Joule Thief
The "Joule
Thief" is a very simple dc-dc boost converter that
drives a white LED from a single cell battery 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.
-- eck 20071103
