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December 09, 2014

A Joule Thief Circuit

ferrite toroid

The Joule Thief

I was looking for a simple voltage boost circuit for one of my projects, when I came across a circuit called the Joule Thief. This cool sounding circuit is astonishingly simple - just a coil which you could wind yourself, a resistor, and a transistor. There seems to be several variations of this nifty little circuit on the net. I had to give this a shot! This is the circuit that I used:


joule-thief

Now let’s look at building the circuit.

Building the Circuit

The tough thing was getting the ferrite toroid. I looked online and couldn’t find any cheap options. My friend Ravi and I wandered around S.P. Road (Bangalore), looking for it, and finally had success at a wholesaler called Jinanica. He reluctantly sold us 10 toroids - darn cheap at Rs.6 per piece.

To make the coil, I wound 10 turns of 26 gauge insulated (yellow) wire and then wound 7 turns of a second (red) wire on it. It matters how you connect the coil to the circuit. For instance, you don’t want to join up the red and yellow wires at the starting point. The way you connect it changes the way the coils are electromagnetically coupled, changing the output. Here is the circuit, assembled on a breadboard:


joule-thief-bb

I hooked up the circuit to a 1.5 V AA battery, and the output to my oscilloscope, and I was shocked to see pulses of 52 V (max) coming out. The pulses were about 2 us in width and 20 us apart, giving an output of about 5.5 V RMS. Wow man, electromagnetism rocks!


joule-thief-osc

Conclusion

The circuit works really well. I need to experiment with smoothing the output a bit. I plan to use it soon for a project of mine, and will post results.

References

  1. Weekend Projects with Bre Pettis: Make a Joule Thief
  2. Wikipedia entry on Joule Thief