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A Joule Thief circuit is a simple voltage booster circuit that can light up an LED using a single AA battery, even when the battery voltage is below the LED's typical forward voltage. It typically consists of a few components: a transistor (usually a bipolar junction transistor), a feedback winding (coil), a base resistor, and an LED.
Components:-
BC 547 npn transistor
1k ohm resistor
1n4007 Diode*2
1.5v / 1v / 3v Battery
LED
Here's how it works:
1. Startup: When the circuit is initially connected to a low voltage source (e.g., a nearly depleted battery), the transistor (usually a NPN type) starts to conduct a small current from the base to the emitter.
2. Inductive Kickback: As the transistor starts conducting, current flows through the primary winding of the coil, creating a magnetic field. When the transistor turns off (due to the lack of base current), the magnetic field collapses, inducing a voltage spike across the feedback winding (secondary winding) of the coil.
3. Feedback Loop: This induced voltage spike is higher than the input voltage and gets fed back into the base of the transistor through a feedback winding, turning the transistor back on momentarily.
4. Repetitive Cycle: This cycle repeats rapidly, causing the LED to flicker on and off very quickly. The average voltage across the LED is boosted, allowing it to light up even when the input voltage is too low to directly power the LED.
5. Regulation: The circuit's oscillation frequency and duty cycle are self-regulated by the characteristics of the components and the transformer action of the coil, ensuring stable operation.
Overall, the Joule Thief circuit is a clever way to squeeze out the last bit of energy from nearly depleted batteries and power low-power devices like LEDs.