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 paralleled resistors in its base-emitter circuit.
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.
An Electronic Circuits Blog with Latest and rear Electronic Circuits for Hobby and Projects
Subscribe to:
Post Comments (Atom)
Featured Posts
Home Electric Circuits
Electric energy required to operate home electric appliances is obtained from the national electric grid. Electric energy generated by ...
Popular Posts
-
By using this Inverter circuit you can convert the 12V dc in to the 220V Ac. In this circuit 4047 is use to generate the square wave of 50Hz...
-
The numeric water level indicator circuit works off 5V regulated power supply. It is built around priority encoder IC 74HC147 (IC1), BCD-to-...
No comments:
Post a Comment