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Circuits – White LED Torch

white led torch Circuits   White LED Torch

           This low-cost circuit is suitable for powering flashlights, emergency lighting and other applications in which it is desirable to power white LEDs from one or two primary cell batteries. The transistor, lk resistor [Rll and the tapped inductor form a blocking oscillator. When the power button is pressed, the transistor is biased on through the lk resistor. Voltage that appears from the tap on the inductor to the collector causes the voltage on this resistor to be even higher than the battery voltage, thereby providing positive feedback. Also because there is voltage across the inductance between the tap and the collector of the transistor, collector current increases with time (this is in addition to a starting value that relates to the current supplied to the base, but this part of the collector current is rather small). Because of the positive feedback, the transistor stays saturated until something happens to change its base current.

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