws 8mA when the base resistor is 68 ohms and the other component used in the test circuit was 3n3 for the capacitor.

It's not simple in operation and needs to be explained carefully to show how the high voltage is generated.
The capacitor charged very quickly via the resistor and when the base of the transistor sees 0.6v, it turns ON.
The resistor is a very low value and it will have the ability to fully turn the transistor ON.
This action takes the collector to about 0.3v above the 0v rail.
The base is at 0.7v and the voltage difference puts current through the inductor to produce magnetic flux. And as the transistor turns ON, the current increases. This produces EXPANDING magnetic flux.
Eventually the transistor turns ON fully and although the current is a maximum, the flux is called STATIONARY flux and it does not cut all the other turns of the inductor to produce a back emf (voltage). The inductor changes its characteristic from a high impedance component  to become a resistor of a few ohms and it discharges the capacitor and the base voltage drops a very small amount.   This reduced the current flowing through the inductor and thus the current cannot maintain the large amount of magnetic flux. This causes the strength of the magnetic flux to reduce and now we call the flux COLLAPSING flux. This creates a reverse voltage in the winding and the voltage that was higher (or more positive) on the base is now LESS and the voltage on the base reduces. It only has to reduce by a small amount and the transistor turns OFF.
The magnetic flux keep collapsing and the collector lead effectively disappears. The only thing left on the right-hand lead of the inductor is the LED.
Because the flow of current has ceased, the magnetic flux collapses very quickly and this produces a very high voltage.  The charging voltage will have been only a few hundred millivolts, but the collapsing voltage can be as high as 10v to 12v.
But the LED starts to turn ON when the voltage reaches 3.4v and so the excess voltage is converted into current to illuminate the LED very brightly.
When all the magnetic flux has been converted, the supply starts to charge the capacitor via the resistor to start the next cycle. The frequency of operation of this circuit can be anywhere between 50kHz and 180kHz.


LED Torch  

$7.00 incl postage
Email Colin Mitchell for details
on buying the kit.

The LED Torch - Joule Thief project comes with a PCB, all parts: QX5252 Chip, tactile switch, 1.5v button cell, tinned copper wire for cell and heatshrink for cell-cover, 100uH inductor, very fine wire and 1M to make your own 100uH, 2 machine pins and length of fine solder, but only 2 LEDs and not the change-over switch.  All for $7.00 posted.
Email Colin Mitchell for details on buying the kit.