 Circuit takes 70mA on LOW brightness and 120mA on HIGH Brightness |
This circuit has been specially designed
for a 6v rechargeable battery or 5 x 1.2v NiCad cells. Do not use
any other voltage. It has many features: The pulse-operation to the two 1-watt LEDs delivers a high current for a short period of time and this improves the brightness. The circuit can drive two 1-watt LEDs with extremely good brightness and this makes it more efficient than any other design. The circuit is a two-transistor high-frequency oscillator and it works like this: The BD139 is turned ON via the base, through the white LED and two signal diodes and it amplifies this current to appear though the collector-emitter circuit. This current flows though the 1-watt LED to turn it ON and also through the 30-turn winding of the inductor. At the same time the current through the 10R creates a voltage-drop and when this voltage rises to 0.65v, the BC547 transistor starts to turn ON. This robs the base of the BD139 of "turn-on voltage" and the current through the inductor ceases to be expanding flux, but stationary flux. |
This turns off the BD139 a little more and the current through the inductor reduces.
This creates a collapsing flux that produces a voltage across the coil in the opposite direction. This voltage passes via the 1n to turn the BC547 ON and the BD139 is fully turned OFF.
The inductor effectively becomes a miniature battery with negative on the lower LED and positive at the anode of the Ultra Fast diode. The voltage produced by the inductor flows through the UF diode and both 1-watt LEDs to give them a spike of high current. The circuit operates at approx 500kHz and this will depend on the inductance of the inductor.
The circuit has about 85% efficiency due to the absence of a current-limiting resistor, and shuts off at 4v, thus preventing deep-discharge of the rechargeable cells or 6v battery.
The clever part of the circuit is the white LED and two diodes. These form a zener reference to turn the circuit off at 4v. The 10k resistor helps too.
The circuit takes 70mA on low brightness and 120mA on HIGH brightness via the brightness-switch.
The LEDs actually get 200mA pulses of current and this produces the high brightness.