This white led torch circuit provides an effective solution for powering white LEDs. It is ideal for flashlights, emergency lighting, and various other applications. The circuit operates efficiently from one or two primary cell batteries.
The core of the circuit involves a transistor (Q1) and a tapped inductor, forming a blocking oscillator. When the power button is pressed, the 1k resistor (R1) properly biases the transistor. A critical voltage then appears on the tap of the inductor. This voltage is subsequently applied to the collector. The voltage at the tap rises, becoming even higher than the battery voltage, which in turn provides positive feedback.
Crucially, voltage also develops across the inductance, specifically between the inductor’s tap and the transistor’s collector. Consequently, the collector current of the transistor increases significantly. As this process continues, the current supplied to the base remains relatively small, while the feedback current is also minimal. Eventually, the transistor saturates, leading to a change in its base current.
At a certain point, the IR drop across the inductor occurs. This drop moves from the tap to the collector, causing the battery voltage to be approached (specifically, the battery voltage minus VCEsat). When this happens, the winding no longer induces voltage from the tap to the 1k resistor. Subsequently, the base voltage begins to drop, forcing it to go negative. This action ultimately switches off the transistor.
Once the transistor is off, the inductor continues to supply current, and the collector voltage rises rapidly. The collector voltage quickly reaches a level sufficient for the LED to conduct and emit light. This state persists for a brief period until the inductor’s current diminishes. Afterward, the collector voltage starts to swing back towards ground. Simultaneously, the base voltage swings positive, which turns the transistor on for a new cycle, thus maintaining continuous oscillation.
For enhanced brightness from the LED at a given current, an additional 1N4001 diode and a 0.1 µF capacitor (C1) are incorporated into the circuit. The tapped inductor is specifically wound with 30 turns of 28 SWG wire on a ferrite transformer core. It also features a tap precisely at 15 turns for optimal performance. This entire circuit typically draws approximately 20mA from a standard AA cell, ensuring efficient power usage.