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[Note: Wait a few minutes when you switch on the simulation, circuit requires few seconds to reach steady state]
>> Introduction:
This is a slightly modified version of the popular circuit aptly named, "Joule Thief" as it is commonly used to squeeze out any remaining energy (joules) out of battery.
It can operate from around 0.3V to 0.7V depending on the transistor semiconductor material and boost the voltage to turn on a LED which has forward voltage of around 2 to 3.5V.
It includes a transformer with 5-10 turns around ferrite torrid, a transistor and a LED. Fundamentally it is a type of primitive boost converter with positive feedback.
>> Theory of operation:
When the circuit is switched on, initially a small leakage current flows through the collector-emittor of the transistor (Ice leakage) which is passed through the transformer's primary coil. As the phasing of the transformer is reversed, it induces a small current through the secondary which in turn flows through the base-emittor path (Ibe). If we recall the property of the bipolar transistor, the collector current is directly proportional to the base current hence it results in the increase in the collector current and the transformer itself. This creates a feedback loop and the current shoots up until the transformer core saturates and the current cannot increase further.
Then the transistor switches off and a sudden self induced EMF in generated in the transformer (with reverse polarity, in the magnitude of 10's or even 100's of volts) which passes through the LED generating light :)
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