Joule thief
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The Simplest Voltage Booster
In amateur electronics, there is sometimes a need to boost the supply voltage, especially when certain components require a higher voltage than what is available. A typical scenario is powering an LED from the lowest possible voltage source, such as a single Leclanché cell at 1.5V. In such cases, we can use a circuit known as a “Joule thief.” This intriguingly named device is actually a very minimalist electronic circuit performing the role of a simple step-up converter.
Construction
To assemble this converter, you will need:
- a BC546 transistor or similar,
- a 10k resistor,
- a small ferrite toroidal core,
- enameled copper wire (0.3–0.4 mm gauge).
You can often salvage the toroidal core from damaged power supplies, older computer motherboards, or purchase one from an electronics store. It’s best to wind the transformer in a bifilar manner—winding both wires simultaneously—to ensure strong magnetic coupling, with both windings containing an equal number of turns (typically 10-30). These components should be arranged according to the schematic:
The dots on the schematic indicate the starting leads of each winding. You can build the converter on a small piece of protoboard:
With the circuit assembled, let's see how it functions. First, try powering an LED from a 1.5V source:
As expected in a standard setup, the LED does not light, since the forward voltage of most LEDs is between around 2V (for red LEDs) and over 3V (for white or blue LEDs). The LED will not illuminate below this threshold.
Is that all there is to it? Let’s now connect our LED to the Joule thief circuit we just built:
The LED lights up! Our tiny converter has boosted the input voltage to a sufficient level for the LED to operate. But how did this happen?
Explanation
The Joule thief is essentially an inductively coupled oscillator, similar to a traditional self-oscillating converter. The exact operating frequency is difficult to determine without measurement, as it depends on the inductance of the homemade transformer and any parasitic capacitances. Typically, it operates at a relatively high frequency. Due to the strong magnetic coupling between the windings (both having the same number of turns), the oscillations produce a near-square waveform, causing the transistor to operate primarily in saturation or cutoff mode. This reduces power dissipation within the transistor. Rapid changes in current through the inductive coil generate a voltage spike higher than the supply voltage, which powers the LED. The circuit can even operate with nearly depleted batteries, extracting almost all the remaining energy.
Enjoy experimenting with this educational circuit!
Further readings:
- Latek W., Zarys maszyn elektrycznych. Tom I, Wydawnictwa Politechniki Warszawskiej, Warszawa, 1974, pp. 26-29
- Hameyer K., Electrical Machines I: Basics, Design, Function, Operation, RWTH Aachen University Institute of Electrical Machines, 2001, pp. 11–12
Marek Ples