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Low-Drop Constant Current Source

for Ultrabright LEDs

Low-Drop Constant Current Source
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.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor Members can exclusively download these files for free as part of their Membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service. Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

R1 = 6Ohm 8 or 18Ohm (see text)
R2 = 33kO
R3 = 820Ohm
R4 = 10MOhm
R5 = 22kOhm or 0 Ohm (see text)
R6 = 10kOhm
C1 = 100µF 40V radial
C2 = 470µF 40V radial
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
S1 = switch (see text)
PCB, order code 040008-1 from the PCBshop
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