Low-Drop Constant Current Source
for Ultrabright LEDs
Published in issue 338, December 2004
Ultrabright LEDs are becoming increasingly attractive for use in lighting and warning-signal applications. LEDs must be operated at a constant current to ensure that they continue to emit light at the same brightness. The usual approach is to use a series resistor, but in order to prevent the non-linear voltage/current characteristic, the NTC property of the LED and variations in the supply voltage from affecting brightness, we’d like to have something better. The circuit described here is a low-drop constant current source for ultrabright LEDs with blinking capability, for use in headlights, taillights, dog blinkers, light chains, car alarms and the like. It provides a constant current of 20 mA with a supply voltage of 4.5–30 V, or 50 mA with a 4.5–12 V supply. The voltage drop of the circuit is only 0.6 V, so practically the entire supply voltage can be used for the LEDs.
Resistors:
R1 = 6Ohm 8 or 18Ohm (see text)
R2 = 33kO
R3 = 820Ohm
R4 = 10MOhm
R5 = 22kOhm or 0 Ohm (see text)
R6 = 10kOhm
Capacitors:
C1 = 100µF 40V radial
C2 = 470µF 40V radial
Semiconductors:
D1,D2 = BAT41 or similar Schottky diode with IF>80mA
D3 = zener diode 15V, 0.5W
D4,D5 = LED (see text)
T1 = BC557B
T2 = BC550B
Miscellaneous:
S1 = switch (see text)
PCB, order code 040008-1 from the PCBshop
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Click below to download a PDF copy of this article from Elektor magazine. Please note. In view of the complexity of international markets, Elektor cannot guarantee the availability of components for this project.
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