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Joule Thief


The "Joule Thief" is a very simple dc-dc boost converter that drives a white LED from a single cell battery.


It is built using only a two-winding coil, an npn transistor, and a 1k resistor. Below are some assembly photos as well as an oscilloscope screenshot of the voltage across the LED while operating.


This circuit works as follows: current flows through the left side of the coil and resistor into the npn transistor's base, saturating it. Current rapidly builds through the right side of the coil and the transistor collector, storing energy as a field in the toroid. The reactance from this stops flow through the other side and switches off the transistor. Without the path to ground through the transistor, the current continues to flow through the right side of the coil, "uphill", as it were, and through the LED. When the inductor's field energy is depleted and current slows, current through the resistor into the transistor base resumes, restarting the cycle.]


The oscilloscope shot shows the voltage across the LED on the top trace and the transistor base voltage on the bottom trace.

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