Joule thief

This self-stroking/positive-feedback process almost instantly turns the transistor on as hard as possible (putting it in the saturation region), making the collector-emitter path look like essentially a closed switch (since VCE will be only about 0.1 volts, assuming that the base current is high enough).

With the primary winding effectively across the battery, the current increases at a rate proportional to the supply voltage divided by the inductance.

Hence, when it reaches a critical collector current, the base drive available becomes insufficient and the transistor starts to pinch off and the previously described positive feedback action occurs turning it hard off.

To summarize, once the current in the coils stops increasing for any reason, the transistor goes into the cutoff region (and opens the collector-emitter "switch").

The magnetic field collapses, inducing however much voltage is necessary to make the load conduct, or for the secondary-winding current to find some other path.

As there is no regulation, any excess of energy not consumed by the load, will be dissipated as heat in the zener diode with consequent low efficiency of conversion.

In the example circuit, the Schottky diode D1 blocks the charge built up on capacitor C1 from flowing back to the switching transistor Q1 when it is turned on.

A conventional joule thief, showing components and how they are connected. This example uses a red LED . A ferrite toroid is wound to form a coil with primary (white) and feedback (green) windings. A PN2222A transistor and 1000 ohm resistor are used
A joule thief with two axial inductors replacing the ferrite toroid, shown on a solderless breadboard
Joule thief with regulated output voltage
Example of a joule thief circuit driving an LED. The coil consists of a standard ferrite toroid core with two windings of 20 turns each using 0.15 mm (0.006 inch) diameter wire (38 swg ) (34-35 AWG ). 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 . The battery voltage is usually 1.5 V . The resistor is ~1 kΩ , 1/4 W . The transistor could be a 2N3904, BC547B, 2SC2500, BC337, 2N2222, 2N4401 or other NPN. V ceo = 30 V, P= 0.625 W.
A closed-loop regulated joule thief
The waveform of an operating joule thief, showing a 30% duty cycle at approximately 40 kHz