Chip manufacturers are always coming up with ever more sophisticated constant current driver chips for LEDs. We have included this design for those of you who prefer a more cheap and cheerful solution.
Current through the LEDs produces a voltage drop across resistor R1. As the current rises to a level to produce a voltage drop of 0.6 V across R1 it will cause T2 to start conducting and shunt the gate voltage of T1 to the ground. This produces a constant current I = 0.6 V/R1 through the LEDs.
The control input allows the LEDs to be switched on by applying a voltage in the range of 5 V up to around 12 V and switched off by applying a voltage of 0 V. When this input is driven by a pulse width modulated signal it gives the possibility to change the LED brightness.
The supply voltage for all the series-connected LEDs can be as high as practical providing the maximum drain-source rating of T2 is not exceeded. The choice of T2 and any necessary heat sink will depend on the power dissipated in this device. This can be calculated from:
(Supply voltage minus the voltage drop across the LEDs) × ILED
- Thermistors.
- Varistor Resistors.
- Photo Resistor or LDR (Light Dependent Resistors)
- Surface Mount Resistors.
