A Joule Thief is an Armstrong minimalist automatic swinging voltage propeller that is small, inexpensive, and easy to build, typically used to drive small loads. This circuit is also known by other names like oscillator lock, joule bell, vampire torch.
You can use almost all power in a single cell electric battery, even well below the voltage where other circuits consider that the battery is completely discharged (or "dead"); hence the name, which suggests the notion that the circuit is stealing energy or "joules" from the source. The term is a play on words.
The circuit is a variant of the blocking oscillator that forms an unregulated voltage converter. The output voltage is increased at the expense of a higher current consumption at the input, but the integrated (average) output current decreases and the brightness of a luminescence decreases.
Operation Description
The circuit works by rapidly switching the transistor. Initially, the current begins to flow through the resistor, the secondary winding and the base-emitter junction (see diagram), which causes the transistor to begin to conduct the collector current through the primary winding. Since the two windings are connected in opposite directions, this induces a voltage in the secondary winding which is positive (due to the polarity of the winding, see the dot convention) that activates the transistor with a greater bias. This automatic feedback / feedback process almost instantly turns the transistor as hard as possible (by placing it in the saturation region), making the collector-emitter path essentially look like a closed switch (since VCE will be only about 0, 1 volts, assuming the base current is high enough). With the primary winding effectively through the battery, the current increases at a rate proportional to the supply voltage divided by the inductance. The disconnection of the transistor takes place by different mechanisms that depend on the supply voltage.
The gain of a transistor is not linear with VCE. At low supply voltages (typically 0.75 V and below), the transistor requires a larger base current to maintain saturation as the collector current increases. Therefore, when it reaches a critical collector current, the available base unit becomes insufficient and the transistor begins to pinch and the positive feedback action described above is turned off.
In summary, once the current in the coils stops growing for any reason, the transistor enters the cut-off zone (and opens the "switch" collector-emitter). The magnetic field collapses, inducing the voltage necessary to cause the conduction of the charge, or for the secondary winding current to find some other path.
When the field returns to zero, the entire sequence is repeated; with the battery increasing the primary winding current until the transistor is turned on.
If the load in the circuit is very small, the rise speed and the final voltage in the collector are limited only by parasitic capacitances and can rise to more than 100 times the supply voltage. For this reason, it is imperative that a load is always connected so that the transistor will not be damaged. Because VCE is reflected back to the secondary, transistor failure due to a small load will occur across the inverse VBE limit for the transistor being exceeded (this occurs at a much lower value than VCEmax).
The transistor dissipates very little energy, even at high oscillation frequencies, because it spends most of its time in the state completely on or completely off, so that the voltage or current through the transistor is zero, thus minimizing losses of switching.
The switching frequency in the opposite example circuit is approximately 50 kHz. The light emitting diode will blink at this rate, but the persistence of the human eye means that the blinking will not be noticeable.