1W LED Driver

This circuit is designed to drive the 1W LEDs
that are now commonly available. Their non-linear voltage to current
relationship and variation in forward voltage with temperature
necessitates the use of a 350mA, constant-current power source as
provided by this supply. In many respects, the circuit operates like a
conventional step-down (buck) switching regulator. Transistor Q1 is the
switching element, while inductor L1, diode D1 and the 100mF capacitor
at the output form the energy transfer and storage elements. The pass
transistor (Q1) is switch-ed by Q2, which together with the components
in its base circuit, forms a simple oscillator. A 1nF capacitor provides
the positive feedback necessary for oscillation. The output current is
sensed by transistor Q3 and the two parallelled resistors in its
base-emitter circuit.

1W LED Driver Circuit

1W LED Driver Circuit Diagram

When the current reaches about 350mA, the voltage drop across the
resistors exceeds the base-emitter forward voltage of transistor Q3
(about 0.6V), switching it on. Q3’s collector then pulls Q2’s base
towards ground, switching it off, which in turn switches off the main
pass transistor (Q1). The time constant of the 15kW resistor and 4.7nF
capacitor connected to Q2’s base adds hysteresis to the loop, thus
ensuring regulation of the set output current. The inductor was made
from a small toroid salvaged from an old computer power supply and
rewound with 75 turns of 0.25mm enamelled copper wire, giving an
inductance of about 620mH. The output current level should be trimmed
before connecting your 1W LED. To do this,
wire a 10W 5W resistor across the output as a load and adjust the value
of one or both of the resistors in the base-emitter circuit of Q3 to get
3.5V (maximum) across the load resistor.

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