Created by
Created April 02, 2012
Last modified April 02, 2012
Tags flashlight   joule   joule-thief   led   light   thief  


Not provided.


A simple Joule Thief circuit that uses bootstrap feedback to drive the transistor for added efficiency. There is a slight under-voltage (-6.6v instead of -5v) at the Base-Emitter junction but the transistor seems to handle it OK.

The coil can be from 50uH to 200uH per leg. It can be made by winding 12ft (3,5m) of a pair of wires around a 1" (25mm) form to make a 50uH air-core inductor. Make sure you connect the circuit to align the dots at the start of each wire.

The transistor must be rated for over 600mA and has a high frequency gain of 150 or more. Vce (at 500mA) dictates efficiency and the 'end-point' of battery life.

The 1N4148 will work OK but best efficiency is with a 1-amp schottky diode at D7.


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