15 Minute Joule Thief

by playmonkey

20K22126

Intro: 15 Minute Joule Thief

This is an introduction into the operation of a Joule Thief and a brief look at it’s history. In November 1999 a simple circuit was published by Z. Kaparnik of a transformer-feedback single-transistor voltage converter in Everyday Practical Electronics magazine. The Joule Thief circuit is based on the blocking oscillator which predates World War II.

The circuit starts when the transistor is off. Current flows through the left hand side of the transformer and through the 1k resistor, into the base of the transistor. The transistor turns on slightly and produces a current in the collector emitter circuit. This allows current to flow in the right hand winding of the transformer and produce magnetic flux. This flux cuts the turns of the left hand winding and produces a voltage that adds to the voltage produced by the battery.

This increases the current into the base of the transistor and the transistors turns on more. This continues and the transistor turns on more and more until it cannot turn on any harder. At this point the magnetic flux in the right hand winding is a maximum but is not expanding flux and thus the left hand winding does not produce any additional voltage. The current into the base of the transistor reduces and the transistor turns off slightly.

The current through the right hand side of the winding reduces and the magnetic energy in the core of the ferrite ring starts to collapse and produce a voltage (in both windings) of opposite polarity. In the left winding, it starts to turn the transistor off completely and in the right winding, it delivers this energy to the LED. Now here’s the clever part. When the current is abruptly switched off, as is the case with this circuit, a voltage is produced in both windings that has opposite polarity to the original voltage and will be higher amplitude than the original voltage.

This voltage can be 10 or even 100 times higher than the original voltage and this is called the “Q” of the circuit. We are not creating something for nothing as the voltage will be higher but the current will be lower than the current drawn from the battery. The voltage produced by this circuit will be over 10v but a white LED has a characteristic voltage of about 3.2v to 3.6v and all the energy in the 10v spike, and the accompanying current, will be delivered to the LED to produce illumination. The LED starts to absorb energy at 3.2v and that’s why the voltage across it never gets any higher than 3.6v.

STEP 1: Quick Video


STEP 2: Components

1) NPN Transistor

2) 3V Super Bright LED (any colour will be fine)

3) Wiring (x2 meters)

4) Breadboard*

5) Ferrite Core

6) 1K Ohm Resistor

The breadboard is not really necessary though for beginners it could be ideal to help with further experiment too.

STEP 3: Circuit Diagram

STEP 4: Construction - Wiring the Toriod

Wiring the toroid is the perhaps the most difficult part of constructing a Joule Thief. Essentially you are creating two windings around the toroid (called a bifilar winding). Take your time and don't rush especially if it's your first time winding a toroid :) It's easier if the wires are different colours too.

1. Take two strands of insulting wire and thread them through the toroid as seen in the first figure above: (Keep around 2cm of wire for the connections.)

2. Begin winding the wire strands around the toroid as seen in the next image. Please note how the wires are wound correctly:

3. Make 15-18 turns of the wire through the toroid as shown in the next image above.

4. Connect the two different coloured wire strands from either side of the toroid and connect them together. Your toroid should look similar to the last figure above.

STEP 5: Construction - Building the Circuit

If you are you using the breadboard please examine the images above on where to place the components. See the circuit diagram in Step 3 to familiarize yourself with the way the components are connected.#

1. Place the breadboard on a flat surface and gather the transistor, resistor and LED

2. Insert the transistor, LED and resistor into the breadboard sockets as shown above

3. Connect the toroid to the breadboard. The twisted wires go to the positive terminal of the battery

4. Connect the orange coloured wire to the resistor

5. Connect the green wire to the collector terminal of the transistor and the long lead of the LED (Anode)

STEP 6: Conclusion - Working Joule Thief

If you have connected the components as shown and wired the toroid correctly your joule thief LED should be shining brightly. Experiment with more turns on the toroid and/or use a larger ferrite toroid. Happy hacking!!

20 Comments

XxelectricmanxX 8 years ago

Dude, You have no idea how much I live joule thiefs, awesome instructable! You should do more!!!!

playmonkey 8 years ago

Thanks :D Joule Thief's are pretty amazing!! Have you seen the famous Fuji Thief? https://www.youtube.com/watch?v=EcJG_6YF1X4

blakehx 8 years ago

Is it possible to have too many turns around the toroid? I probably have 25-30 turns of magnet wire around my toroid but can't get it to work with any voltage! Can't think what else to try! Any help greatly appreciated thanks!

colin55 1 year ago


WHY A JOULE THIEF DOES NOT WORK

Many readers are saying their Joule Thief circuit does not work.
There are a number of reasons for this and one of the is highly technical.
The Joule Thief circuit uses a transformer and to get the output of the transformer to work properly one of the leads of each winding must be 'HELD RIGID" so the signal can come out the other end.
This might sound surprising but the output signal (waveform) produced by a winding will come out both ends equally. In our case the end connected to the LED is like pushing against a solid brick wall while wearing roller skates. You cannot provide much effort.
And the winding pushes most of its energy out the top of the winding.
If the top wire is connected to a long wire and then to a battery, this long wire will have very little resistance to the signal and the signal will be lost in the wire.
By connecting the battery very close to the circuit you reduce the IMPEDANCE OF THE POWER SUPPLY and the circuit will start to work.
The battery is creating a low impedance for the supply rails and this effectively 'TIGHTENS UP THE CIRCUIT" and makes it work.

Another cause for the circuit not working is too few turns on each winding.
I have seen and designed circuits with 10 to 25 turns and the transformer wound on a nail, steel bolt or brass bolt 3mm in diameter so there is a wide range of successful designs.
I have designed a flashing circuit and one that is 300% more efficient on talkingelectronics.com website (Joule Thief).

JYon 7 years ago

There may be too much power being wasted, as magnet wire is made for creating a magnetic field, and a magnetic field would drain a bit of power. Try using a normal wire?

playmonkey 8 years ago

It should not make any difference really, power the JT using a new battery and see if it works? Otherwise I would re-check all the connections and especially the way the toroid is wired and connected. Hope that helps??

_Vehner_ 8 years ago

I made my joule thief. I made it work with a simple dry run,but after that it stopped working? Anyone got any ideas?

playmonkey 8 years ago

How long did it work for? What voltage is the battery giving? Try another battery and see if it works?

eyal_hadar 8 years ago

Worked for five hours.
Although the batteries were not full, instead of 3 volts was 2.5
The light seems incredible that taken to increase the light is weak.
Increased exposure, otherwise the camera does not catch the frequency of the LED.

eyal_hadar 8 years ago

j.t in work only 2 AA 600 led 10m long

Malkaris 8 years ago

Thank you very much, I've been looking for a well documented explanation of this device!

playmonkey 8 years ago

Thank you, glad the instructable helped :)

pucksurfer 8 years ago

Is this how they make it so you can charge your phone with just 1 AA battery?

playmonkey 8 years ago

It's not really possible to charge it from a single AA 1.5v battery as most mobiles need around 500mA @ 5v. As the JT output voltage increase the current decreases so it may generate 5v there will not be enough current to charge the mobile. Hope that helps?