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Joule Thief Kit

PostPosted: Wed Oct 27, 2010 10:10 pm
by Kirk
A little info on the Joule Thief for those of you that have not heard about it. In the article "One Volt LED - A Bright Light" written by Z. Kaparnik from Swindon, Wilts, UK published in the November 1999 issue of the magazine "Everyday Practical Electronics" in the section "Ingenuity Unlimited"; described how to make a "Micro-torch circuit" using a very compact high frequency, high efficiency DC-DC converter design. The circuit consisted of a hand wound micro toroid, a 10K resistor and a ZTX450 transistor. The circuit was designed to run a LED that had a forward voltage drop greater than 1.8VDC from a single cell battery (1.5VDC) and could run as low as 750mV, this meant it could run from nearly dead batteries. This circuit has been propagated, experimented with, and changed at numerous sites and discussion boards and most significantly can now run white LEDs that have forward voltages greater than 3V. Someone nicknamed the circuit "Joule Thief" and it has stuck ever since.

A Joule Thief circuit is a simple three component, low Voltage DC-DC boost converter. The circuit can run on voltages as low as 300-400mV depending on the transistor used and windings on the transformer. The output voltage and current depends on the three components used in the circuit. As a minimum the transistor must have high enough gain and should have a Collect-Emitter Voltage rating that is well above the maximum output peak voltage on the secondary winding ( I like at least a 25% margin ). The resistor is chosen so that it limits the maximum circuit current, by limiting the current to the base of the NPN transistor. The transformer can be wound 1:1 for simple operation or can be wound with more than two coils ( A third winding may be wound to create high voltages for running EL devices, nixie tubes, neon bulbs, etc... )

I decided to try and experiment with this circuit myself. I have made many variations of the circuit, some that could run to voltages as low as ~350mV and still produce 12V out (not much current though with such a low input voltage). In making the circuit, I decided it would be nice to have a PCB so that I did not have worry about problems that I was having with the air wired experimental circuits. The air wired circuit had several problems due to wiring shorts or opens, and was not robust enough to carry around. I thought it would be nice to produce some kits for other experimenters so that it would be easy to assemble and not have to worry about wiring problems. Visit the products page or see Joule Thief kits here: ... ief%20Kits


So I have created my very first PCB and sent out for fabrication on Oct 17th. The PCB's finally arrived today (Oct 27th). I built the first Joule Thief kit using a 2N2222 T0-39 can package as I am waiting for the T0-92 plastic package 2N4401 to arrive tomorrow. I tested this one on the bench it starts up at 520mV and shuts down at around 470mV. At 1.55V in the circuit was drawing 100mA. I cannot wait to get the 2N4401 and build another one to see where it is running at.

For the JT_001b_ASM user manual click here: ... manual.pdf

For JT_Kit001b assembly instructions click here: ... ctions.pdf

To read about the homemade Zinc Copper Battery Experiment click here:

Re: Joule Thief Kit

PostPosted: Wed Oct 27, 2010 10:16 pm
by Kirk
I almost forgot to include a photo of the bare PCB.

Re: Joule Thief Kit

PostPosted: Thu Oct 28, 2010 6:49 am
by Kirk
I created a visual aid with the Joule Thief schematic to make the Joule Thief kit component connection easier to identify.

Re: Joule Thief Kit

PostPosted: Wed Nov 24, 2010 1:02 pm
by Kirk
I thought a simulation would be nice so that people could see a graphic representation of how the Joule Thief works. I created a simulation using LTspice (download for free from Linear Technology here: )

If you want to see the simulation run yourself please download the simulation files here:
LTspice sim files
(43.97 KiB) Downloaded 6898 times

After you install the LTspice software and open the file JT.asc just click the little running man icon. Then click individual nodes to display what the simulation produced. The probe will show voltage. If you mouse over each portion of the components you will see a current probe that will show the current flowing in the simulation where you click.

click image to see full view
Joule Thief Simulation in LTspice

click image to see full view