BUCK CONVERTER for3watt LED
This circuit drives a 3watt LED.
You have to be careful not to damage the
LED when setting up the circuit.
Add a 10R to the supply rail and hold it in
your fingers.
Make sure it does not get too hot and monitor the voltage
across the resistor. Each 1v represents 100mA.
The circuit will work and
nothing will be damaged.
If the resistor "burns your fingers" you have a
short circuit.
The BC557 multivibrator has a "mark-to-space ratio" determined by the
22n and 33k, compared to the 100n and 47k, producing about 3:1 The
BD679 is turned ON for about 30% of the time.
This produces a very
bright output, and takes about 170mA for 30% of the time.
You cannot
measure this current with a meter as it reads the peak value and the
reading will be totally false.
The only way to view the waveform is on a
CRO, and calculate the current.
The 100-turn inductor allows the BD679 turn turn ON fully and
"separates" the voltage on the emitter of the BC679 from the voltage on
the top of the 3watt LED.
When the BD679 turns ON, the emitter rises to about 10v.
But the top of
the LED NEVER rises above 3.6v.
The inductor "buffers" or "separates"
these two voltages by producing a voltage across the winding equal to
6.4v and that's why the LED is not damaged.
When the transistor turns off (for 60% of the time), the magnetic flux
produced by the current in the inductor collapses and produces a voltage
in the opposite direction.
This means the inductor now becomes a
miniature battery and for a very short period of time it produces energy to
illuminate the LED.
The top of the inductor becomes negative and the
bottom is positive.
The current flows through the LED and through the
Ultra High-Speed 1N4004 diode to complete the circuit.
Thus the circuit
takes advantage of the energy in the inductor.
A 500R pot is placed across the LED and a voltage is picked off the pot
to turn on a BC547 transistor. This transistor "robs" some of the "turn-on"
for the BD679 transistor to reduce the brightness of the LED.
Because the circuit is driving the LED with pulses, very high brightness is
obtained with a low current.
Our eyes detect peak brightness and you can compare the performance
of this circuit with a DC driven LED.
