How to Make a Joule Thief

This project goes through the step by step process of building a joule thief. A joule thief is a minimalist self-oscillating voltage booster that is small, low-cost, and easy to build, typically used for driving small loads, such as driving an LED using a 1.5 volt battery(wikipedia). For our project we built a joule thief that amplified our input voltage to power an LED.

The Tools used for this project include:

1. Tripple A battery
2. Wires
3. Ferrite toroid core with two windings of 20 turns each using 0.15 mm (0.006 inch) diameter wire (38 swg) (34-35 AWG)
4. NPN transistor
5. 1 1kohm resistor

The first step is to make the coupled transistor by winding insulated wire around the toroid core. The toroid can be purchased directly, but we sourced ours from an old toroid inductor. 

You will need 2 strands of wire which will be wrapped around the core simultaneously. Make sure to mark the wires so you know which is which after wrapping. We used red and black wire so we could tell them apart. The insulated wires should be wrapped around the inductor as many times as possible and in a 1:1 ratio. We were able to wrap ours approximately 10 times but this depends on the size of the toroid and the wire.

Assemble the circuit according to the diagram using the 1 KOhm resistor. You might need to play with the resistor value based on your particular inductor. The best resistor value depends on the mutual inductance of the coupled inductor which depends on geometric properties such as wire spacing and coil radius. We began with using a 100 Ohm resistor, but this slowed down the oscillation such that the average voltage was below the LED forward voltage of 1.7V and it did not light up. A switch to a 1 KOhm resistor solved this problem.

Verify that the input voltage is being amplified and able to power the led. Use an input voltage of less than the forward voltage of the diode. As well, ensure that the output is a periodic waveform.

Once everything has been tested, your circuit should be capable of powering a diode