Improved Joule Thief

This is a improved version of classical joule thief self oscillating step up converter. This sample works in constant current mode by replacing current shunt with a zener diode or voltage reference converter can be used in constant voltage mode.

I used this circuit for a flashlight. It was cheap LED flashlight with 3 AAA batteries, a switch, a 3 ohm resistor and an half watt LED. I replace this old circuit with my new circuit, Now this flashlight shines more brightly uses almost all energy from the batteries. Also it drives the LED's at constant current for almost entire discharge cycle of of the batteries.

The components are soldered without any PCB and tightly because of space constraints in the flashlight body.

1 power transistor NPN (I've used tip31c)

1 small signal NPN BJT (2sc1815, 2n3904, bc547 or similar)

Ferrite core

330ohm base resistor

Enameled copper wire

1Watt power LED's

1n5819 Schottky diode

100uF 16v electrolytic capacitor

shunt resistor for current setting

This is the schematic of the converter. The transformer is made of some enameled copper wire and a ferrite rod. A ferrite ring will lead more efficient converter. But I dont have a ferrite ring in appropriate size so I've used a ferrite rod.

The oscillator consist of transformer, 330ohm resistor and TIP31C. The oscillator creates pulses of high voltage (higher than battery voltage). These pulses then rectifed and smoothed by schottky diode and capacitor. Then chain of 3 power LEDs recevies this smoothed voltage.

The current of the LED is set by 7 ohm shunt resistor (which consist of three 22 ohm resistor in parallel). When voltage drop on the shunt resistor reaches to base-emitter voltage of the small transistor (0.6 - 0.7V) it opens the small transistor and shunts the oscillator.

From 4.8V (1.6V per battery when they are new) about 3.6-3.5V circuit regulates the current at about 100mA below 3.5 V current starts to drop as well as the LED brightness.

In the first photo with input voltage of 4.5V, we can see that the circuit regulates the current by closing oscillator earlier in each cycle.

In the 2nd photo, we can see LED current at the 4.5V input voltage

In the 3rd photo, we can see input voltage and current

In this chart we can see input voltage, input current, output voltage, output current and efficiency. For most part of discharge of the batteries efficiency is above 60% (which is not so bad considering I've made this circuit using some old parts and I did not experiment with numbers of turns in the transformer. I've just wrapped 30-40 turns for collector and 10-20 turns for base winding.). Probably with a ferrite ring and much more thought transformer, efficiency would be higher