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The circuit is specifically designed for driving 1W LEDs that are readily available nowadays due to their non-linear characteristics and voltage variations. To address this, a 350mA constant-current power source is essential for the supply. Functioning similar to a traditional step-down (buck) switching regulator, transistor Q1 serves as the switching element with inductor L1, diode D1, and a 100mF capacitor at the output acting as energy transfer and storage components. The pass transistor (Q1) is controlled by Q2 to create a basic oscillator along with its base circuit components.
A 1nF capacitor offers the required positive feedback for oscillation. Transistor Q3, in conjunction with the two resistors in its base-emitter circuit, senses the output current. Once the current reaches approximately 350mA, the voltage drop across the resistors surpasses the forward voltage of Q3, enabling it to switch on. Consequently, Q3's collector pulls Q2's base down, thus turning it off and subsequently the main pass transistor (Q1). A time constant introduced by the 15kW resistor and 4.7nF capacitor linked to Q2's base introduces hysteresis to ensure stable regulation of the output current.
The inductor, fashioned from an old computer power supply's small toroid, was rewound with 75 turns of 0.25mm enameled copper wire to provide an inductance of approximately 620mH. Before connecting the 1W LED, it is advised to adjust the output current level by wiring a 5W 10W resistor across the output as a load and modifying one or both of the resistors in the base-emitter circuit of Q3 to achieve a maximum of 3.5V across the load resistor.