Joule thiefFrom Wikipedia, the free encyclopedia

Jump to: navigation, search

"Joule thief" is a nickname for a simple switched-mode power supply that is easy to build and can use nearly all of the energy in a battery, even far beyond the point where other circuits consider the battery "dead". The circuit uses a variant of the blocking oscillator as an unregulated boost mode switching power supply.

Contents

[hide] 1 History 2 Description 3 See also

4 External links

[edit] History

In the November 1999 issue of Everyday Practical Electronics (EPE) a simple circuit was published by Z. Kaparnik that consists of a transformer-feedback single-transistor switched-mode converter. Due to its simplicity and usefulness the circuit has gained popularity.

[edit] Description

The transformer consists of a standard ferrite bead with two windings using 0.15 mm diameter wire (38 swg). The switch frequency is about 50 kHz. The circuit can utilize an input voltage down to about 0.35 V and can run for weeks using a 1.5 V LR6/AA battery if made correctly.

The x1 and y1 connections are at the same side of the transformer. Same goes for x2 and y2. The way the wires come out of the ferrite bead is extremely critical for the circuit function.

The battery voltage is usually 1.5 V. The resistance (above x2) is ~1 kΩ, 1/8 W. The transistor is a BC549 (a 2n3904 will also work) Vceo=30 V Ic=0.1 A P=0.625 W. A white

light-emitting diode with Vf=3.2 V might be used. The light-emitting diode will blink nearly 50 000 times a second.

[edit] See also